Figure 2. Mint production areas of the Midwest.

Mint was introduced into the U.S. in colonial times and was first grown in Massachusetts. Mint growing gradually moved westward with the early settlers. The soils and climate of southern Michigan and northern Indiana were so well suited to mint culture that these areas became the major mint-producing areas of the United States by about 1920.

In 1924, Verticillium wilt was diagnosed in a field near Mentha, Michigan. This disease gradually spread and became more severe, eventually causing many midwestern growers to abandon mint production. After 1950, peppermint acreage increased greatly in Oregon and Washington, until this area is now the largest mint-producing area in the U.S. All mint on the west coast is grown on mineral soils (most of them light-textured) under supplemental irrigation. Midwestern mint is grown almost entirely on drained organic (muck) soils, usually with some supplemental irrigation. Wisconsin now has a significant acreage of mint on mucklands which were cleared and drained fairly recently.

The end product of mint culture is the oil produced in special glands on the mint leaves and stems. This oil is collected in pure form by steam distilling the mint hay. Peppermint oil is used to flavor chewing gum, candy, toothpaste, and medicines. Spearmint oil has a smaller market, and is used mostly to flavor toothpaste and chewing gum.

The main types of mint grown in the Midwest are peppermint (Mentha piperita ) and two species of spearmint: the so-called "native" spearmint (Mentha spicata ) and Scotch spearmint (Mentha cardiaca ) (Fig. 1). There are no improved varieties of native or Scotch spearmint at this time.

There are three varieties of peppermint: the Black Mitcham variety, named after its place of origin (Mitcham, Surrey County, England); Todd's Mitcham, named after the A.M. Todd Co., Kalamazoo, Mich., buyers of mint oil; and Murray Mitcham, named after Dr. M. J. Murray who was the leader in developing the Todd's Mitcham and Murray Mitcham varieties. These last two varieties arose from some 100,000 pieces of stolon exposed to ionizing radiation to induce mutations, and were selected after much field testing for their tolerance to Verticillium wilt and acceptable oil flavor and quality. Murray Mitcham is generally considered to be more vigorous, especially the first year after planting in the Midwest, and to be slightly more resistant to Verticillium wilt than Todd's Mitcham.

Geographical Location: In the Midwest, most mint is grown on organic (muck) soils in Indiana, Michigan and Wisconsin north of the 41st parallel (roughly the latitude of Fort Wayne, Indiana) (Fig. 2). Mint needs a midsummer day-length of at least 15 hours for satisfactory yields and oil quality. This day-length cannot be achieved south of the 40th parallel (roughly the latitude of Indianapolis, Indiana).

Soil and Nutrition: The mint plant is shallowrooted and needs abundant water and a well- drained, rich, loose-textured soil for good growth. Since drained muck soils meet these criteria better than most other soils, they are most commonly used for mint production in the Midwest. Drained muck soils have an additional advantage: the water table can be controlled in the fields by raising or lowering gates in the drainage ditches. This practice reduces the need for supplemental irrigation. Maintain the water table about two feet from the soil surface until just before harvest, and then lower it to three feet below the surface. Loose-textured mineral soils with abundant humus can also be used for mint production, but will require more irrigation.

Muck soils are subject to wind erosion during dry, windy periods in the spring (Fig. 3). Windbreaks, usually willow, are often planted in north-south rows at field margins to help prevent wind erosion.

Since mint is grown as a perennial crop, it is important to have the soil as weed-free as possible prior to planting, especially of troublesome weeds such as Canadian thistle, field bindweed, and red root pigweed. Fall-plow the soil and disk and harrow thoroughly before planting to reduce weed populations.

Figure 3. Wind erosion on muck soils.

Keep the pH of muck soils between 5.5 and 6.5 for mint production. If the soil pH is below 5.5, add lime to raise the pH. Have soils tested by a soils testing laboratory to determine fertility levels and appropriate fertilization rates. If soil test levels are lower than 100 pounds available phosporus per acre or 400 pounds available potassium per acre, fertilize prior to planting first year mint.

With favorable soil fertility conditions, the equivalent of 300-500 pounds of 5-20-20 fertilizer is usually applied per acre for first year mint on Midwest muck soils. Nitrogen is sidedressed at 20 to 25 pounds per acre when plants are 8-12 inches tall. On established plantings, the equivalent of 250- 500 pounds of 5-20-20 fertilizer are applied broadcast and harrowed-in prior to the time growth begins in the spring. Add additional nitrogen as a top dressing if needed.

