Preemergent Herbicides for Aisles between Plastic Covered Beds

Suggestions for smaller scale vegetable growers

Published: February 27, 2020

Commercial vegetable production throughout much of this country relies extensively on raised beds covered with plastic mulch. Some longer season vegetables in this system may be in place for three to four months, making season long weed control more challenging. Examples are crops like cantaloupes, peppers, tomatoes, and watermelons. A common weed control tactic has been to repeatedly spray between these beds with glyphosate, using shields over the nozzles to prevent drift onto the crop. This can give season long control to upright crops (e.g. tomatoes) but for vining crops this practice is limited to about the third week following transplanting, when vines run off the raised beds. Other disadvantages on relying on glyphosate repeatedly are:

• Some weeds (e.g. water hemp) are becoming resistant to glyphosate.
• Glyphosate is less effective on certain weed species than other herbicides. As an example, Sandea provides better control of yellow nutsedge, with Sandea than glyphosate.
• More applications of glyphosate required than when other herbicides are use. For example, weed control program based on preemergent herbicdes should be possible with two applications. For glyphosate, during a rainy season, four or more applications may be required.

Melon plot on June 11, which was 25 days after the beds were formed and 22 days after preemergent herbicides were applied. Some weeds were becoming noticeable.

Melon plot at end of June. A second round of preemergent herbicides (and a burndown) was applied about 2 weeks prior (the day of the photo above). A light layer of straw was then applied, mostly to facilitate harvesting during rainy conditions, but it likely suppressed some weed growth.

Failure to control weeds earlier in the season is more harmful to the crop growth, as the weeds significantly compete for moisture, nutrients and light. Later in the season, weeds are less impactful, but continue to harbor insect pests and restrict airflow, thus increasing disease pressure. Weeds are also likely to increase the amount of protective pesticides needed, as they generally will increase the total leaf surface in a given area.

Growers may be reluctant to use preemergent herbicides; they are more complicated to use in an integrated program and hand applying them can challenging. Tilling the soil after applying can only be accomplished with very specialized equipment, so rain activated products are generally used. This article provides some very simple suggestions on products to use, when to apply and scaling to smaller areas. Missouri lost its special use permit for Dual Magnum on cantaloupes and watermelons this year, so it is important for growers make adjustments. League is an herbicide now labeled for tomatoes and peppers, but it does not control grasses, so was not included. All herbicides in the table below control grasses and small seeded broadleaves, except Sandea, which controls large seeded broadleaves.

Table 1* Preemergent herbicide options or combinations for consideration between plastic covered beds of described vegetables.

* See the 2020 Midwest Vegetable Production Guide for Commercial Growers for complete details. Pay special attention to the description on weed control in the chapters specific to these crops.
** While labeled for cucumbers, Command has a 45 day preharvest interval. This may limit its use for cucumbers that bear quickly.
*** Use shields to protect vegetable crop when using a burndown. Glyphosate is more likely to damage crop (as it is a systemic) than a product like Gramoxone SL 2.0. If only grasses are present, use of Poast or Select would be sufficient and pose less risk to adjoining crop.
**** Dacthal is an option but only controls grasses. Command is only labeled for cabbage.

A major limitation to using preemergent herbicides is their effect on subsequent crops. Typically several months up to a year or more may be required to elapse before sensitive crops are planted. Thus, if a grower wants to replant a mixture of vegetable crops back to the same area of land, this could be limit use of preemergent herbicides. It is up to the grower to become familiar with the rotational crop restrictions, which can be significant. An example is with Sandea, which should not be followed with Cole crops for 15 to 18 months.

Calculating the area to apply and amount of spray solution
(example in parenthesis):

• Determine the row length (150 ft), the area between the plastic beds (4 feet) and number of row middles (7). (150 feet x 4 feet x 7 = 4200 sq ft.)
• This area (4200 sq ft) is very close to 1/10th of an acre (43,560 sq ft per acre). To mix any herbicides, add one tenth of the acre amount into the solution that will be applied.
• To calculate the amount of solution to mix to apply on that area, start by determining how much a certain amount of water will cover. A good amount would be about ½ gallon.
• It is VERY important to use a properly functioning sprayer with a flat tip nozzle, to get the right spray pattern. Keep a good level of pressure in the sprayer and walk at an easy pace you can mimic later.
• Example- ½ gallon of water covers 2 of the row middles and 1/3rd of another. (This is 1400 sq ft; 150 x 4 x 2.33 ).
• This is 1/3 of the area that will need to be sprayed (1400/4200= 0.33). One would need to mix up 3 times that amount to treat the area or 1.5 gallons of spray solution (3 x ½ gallon). A cup or pint of additional water might be good to make sure it doesn't run out just before finishing.