How to Manage Pests

UC Pest Management Guidelines

Peppermint is grown in northeastern California for its oil. This perennial crop is established by transplanting either greenhouse-grown plants in spring or field-grown roots in fall. Greenhouse-grown plants are usually planted in spring in rows 40 inches apart with plant populations of about 10,000 per acre. Over the course of the first summer, stolons spread between rows to create an established stand. The wider rows allow limited cultivation for weed control before plants fill across rows. The crop is swathed, chopped, and distilled to extract the oil when it begins to bloom in August. Increases from nursery fields occur in November when field-dug roots are transplanted in 20-inch rows.

Northeastern California is an ideal location for growing peppermint. The growing conditions in this region produce high quality oil, and there is a low incidence of strains of the soil-inhabiting Verticillium fungi that reduce stand life. The major limiting factor to a profitable crop is the development of dense weed populations, especially those weeds that affect the quality of the mint oil.

Oil quality problems are more serious with broadleaf weeds than with grasses. In addition to contaminating the oil, weeds also reduce yield of a mint crop. Oil quality problems are most severe when pigweed, mayweed chamomile, prickly lettuce,and salsify are present.

Newly established mint grows slowly and is susceptible to weed competition. Competition is most severe in mint during the first and second season before the canopy closes over. Even after canopy closure, winter annual and perennial weeds can be major problems. It is important to reduce weed populations before the crop is planted because the herbicide options for newly established mint are limited.

MONITORING

Avoid planting mint in fields with a recent history of high weed populations. During the 2 years before planting a mint crop, observe and identify weed seedlings in fields on a quarterly basis. Continue this practice after the field is planted and keep written records. Pay particular attention to the presence of hard-to-control weeds or weeds that produce oil contaminants.

WEED MANAGEMENT BEFORE PLANTING

Eliminate weeds before they go to seed in rotation crops to prevent the buildup of weed seed in the soil. Planting an annual crop such as grain or sudangrass for two seasons preceding the establishment of a mint field is a good strategy because herbicides registered for use in these crops effectively control the broadleaf weeds that are most troublesome in mint. Seedbed preparation activities before planting will eliminate many annual weeds as will an application of paraquat (Gramoxone) just before planting. Perennial weeds can be controlled by crop rotation, by fallowing in conjunction with repeated cultivations, or by herbicide applications. Begin managing perennials at least 1 year before the mint is to be planted and use repeated cultivations and/or multiple applications of an herbicide such as glyphosate (Roundup, Touchdown) to control actively growing weeds. Fall applications of these herbicides are often best for controlling perennial weeds because the herbicide is translocated into the roots at this time.

Choose planting times carefully; transplants should be dug in fall after plants go dormant to avoid competition from summer weeds. Transplanting in late winter/early spring results in more competition from summer weeds.

WEED MANAGEMENT AFTER PLANTING

The primary method of weed management after a crop is established is the use of herbicides. Because of the narrow planting arrangement of this crop and the rapid spread of plants by stolons, close cultivation is generally not practiced after the first year. Herbicide selection depends upon the weed species present (see SUSCEPTIBILITY OF WEEDS TO HERBICIDE CONTROL) and the crop growth stage (see Table 1. Timing Herbicide Applications). Restrictions include legal limits on the total amount of material that can be applied in 1 year and the interval between last application and harvest.

Handweeding, though costly, is also frequently necessary before harvest to eliminate mature broadleaf weeds such as mayweed chamomile, pigweed, or prickly lettuce that have escaped herbicide treatments. These species can produce oil contaminants.

Helpful Hints to Improve Weed Control in Mint
Excellent timing of weed control in mint is critical to success. Good timing for weed control often results in high levels of weed control at the lowest cost. Also, good timing for weed control usually means early weed control when weeds are not yet emerged or very small. A delay of only a few days can sometimes mean a reduction in control from good to poor. Common timing schedules for weed control in mint in Northeastern California are listed in Table 1.

Multiple efforts to control weeds in mint produce best results. Weeds may appear in mint continually throughout the season, usually requiring multiple efforts in weed control -- multiple treatments of herbicides and more than one visit to handweed the escapes. Plan on scouting mint fields several times during the dormant and growing season to assureweeds are controlled in a timely manner.

Use a combination of techniques for best weed control. No single method will control all weeds. The most successful growers begin with a good crop rotation to reduce the number of weeds initially present in baby mint. Supplement chemical weed control with handweeding to reduce weeds to low, manageable populations.