Mint is seldom responsive to additions of micronutrients except on newly reclaimed acid mucks where copper applications (5 pounds per acre) may be beneficial. Mint grown on muck soils above pH 6.5 may benefit from manganese (5-6 pounds per acre) included in the drilled-in fertilizer or from foliar manganese applications (1-2 pounds per acre).

Propagation and Planting: Mint is planted in late April-early May by dropping pieces of mint stolons (also called runners or roots) in the row with a mechanical planter (or occasionally by hand), and covering with 2-3 inches of soil. Single stolons laid end-to-end in the row provide a sufficient stand for good row mint establishment. Roots and shoots soon grow from the nodes on the stolons and establish new p lants.

Stolons are generally dug from a nursery bed or from an existing planting in the spring, using a stolon digger (Fig. 4) or potato digger. Dig stolons just before planting in the spring or late in the fall. Do not plant stolons if they have sprouted beyond 2-3 inches.

The quantity of stolons required to plant an acre depends on row spacing and how thickly stolons are planted within the row. An acre of well-established mint that has not had significant loss due to winter damage or other factors will usually yield enough stolons to plant 10-l 5 acres, or sometimes as much as 20 acres.

If stolons are dug and not planted immediately. place them in piles of moderate size and cover with soil until used. Stolons exposed to sun or wind will wilt and lose vigor. Cover stolons as soon as possible after they are dropped in the row, and do not plant in dry soil. Do not make furrows too far ahead of time so that they remain moist when stolons are planted.

Figure 4. Mint stolon harvester (custom-built).

Figure 5. Crop damage due to terbacil (Sinbar) residues in soil.

Fields may be started with young plants if a late planting is made, or if planting stock is scarce. When mint shoots are 4-6 inches tall, loosen the soil around the base of the plants with a spading fork, and pull the shoots (with some roots attached) from the soil. These plants can be set in the ground with a celery or vegetable transplanter. Heavily irrigate plants immediately after transplanting.

Transplants allow later planting of the crop, which insures against frost damage. However, transplants are more apt to be damaged by hot, windy, or dry weather. If the soil is infested with Verticillium, transplants are more susceptible to wilt damage in the young bed than are plants growing from stolons. Transplants are also more likely to become infected with rust in first year plantings.

Irrigation and Drainage: If mint is grown in muck or heavy mineral soils in the Midwest, the soils must be well-drained. While mint requires great amounts of water during the growing season, it does not tolerate water-logged soils.

Mint requires about 5 acre-feet of water per year for grovvth in the West where there is little rainfall. In the Midwest, sprinkler irrigation is used to supplement rainfall, or at times to reduce frost damage or wind erosion of muck soils. Mint is a very shallow-rooted crop, so irrigate whenever the top 4 inches of soil is so drv that it will not adhere and retain its shape when squeezed in the hand. The water table can be controlled by adjusting the height of the headgates in the drainage ditches.

Mint is usually plowed under after the first killing frost to protect the stolons from winterkill. Do not plow stolons deeper than 4-5 inches, or they will not survive. Try to plow at a speed that turns under stolons and stubble cleanly with a minimum exposed on the soil surface.

The year before mint is planted, it is advisable to either (1) plant the field to a crop which can be kept weed-free with tillage and/or herbicides, or (2) to fallow the field with frequent disking to eliminate as many troublesome weeds as possible. Excessive weed populations compete with mint and reduce yields, and may contribute off-flavors to the mint oil at harvest, resulting in lowered oil quality.

Two herbicides, terbacil (Sinbar®) and bentazon (Basagran®) are currently registered for use in mint crops. Terbacil can be applied as either a preemergent or postemergent spray in the spring. For best results, terbacil must be applied before weeds are more than one-inch tall and followed by at least 1/2 inch of rainfall or irrigation within 10 days. Terbacil will control most common weeds found on muck soils if applied at the recommended rate before weeds get too large. Redroot pigweed, for example, can be very hard to control if terbacil is applied after plants are more than 2 inches tall.

