How Can You Control Greenhouse Weeds?

Weeds can be a persistent problem in greenhouses with natural floors. Weeds are a pest in their own right, but also harbor insects like whiteflies, aphids, thrips, mites, slugs, and pathogens like impatiens necrotic spot virus (INSV), tomato spotted wilt virus (TSWV), and others. Weed seeds can be brought into the greenhouse on equipment, clothing, unfiltered irrigation water, and can blow inside from the wind. Weeds can be pervasive, and reproduce several times in one year. They produce an ample amount of seed that can easily spread throughout the greenhouse. 

Most Common Greenhouse Weeds 

• Chickweed (Stellaria media)
• Creeping woodsorrel (Oxalis corniculata)
• Bittercress (Cardamine hirsuta)

Greenhouse Weed Prevention

Using weed blocking fabrics to line the greenhouse floor provides a physical barrier that prevents weeds from establishing. To maintain the effectiveness of the weed block fabric, it must be taken care of. If the fabric becomes worn or gets a hole it should be patched or replaced. Any spilled growth medium should be cleaned up promptly, and fabric should not be covered with gravel. Covering fabric with gravel gives weeds a place to take root and also does not allow for the fabric to be swept and cleaned. Most weed fabric is black, but white varieties have the added advantage of reducing cooling costs since they do not absorb as much heat.

• Prevent weeds near the greenhouse by mowing around the greenhouse regularly to reduce the likelihood that they will be brought inside. 
• Landscape care prevents vegetation, including weeds, from flowering. 
• If weeds get inside the greenhouse, be sure that they are removed before they flower and reproduce. 
• Always use a clean growth medium (soil or other) and clean plant material to help prevent weeds.
• Keep growth medium covered when not in use to prevent seeds from getting in.

Greenhouse Weed Control: Herbicides

Hand weeding to remove weeds in the greenhouse is good for when there are just a few weeds, but in some cases, an herbicide is needed. Herbicides may be chemical or natural based. Preemergence herbicides are applied to prevent weed seeds from germinating. However, there is no greenhouse approved preemergence herbicide, so post-emergence herbicides are the only option. Post-emergence herbicides kill weeds after they have sprouted. If weeds are allowed to get large, the herbicide will only burn them instead of killing them completely. Contact herbicides applied to small weed seedlings are best. Remember to have a safe chemical storage plan for herbicides used in the greenhouse.

Natural Herbicides

There are some natural herbicides that are approved for organic use. They are most often made from acetic acid, citric acid, citrus oil, and/or clove oil (AKA eugenol). These herbicides are not selective and work on contact with the plant. They generally require multiple applications for controlling perennial weeds. Different product names exist for natural herbicides, and if you are unsure of their organic status, you can check the Organic Materials Review Institute website. Be aware that natural products can still present human health hazards when directions and precautions are not followed.

Chemical Herbicides

Chemical herbicides present potential hazards when used in greenhouses so few are labeled for use in greenhouses. It is important to avoid using volatile herbicides because the greenhouse ventilation can drift the spray back on the worker, and from weeds on to crops. Even outside of the greenhouse there is a risk of exposure to sprays. 

Herbicide Injury Plant Symptoms

• Discolored and thickened leaves
• Stunted leaves
• Injured growth tips
• White discolored foliage
• Cupped leaf shape
• Plant death

Dedicated sprayers should be used to apply greenhouse labeled herbicides to ensure the sprayer is not re-used for applying another material to crops. Even traces of sprays intended for use on greenhouse weeds can be a hazard to crops. The best time to treat a greenhouse with herbicide is while it is empty between crops and ventilation can be turned off. 

Chemical Greenhouse Labeled Herbicides

• Non-selective, systemic, no residual control or preemergence activity

• For use on greenhouse floors in an empty greenhouse

• Ammonium nonanoate: Axxe
  • Non-selective herbicide, works by contact
• Clethodim: Envoy Plus
  • Selective, postemergence herbicide, for the control of grasses only, works by contact
• Glufosinate-ammonium: Finale
  • Non-selective, postemergence, systemic herbicide
• Fluazifop-P-butyl: Fusilade II
  • Selective, postemergence, systemic herbicide, for the control of grasses only
• Diquat dibromide: Diquat SPC 2L, Reward
  • Non-selective contact herbicide
• Pelargonic acid: Scythe
  • Non-selective, postemergence, contact herbicide

Outdoor Herbicide Use

When using herbicides outdoors around the greenhouse, a knockdown insecticide should be used to avoid stirring them up towards vents and into the greenhouse. A horticultural oil does a good job of this. Next, you can mow the area and apply a non-selective postemergence herbicide. Auxin-containing and volatile herbicides should not be used near a greenhouse.