Consider the costs of the different weed control strategies. For additional information, see Cost Study Analysis for Weed Management in Peppermint (392 KB, PDF) .

CHARACTERISTICS OF HERBICIDES AVAILABLE FOR USE IN MINT

Timing of application and herbicide rate must be carefully matched with emergence or maturity of weeds. Residual products such diuron must be applied before weed emergence (only Drexel Diuron 4L is registered in California for use on mint grown in the northeastern part of the state in Del Norte, Humboldt, Lassen, Modoc, Shasta, and Siskiyou counties). Herbicide application rates of many other products should be carefully matched to the size of weeds (bromoxynil, bentazon, oxyfluorfen, and sethoxydim). Slightly larger weeds can be controlled with paraquat or paraquat and oxyfluorfen combinations at moderate herbicide rates. Early weed control when weeds are still small is nearly always less costly and more effective than weed control later in the season, even if a second application is needed to control late-emerging weeds. Sometimes more than one herbicide is needed to control combinations of weeds. However, mixing herbicides should be approached with caution as occasionally either poor performance of the products or injury to the crop can occur. The characteristics of herbicides registered for use in California are summarized in Table 2.

Newly Planted Peppermint (Less than 1 Year Old—Often Referred to as "Baby Mint")

• Paraquat (Gramoxone Inteon) is an herbicide used in young peppermint stands. It is a contact herbicide that is used before peppermint emerges to control small annual weeds. Paraquat is excellent on emerged winter annual weeds such as cheatgrass and shepherd's-purse. It may be used on both baby and established mint but is poor on volunteer cereals, salsify, and filaree. Combining paraquat at low rates (0.38 lb a.i./acre) with low rates of oxyfluorfen (0.13 to 0.25 lb a.i./acre) enlarges the spectrum of weeds controlled and enhances weed control. Use with a nonionic surfactant at 0.25% volume by volume.
• Clethodim (Select Max) may be used on dormant and nondormant mint for control of grasses, including annual bluegrass. It can be used for baby mint and established mint. It is absorbed by the leaves of grasses and has no soil activity. It will not control broadleaf weeds. Use the high rate for heavy grass pressure, for annual bluegrass, or when grass height exceeds the label recommendation. Always add crop oil concentrate to improve absorption into the plant.
• Oxyfluorfen (Goal) used at low rates (0.13 to 0.25 lb a.i./acre) and/or paraquat (Gramoxone Inteon) may be used in baby mint before emergence of mint. Oxyfluorfen has both residual and postemergent activity. In baby mint, low rates (0.13 to 0.25 lb a.i./acre oxyfluorfen) are used for control of weeds primarily by contact activity. Oxyfluorfen is particularly effective on cheeseweed (Malva parviflora) and filaree (Erodium spp.). Contact weed control by oxyfluorfen may be improved when combined with paraquat.
• Sethoxydim (Poast) may be used after the crop has emerged for control of grasses. It is the only registered herbicide that specifically controls grassy weeds. It is absorbed by the leaves of grassy weeds. It will not control broadleaf weeds, bluegrass, or fine fescue. It may be used in either baby and/or established mint. The most common use of sethoxydim is to control grasses missed by residual herbicides. If residual herbicides are applied early in fall, their effectiveness is usually gone by late spring and sethoxydim may be used to control summer grasses such as barnyardgrass (watergrass). Sethoxydim may be applied in combination with bentazon (Basagran). Always use with methylated seed oil or crop oil concentrate to improve contact and absorption of both materials.
• Bentazon (Basagran) may be used after the crop has emerged for control of broadleaf weeds. It controls only broadleaf weeds and is the safest in baby mint. It is relatively expensive because of the higher rates required for effective control, and it is weak on mayweed chamomile. It may be tank mixed with sethoxydim for control of both grass and broadleaf weeds.

Established Peppermint (Over 1 Year Old)
These products can cause crop injury, particularly at high label rates, and must be used with caution.