Terbacil has a very long residual life in soil. While this makes season-long weed control possible with only one application of material, it can also cause toxicity and damage in other crops following mint (Fig. 5). Do not use terbacil within 24 months of the planting of crops other than mint. Never apply more than 11/2 pounds of terbacil (active ingredient) in any one year.

A simple test for terbacil residues can be done before planting a different crop. Collect soil from various parts of the field to be planted. Plant either oat or cabbage seeds (very sensitive to terbacil [Sinbar] ), in small containers of these samples, and place them in a warm (60-75°F) well-lighted place. Plant some seeds in soil which has never had an application of terbacil

Figure 6. Diagnostic symptoms of Verticillium wilt.

for comparison. If the test seeds germinate normally and seedlings are green, there should be no residue problem. If seedlings germinate, and then the leaves become scorched looking, there may be a terbacil residue problem, and sensitive crops should not be planted.

For some of the troublesome broadleaf weeds that may escape control with terbacil, bentazon (Basagran®) may be used postemergence. It will control such weeds as Canada thistle, field bindweed, ladysthumb, Pennsylvania smartweed, wild mustard, and nutsedge. For best results, make two applications about 10 days apart, applied when soil moisture is high and temperatures are warm. Weeds must be relatively small and actively growing for good results.

Figure 7. Verticillium wilt killing plants in a stand of mint.

Apply herbicides in 30-50 gallons of water per acre at 40- 60 pounds pressure using flat fan nozzles.

Diseases: Verticillium wilt, caused by the soilborne fungus Verticillium dahliae , is the most serious and destructive pest faced by mint growers. This fungus can survive for 10 years or longer once its numbers have been built up in the soil, even if the soil is continuously fallowed. The fungus can also survive indefinitely on the roots of many plants on which it does not cause disease, including many weed species. Thus, crop rotation is only effective in preventing the buildup of Verticillium wilt if it is practiced from the beginning of mint production -before the fungus is widespread and wellestablished. Profitable mint production may be maintained almost indefinitely by using short rotations of three years in mint followed by three years of other crops (onion is a particularly good rotation crop).

Verticillium wilt can be recognized by its rather distinct symptoms. Plants are stunted because of shortened internodes, leaves turn yellow and may eventually turn red, and opposite leaves bend toward each other because of asymmetric growth (Fig.6). When the stem is split with a knife, dark, discolored areas can usually be found inside. Stands become thinned out as individual plants are killed by the disease (Fig. 7).

Two peppermint varieties (Todd's Mitcham and Murray Mitcham) are tolerant to Verticillium wilt and are available, in addition to the standard Black Mitcham variety. Murray Mitcham seems to thrive better in the Midwest, and is recommended for midwestern muck soils. These new varieties are not immune to Verticillium wilt and may show wilt symptoms when planted in infested soil. especially the first y ear. However, the level of disease will be less than if the field were planted to Black Mitcham. Murray Mitcham usually yields better than Todd's Mitcham in first year plantings.

Figure 8. Mint rust lesions on spearmint leaves.

If mint is to be planted on soil which has never grown mint. use disease-free planting stock. Small amounts of this stock can be obtained either from Michigan State or Purdue Universities for multiplication until you have sufficient rootstock to make a commercial size planting. Also, rootstock certified to be disease-free can be purchased from certified rootstock growers in Oregon by contacting the Oregon Department of Agriculture, Salem, Oregon.

Mint rust (Puccinia menthae ) may be damaging to spearmint plantings in the Midwest. The rust fungus causes light-yellow, blister-like lesions on young shoots in the spring, and brownish-red spots surrounded by a yellow halo on the leaves later in the season (Fig. 8). Rusted leaves may eventually drop off and defoliation of plants can be severe. If rust has infected young shoots, the shoots are usually twisted and distorted and break off easily at the point of infection. In late summer and fall, leaf spots become a deep- chocloate brown as the overwintering spores of the fungus are produced.

There are two different types of mint rust. The rust that infects peppermint will not infect native (common) spearmint, nor will the rust which infects native spearmint infect peppermint. Either type of rust can infect Scotch spearmint. However, in the Midwest. rust unexplainably occurs only in spearmint.

Clean fall plowing of mint beds is the key to rust control in the Midwest. The overwintering spores on the stubble, on regrowth, or on the soil surface will be buried so that these spores cannot germinate and infect mint shoots or leaves the following year. Eliminate wild mint or volunteer mint in field borders so it will not serve as a source of infection for commercial mint. There are currently no fungicides registered for rust control in mint. An application for label of one fungicide on mint is now pending. Consult your county agent for most recent rust control recommendations.

