Herbicides and Plastics

With the increased use of plastic culture for the production of various vegetable crops, it is very important to keep in mind a few points for proper weed management. Here are some tips:

  • If methyl bromide or Vapam fumigation is used under the plastic strips, there is no need for a herbicide under the plastic. If soil fumigation is not used and black plastic is, it may not be necessary to put a herbicide under the plastic (unless nutsedge or other weeds are excessive). If you use clear or white plastic, without soil fumigation, then you need a herbicide under the plastic mulch. To get the best results, take the following steps for herbicide application:
  • Prepare a pressed bed.
  • Apply the herbicide to the bed surface.
  • Activate the herbicide with rainfall or overhead irrigation (at least 1/2"); if the herbicide needs to be mechanically incorporated, do so and repress the bed.
  • Apply the plastic mulch. Note: Some herbicides may volatilize and cause crop injury when used under plastic mulches or row covers.  Some herbicides are prohibited from use under plastic. Be sure to observe any cautions on the product label. Wherever possible, try it on a small scale first.
  • If weeds are present between the plastic strips before planting, use a banded application of Gramoxone plus surfactant, or Roundup (if registered for stale bed use) to kill all existing vegetation. Depending on the level of weed vigor, a second application of Gramoxone plus surfactant may be needed before planting. Do not broadcast apply these herbicides to the surface of the mulch.
  • For weed control between the plastic strips, after planting, use a registered preemergence herbicide for that crop. Do not use nonregistered herbicides between strips as the crop roots will grow in that zone and, in many cases, the crop will be damaged or killed. Do not spray the surface of the plastic with any preemergence herbicides! During a rainfall the excess herbicide will wash into the holes where the crop was planted and will greatly concentrate the herbicide. Thus, the crop will be damaged or killed. When banded applications of herbicides are used, remember to adjust the rate of the herbicide downward to conform to the actual ground area being sprayed (with 30" beds and 30" areas between strips, only half the normal amount of herbicide would be required per acre since only half the amount of soil area per acre is actually being sprayed).
  • Backpack sprayers or modified boom sprayers are often used for herbicide applications in plastic culture systems. Be sure to calibrate them properly and maintain a constant pressure using a pressure gauge, especially on hand-pumped models. Maintaining a constant speed or walking pace is also necessary. Three miles per hour is a good speed (23 seconds per 100').
  • Use caution when spraying Gramoxone with a backpack or hand-held sprayer. Wear rubber boots, gloves and a mask or respirator. Avoid skin contact with the spray. If contact occurs, immediately rinse the area with water or rub soil on your skin. Soil will help soak up and bind the chemical, preventing its absorption into the skin. Spray when the wind is calm to avoid spray drift. Also, always use a surfactant and plenty of water with Gramoxone for good control.
  • Vapam is not recommended for use in the spring if the plastic is to remain in place. Under the cool soil conditions at this time of year, the soil will not aerate properly and the crop will be damaged. In plastic culture, these materials are best used if the plastic is applied during the fall before planting. Be sure with all fumigants to space the injection shanks correctly. The label recommends a 6" spacing with Vapam.
  • Herbicide use under row covers can be dangerous. Although several herbicides can be safely used under row covers, some can cause crop injury and even crop death. Generally, ventilated covers are safer than solid covers or hot caps, from an herbicide injury standpoint. This is especially true with an herbicide that is moderately or highly volatile. Test on a small scale before you apply it on a large scale.