• Diuron (Drexel Diuron 4L) is a residual herbicide used to control a wide range of broadleaf weeds and grasses and is especially effective for control of mayweed chamomile and pigweed. Diuron cannot be used in baby mint. However it is the mainstay of weed control in established mint. It should be applied in fall before weeds germinate—particularly cheatgrass. Nearly all of the activity of this herbicide is through absorption from the soil. It is weak on control of little mallow, also called cheeseweed or malva (Malva parviflora). A popular herbicide combination is diuron, paraquat, and oxyfluorfen, with the oxyfluorfen providing good cheeseweed control.
• Flumioxazin (Chateau) provides burndown of emerged weeds as well as residual control. Must be applied to dormant mint or unacceptable injury may occur. Apply anytime when mint is dormant before March 1. May be tank mixed with paraquat for additional burndown activity. The addition of a nonionic surfactant at 0.25% volume by volume is needed for controlling emerged weeds. Ammonium sulfate (2–2.25 lb/acre or about 17 lb/100 gallons) may be added to improve control of emerged weeds but should not be considered as a replacement for the surfactant.
• Bromoxynil (Buctril), a contact herbicide, is effective in controlling many broadleaf weeds, including sunflower; it works best on weeds that are less than 1 inch in height. Bromoxynil should only be used on established mint. It will often cause injury when temperatures exceed 70°F or spray volumes are low. Do not add a surfactant to bromoxynil or injury will be increased. Applying bromoxynil through a sprinkler system improves effectiveness and reduces injury to mint. Bromoxynil is popular for control of pigweed, lambsquarters, or sunflower at young growth stages (less than six leaves) and when ground application of other herbicide alternatives is not possible (wet soils caused by continuous irrigation or dense mint growth in June to July).
• Clopyralid (Stinger) is particularly effective for controlling salsify and more mature sunflowers. Use clopyralid to control composites (such as salsify, mayweed chamomile, and prickly lettuce) and legumes (clovers—Trifolium spp.). It controls a narrow, but critical, spectrum of weeds and has residual soil activity. It can injure mint at high rates; therefore, it is important to apply clopyralid early when weeds are small and at relatively low rates (0.12 lb a.i./acre) to reduce cost and damage to mint. Early application of clopyralid also takes advantage of the material's residual activity to control weeds germinating after the initial control of emerged weeds. Avoid use of clopyralid in last production season because only a few crops, such as wheat, grasses, and sugarbeet, can be planted in rotation within 12 months of a clopyralid application. Do not use with a surfactant because this may reduce selectivity to mint and result in crop injury. Do not feed mint slugs to livestock. Do not use for compost in susceptible crops such as legumes (it can be used as compost for grass hay such as timothy, orchardgrass, or fescue).
• Oxyfluorfen (Goal) controls many broadleaf (filaree, little mallow, and mustards) and some grass weeds, but must be applied when the mint is dormant (i.e., no visible green leaves). Oxyfluorfen has both residual and postemergent activity. It should always be applied when mint is fully dormant (no green tissue present) because the product causes injury to green mint tissue. In baby mint, low rates (0.13 to 0.25 lb a.i./acre oxyfluorfen) are used for control of weeds primarily by contact activity. On established mint, higher rates (0.5 lb a.i./acre oxyfluorfen) are used to provide residual weed control. Oxyfluorfen is particularly effective on cheeseweed and filaree (Erodium spp.). Contact weed control by oxyfluorfen may be improved when combined with paraquat.
• Pendimethalin (Prowl H20) should not be applied to first year mint. Best performance is obtained by applying to fully dormant mint in late fall before the end of December. Do not exceed 4.0 pints/acre/season. Rainfall or sprinkler irrigation after application and before weed emergence improves performance. Do not apply using an irrigation system. As mint breaks dormancy in the spring, weed control is reduced and risk of injury to mint increases. Do not feed mint slugs to livestock.

ORGANICALLY MANAGED PEPPERMINT

In organically grown stands of peppermint, much of the weed management must be accomplished before the crop is planted and consists of fallowing and cultivation. Because this crop grows rapidly and is planted in narrow-spaced rows, close cultivation is not practical after the crop is established. The primary method of weed control after planting is handweeding. Another method used on a limited basis is flaming with a propane burner. Flaming works best with small weeds during dormant periods. Geese and sheep have also been used to manage weeds in mints, with geese eating primarily grasses and sheep eating a wide range of weeds but avoiding mint.

PUBLICATION

UC IPM Pest Management Guidelines: Peppermint
UC ANR Publication 3457
Weeds
D. B. Marcum, UC Cooperative Extension, Shasta/Lassen counties
W. T. Lanini, Weed Science/Plant Science, UC Davis
R. G. Wilson, UC Cooperative Extension, Lassen County

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