Mint anthracnose or leopard spot, caused by the fungus Sphaceloma menthae , is occasionally damaging to peppermint, especially in Indiana. Small. sunken brown spots appear on the lower leaves, stems and stolons, and these enlarge to form oval lesions with light gray centers and reddish-brown borders (Fig. 9). The spots may unite, causing defoliation (Fig. 10) and large cankers which may lead to splitting of the stem. Similar brown spots appear on the lower leaves, and develop light brown centers with a dark border. Heavily infected plants are weakened and oil yields are reduced. Anthracnose may become severe during wet seasons when mint foilage becomes rank and remains wet for long periods.

The anthracnose fungus overwinters mainly in mint refuse rather than in the soil. Clean fall plowing will usually prevent disease the following year.

Septoria leafspot (Septoria menthae ) may cause minor damage but is usually not a major problem.

Nematodes: The root lesion nematode (Pratylenchus penetrans ) is commonly found on peppermint and spearmint in the Midwest. These small microscopic eelworms feed on the roots of mint plants, which inhibits root and plant growth. Large numbers of these nematodes may not only stunt plant growth, but often cause plants to show more severe Verticillium wilt symptoms than they would without nematodes. A nematode problem is often indicated b_ localized areas of stunted, unthrifty plants in a field. To determine if nematodes are present, collect samples of soil containing roots from suspected areas and submit to a nematode diagnostic laboratory for analysis.

If root-lesion nematodes have been a problem on any crop in a field which is to be planted to mint, use a recommended soil fumigant prior to planting. It is also important to use clean, healthy planting stock from an area as free as possible from the root lesion nematode.

Several insect pests are troublesome on mint in the Midwest. Most are sporadic and unpredictable. Regular scouting of fields is necessary to observe the presence of most of the following pests:

Mint flea beetle: This beetle was first discovered in mint in Michigan in the early 1920's. Annual production in a field with heavy infestation of flea beetle larvae dropped from 44 pounds to 24 pounds to 2.7 pounds. By 1929, mint flea beetle had become a serious problem throughout mint fields in Michigan and northern Indiana. Flea beetle problems continued in severity through the 1930's and early 1940's, although calcium arsenate was being used as a foliar treatment for adults.

By the mid-1940's, mint fields, especially peppermint, were planted in 2 year rotations due to the severity of Verticillium wilt. This short rotation had a definite effect on flea beetle populations. From 1945 until the early 1970's, flea beetle problems were almost nonexistent in the Midwest, due to short rotations and the widespread use of DDT. The advent of Verticillium-resistant mint varieties made longer rotations possible and increased the severity of damage from flea beetle.

The adult flea beetle is very small, about 0.25 cm (¹/₁₀ inch) long and light tan to bronze (Fig.11). It can usually be found in mint fields beginning about the middle of July and appears in increasing numbers until early August. Egg laying begins after a 2 to 3 week feeding period, most of which is spent on the lower leaves of the plant. Because most of the adult flea beetle activity is confined to the bottom half of mint plants, their presence and damage may not be noticed until they are quite numerous. Adult feeding damage appears as small, 1 to 3 mm (¹/₂₅ to ³/₂₅ inch) holes, scattered across a leaf (Fig. 12). When these feeding holes are numerous, the damage appears as shot-hole type damage. Because these beetles feed on lower leaves, it is quite difficult to get an accurate assessment of flea beetle populations. However, if 5 or more adult beetles are taken per sweep with a sweep net, a field probably has a serious problem.

Figure 11. Mint flea beetle adult.

Figure 12. Mint flea beetle feeding damage.

Female flea beetles lay eggs in the soil near the crowns of plants sometime in late July to early August, and continue laying eggs until late fall. The eggs do not hatch in the fall, but remain dormant in the soil until late April or early May. After hatching, the young larvae feed first on the fine hair roots and then tunnel into stolons and underground parts of the stem, where they continue to feed, for about 4 to 5 weeks. Damage on underground parts is easily visible as distinct tracks or tunnel marks (Fig. 13). In June, the larvae stop feeding and pupate in the soil with adults emerging 3 to 4 weeks later.

The larval damage is far more serious for mint production than adult feeding. In addition to interfering

with water and nutrient uptake, the feeding damage provides entrance for plant pathogens and produces generalized stress on the plants. As with other stress factors on mint, aboveground symptoms include stunting, perhaps some wilting under extremely dry conditions, and a general reddish-purple discoloration of new growth. This reddish discoloration is not, however, restricted to flea beetle feeding, nor does spearmint characteristically show this reddish coloration.

Because larval control is currently impractical, the adult stage must be controlled in order to minimize damage. Crop rotation will generally reduce buildup of populations of mint flea beetle. Also, if volunteer mint on roadsides, ditch banks, and other areas is kept down, flea beetle populations will not build up and move into new fields.

When chemical control is used, timing is critical. Apply labeled insecticides to the crop when the majority of adults have emerged from the soil, but when a minimum number of eggs have been laid. While the complete details for this have not yet been worked out, the timing of sprays for adult flea beetle control probably falls between July 25 and August 10. Make applications in late afternoon or evening to reach the adults when they are most actively feeding, usually at night and early morning. Sprays may be applied before harvest, but insecticides may not be applied beyond the legal cut- off date specified on the label. Sprays may also be applied after harvest.

Treat stubble within 3 days of harvest if the field had a flea beetle problem before harvesting. Adult beetles will leave a field soon after it is cut and migrate to adjacent volunteer mint or fields in production. They will return as new growth appears or when adjacent fields are cut and resume egg- laying.

Figure 13. Mint flea beetle larval damage.

Figure 14. Mint looper larva.

Malathion is currently registered for control of flea beetle on mint. It should be used at 1 pound active ingredient per acre. It has a 7-day preharvest interval. Methomyl (Lannate, Nudrin®) is registered and effective for control of variegated cutworm, looper and flea beetle on mint. Methomyl cannot, however, be applied closer to harvest than 14 days. Consult your county agent for the latest pesticide recommendations on this and other insect pest problems.

Mint Looper: Several caterpillars, called loopers because of their inchworm or looping habit, can be found feeding in both spearmint and peppermint fields (Fig. 14). These include the mint looper, the celery looper, and the cabbage looper. The adults of these caterpillars are dark, nondescript moths which fly at night. Eggs are laid singly on the leaves. Upon hatching, larvae feed on leaf surfaces. When the larvae are larger, about 1 to 2 cm, they begin to eat ragged-edged holes through leaves distributed from top to bottom of the plant.

Often, the damaged leaves seen on the tops of mint plants are from loopers. Because of the feeding damage to the leaves, large populations may seriously affect oil accumulation and yield. While some loopers may be found throughout the summer, the mint looper, the most common and serious of the caterpillars, appears first about June 20 with a second brood appearing around August 10-15. The first brood larvae of the mint looper can do serious damage in this preharvest period.

When fully grown, a mint looper may be as long as 5 cm with alternating light and dark green (almost black) longitudinal stripes. The head is likely to be dark brown to black. Other loopers are less strongly marked than the mint looper. When necessary, loopers may be controlled effectively with Methomyl (Lannate, Nudrin®). If treatment is necessary closer than 14 days before harvest, Dipel will probably give satisfactory control.

Cutworms: While several species of caterpillars, called cutworms, can be found in peppermint and

spearmint, the most serious of these in the Midwest is the variegated cutworm (Fig. 15). This species undergoes 3 to 5 generations per year and attacks a number of crops, including mint, potatoes and tomatoes. The moths of this caterpillar fly at night, laying eggs in bunches throughout the field.

Thus, caterpillars are found infesting circular areas throughout the field. When infestations become more serious, these patches run together giving the appearance of a uniform infestation.

Figure 15. Variegated cutworm larva.

The first generation of larvae usually appears in fields about May 25 to June 5.

After completing development, these larvae pupate in the soil with a second group of moths appearing and larvae showing up in fields about the first to second week in July. A third generation may be present from the middle to the end of August.

The larvae feed on all portions of the plant, but are usually found under the canopy rather than on the top of the plants. It is not uncommon, however, to find larvae feeding on the terminal buds. The mature variegated cutworm larva is about 5 cm long and varies from pale gray to dull brown. Larvae are marked with distinctive pale cream to yellow spots down the middle of the back. Cutworms must be controlled before they become more than 3/4 of an inch in length.

Figure 16. Two-spotted spider mites.

Two-spotted Spider Mite: Spider mites are not true insects, but are more closely related to spiders, having 8 legs instead of 6. They are only occasional pests in mint, but in hot, dry years may be quite severe in some fields. Spider mites are tiny (about 0.5 mm, the size of a period on this page) (Fig. 16). Adult mites are pale yellow and have 2 dark spots on the body. Spider mites in low-to-moderate populations are generally found on the undersides of leaves and feed by sucking out cell contents. As chlorophyll is removed, the plant takes on a mottled appearance, and eventually bronzing and browning occurs. Heavy infestation results in leaf drop and reduced plant vigor.

When mite buildup is heavy, it is often possible to see silken webbing on leaves and stems. The mites may be almost impossible to see without examining the leaf surfaces with a hand lens or microscope. One way to detect mites is to hold a white piece of paper under the leaf or plant and strike the plant sharply. The mites will be dislodged and can be observed crawling about on the paper.

Mites overwinter in mint fields in soil and in debris as females. In the spring, these females begin laying eggs on undersides of leaves. Eggs hatch in 4 to 5 days and the resulting larvae mature in 1 to 3 weeks depending upon temperature. With this rapid development, many generations of mites may be completed in a single season and numbers can reach damaging levels under ideal conditions. Research data from Oregon suggest that if an average of 5 active mites are found per leaf on a 45-leaf sample taken from 5 sites in the field, miticide should be applied to avoid significant mite damage. Materials to be used

for mite control are Kelthane at 1 pound active ingre per acre applied no closer than 30 days before harvest, with one application per season; or Metasystox-R, 3 pints per acre. It will generally take 2 applications of Metasystox-R 10 to 14 days apart to clean up infestations, and the second or last application should not be applied closer than 14 days before harvest. Malathion, if used for flea beetle control, can be expected to give only partial mite control.

Miscellaneous Insects

A number of other insects may be found at one time or another in peppermint and spearmint crops. European corn borers can be found in growing terminals of mint plants in mid- to late June and in late August. In such cases, the top 3 to 4 inches of a stem will be found laying over and wilted. If the stem is split at the point where wilting is started, a European corn borer larva will often be found. Corn borers are seldom damaging in fields and therefore no control is recommended.

A number of leafhopper species are commonly found in mint, including the potato leafhopper and the six-spotted leafhopper. No information is available on the damage these species may cause to mint nor are recommendations for insecticides available. Any of the insecticides used for control of other insects in mint should also control leafhoppers. One problem with leafhopper that may occur is their movement to other crops when mint is harvested. A serious problem can arise in potatoes, alfalfa, beans, carrots, or some leafy vegetables when leafhoppers migrate after a mint crop is cut.

Figure 17. Rotary windrower.

Mint is cut and windrowed in much the same fashion as alfalfa hay, generally with a swather or sickle bar windrower. Recently, some growers have begun using a rotary windrower imported from Germany (made by Klockner-Humboldt-Deutz AG) (Fig. 17). The rotary windrower is said to have three advantages over the conventional windrower: it moves through the field at a high rate of speed (up to 7 mph) so that cutting is accomplished quickly; it cuts closer to the ground so that oil yields per acre are increased; and it leaves the mint hay in smaller windrows so that it dries faster and is ready to distill sooner. Also, the rotary windrower is tractor- mounted and requires a lower initial investment. It remains to be seen if this new equipment will be widely accepted.

After the mint is cut and windrowed, it is allowed to cure until the leaves are thoroughly wilted and partially dried (24-36 hours). The hay should not be allowed to become too dry or leaves will shatter and

some of the oil will be lost. If the hay is too green. it will be difficult to distill. The hay is picked up, chopped, and blown into the distilling tub by a conventional field chopper (Fig. 18). Growers have gradually shifted from coarsely chopped to finely chopped (1/4 inch pieces) hay for faster and more complete distilling of oil.

The Murray Mitcham and Todd's Mitcham varieties are ready to harvest 3-6 days earlier than the older Black Mitcham variety. Carefully observe the state of maturity and cut the hay before the plants are in 10% of full bloom. Biosynthesis of mint oil decreases with the onset of flowering, and the rate of oil degradation increases at the same time. Menthone, a major component of mint oil, can be lost during bloom due to conversion to a water- soluble, nondistillable glucoside. Also, oil glands become leaky as bloom progresses, which can result in loss of oil during rainy periods. Harvesting after 10 percent bloom can thus result in both lower yields and lower quality of oil.

Figure 18. Mint being picked up, chopped, and blown into the distillation tub.

Figure 19. Mint distillation unit.

Mint distilleries consist of mint distillation tubs, condensers, receivers, and (often) redistillation units (Fig. 19).

Mint tubs

Mint tubs are, for the most part, custom built by growers. They are fabricated from 10-gauge steel plate on heavy-duty wagon running gears with large balloon tires (Fig. 20). Tubs are rectangular in shape, measuring about 6- feet deep, 7¹/₂-feet wide, and up to 12-feet long. Tubs are made about 6 inches wider at the top than at the bottom so that the chopped mint is self-packing as it is blown in. This also allows the entire charge to settle in the tub when steam is applied and the charge shrinks. This helps prevent formation of channels for steam along the sides of the tub. These channels prevent good steam penetration of the entire load.

Figure 20. Mint tub hooked up to the distillery.

A series of perforated pipes for steam are fitted beneath the perforated false bottom of the tub for introduction of steam. Total surface area of the perforations in the steam pipes should not exceed the surface area of a cross-section of the steam line into the tub, or there may be difficulty in maintaining pressure. A gasketed door fitted with an 8-inch flexible hose is clamped to the opening at the top front of the tub where the chopped hay was blown in. This is where the steam plus water vapor and oil exits the tub and is taken to the distillery condenser. Large doors in the rear of the tub, opening either from the top or the side, are used to dump the spent charge from the tub after distillation is completed (Fig. 21). The doors are fitted with gaskets made of close-celled neoprene.

Figure 21. A spent charge of minthay being dumped after distillation.

At least two tubs are needed for each station at the distillery; one tub is being filled while one tub is being distilled. If the tubs have to be transported any distance from field to distillery, three tubs per station may be needed. A tub generally will hold the hay from ³/₄ to 1 ¹/₄ acres of land, depending on how well the crop has yielded.

At least one grower has designed an automated hitch for his tubs so that the tractor operator can hitch and unhitch tubs from the tractor without dismounting from the tractor. This requires a spring-loaded tongue on the wagon chassis to keep the tongue in an upright position, a sheet metal device to guide the tongue into the hitch, and a hitch that can be released from the tractor seat.

Boilers

A high pressure boiler is needed for generation of steam in a distilling unit. The size of the boiler will depend on the number of tubs to be distilled at any one time. The rule of thumb used is to provide 100 horsepower for each tub. Thus, a 4-station still should be powered by a boiler of about 400 horsepower (Fig. 22). This leaves some surplus steam capacity for distillation of wet or green hay, which is harder to heat to oil evaporation temperature than cured hay.

Boilers are generally run at 100-125 p.s.i. at the boiler; 60-80 p.s.i. is applied at the tub until condensate begins coming through the condenser, and then the pressure is cut to 10-15 p.s.i. to complete distillation of the charge. Excessive amounts of steam after the charge is heated will cause some oil to pass through the condenser without condensing. About one hour is generally required for distillation of a tub, but this time varies with tub size, hay condition, and steam pressure.

Figure 22. A modern 4-tub distillation unit.

CONDENSER

Figure 23. A horizontal header-type condenser.

Condensers and Receivers

Once the mint oil is vaporized from the mint charge, it passes through the vapor hose, along with vvater vapor, into the condenser (Fig. 23). The condenser is a continuous series of pipes immersed in water so that vaporized oil and water will recondense for collection in the receiver. Condensers are usually either of the horizontal tank type, which is most popular, or the tubular, upright type. Horizontal condensers provide more condensing capacity per unit volume of space occupied. Usually, one horizontal condenser tank will serve three or four stations (one tub per station). A tubular, upright condenser is usually needed for each station or for every two stations. The header- type horizontal condenser (Fig. 23) is replacing the worm-type horizontal condenser. The temperature of the water in which the condenser is immersed should be maintained at 110°F This is done by means of a thermostatically controlled valve which automatically controls the flow of cold water into the condenser tank. Aluminum is the best material for construction of both condensers and receivers because of its heat transfer properties and resistance to corrosion.

Receivers are designed to collect the water and oil from the condenser, and to allow the lighter mint oil to separate and float on top of the water so it can be drawn off in relatively pure form (Fig. 24). The level of the oil in the receiver is controlled by raising or lowering the level of the end of the drain pipe where water leaves the receiver. At the top of the receiver, oil can be periodically drained off through a spout. BN7 raising the end of the water drain pipe, the water level is raised, causing the oil to flow off. Care must be taken to stop collecting oil before the water level in the receiver is reached.

To prevent churning of the oil in the receiver. which affects oil separation, baffles are usually installed below the inlet where condensate enters the receiver. The baffle plates direct the separated drops of oil toward the surface. If the receiver is properly constructed and care is taken, the oil will be entirely clear and can be placed directly in.drums for shipment.

Boilers, condensers, and receivers are built commercially and can be obtained from several sources (see appendix). Buying a used boiler can often mean considerable savings.

Redistillation Units

Many growers now install a small unit on their distilleries consisting of a small steam-heated distillation chamber and a condenser to recover additional oil from the effluent from the receivers. The effluent from the receiver is fed into the redistillation chamber, re-vaporized by steam heat, and re-condensed to encourage separation of oil which passed through the receiver on the first distillation (Fig. 19). The condensate is fed back into one of the receivers for col lection. Most growers feel that they recover 1-3 pounds of oil per tub by redistillation.

Figure 24. Mint oil being collectedfrom the receiving can.

Yield of Oil: The yield of oil from mint varies with the season, geographic location and cultural conditions. The average oil yields per acre for peppermint and the spearmints in the major Midwest mint growing states usually fall in the 30-35 pound per acre range (Table 1). As indicated earlier, a midsummer day-length of at least 15 hours is required for maximum yields. Also, periods of clear, sunny weather. especially as the crop approaches harvest, are essential for maximum yields. By contrast, cloudy, wet weather for periods of several days prior to harvest will reduce yields. A particular problem occurs if there are heavy rains when the mint hay has been cut and is drying in the field. Such rains can actually wash oil from the foliage, causing further losses in yield.

Other factors which may reduce oil yields include poor or irregular stands, dense stands that prevent full leaf development and encourage premature leaf drop, rough handling, weed competition, damage by insects and diseases, drought, and other weatherrelated factors.

Price of Oil: Prices received by farmers for peppermint and spearmint oils depend on demand, quality and carryover from previous crops (Table 2).

Capital Investment and Production Costs: The equipment and machinery needed to grow the crop and distill the mint oil are expensive. The distilling operation, in particular, requires specialized equipment that may have no other use on the farm. This presents a special problem for the new grower, so that it is often advantageous to have the first crop or two distilled by established local growers.

The cost of the distillery varies considerably depending upon location and availability of used equipment from other growers. For example, much of the equipment, such as the portable tubs, are custom built either by local tradesmen or on the farm. The most costly item is the boiler, which can often be purchased secondhand at considerable savings.

Estimated cost for new equipment for a 2-tub distillery, including four portable tubs, condensers, receivers and other necessary equipment (less the boiler) are currently as follows: four portable distillation tubs, $20,000; two condensers and tank, $2,000; two receiving cans, $800; pipe fittings, fuel tank, pumps, valves, hoses, gauges, wiring, and labor, $3,000; building to house the distillery, $6,000; and well and pump, $1,500. Total estimated cost, $33,300. Larger distilleries are expensive because the multitub unit requires a correspondingly larger boiler; however, the boiler should be large enough to handle any anticipated expansion.

Production costs vary greatly among farms, depending upon acreage, labor, cropping program and other factors. In an Indiana study in 1980, the total cost of producing oil from first-year peppermint ranged from $345 to $460 per acre. This cost included land, stolons for planting, fertilizer, pesticides, labor and equipment costs to plant, cultivate, weed, harvest and distill the crop along with all overhead costs. Second and subsequent year costs ranged from $300 to $410 per acre, since there is considerable reduction in labor costs and the cost for planting stock (stolons or plants) is a non-recurring expense.

To increase production, supplemental sprinkler irrigation has become increasingly common on mint in the midwestern region. Growers may apply from 4 to 10 acre- inches of water, depending upon the season. In the Indiana study, the cost for irrigation was $7/acre-inch of water when water was drawn from existing wells or drainage systems.