First Line of Defense – Cash Management

Weeds are a shelter for insects. Keep the greenhouse weed free to prevent insect infestations.

Photo by Lee Stivers, Penn State

Sanitation is the first line of defense in any crop protection program and can reduce potential pest and disease problems from insects and mites. The effectiveness of insecticides and miticides or biological control agents depends on the implementation of a strict remediation program.

Sanitation includes removing weeds, reducing algae and removing plant debris from both inside and outside the greenhouse.


Weeds inside and outside the greenhouse provide shelter for insect and mite infestations such as aphids, leafminers, thrips, spider mites and whiteflies, allowing these pests to survive and spread to the main crop. Weeds that can provide shelter for insects include sow thistle, sonchus spp. (aphids and whitefly); oxalis, Oxalis spp. (thrips); and dandelion, Taraxacum officinale (White fly).

In addition, many weeds serve as reservoirs for pathogens, such as viruses that can be acquired by insects and then transferred to the main crop during feeding. Consider installing landscape or fabric barriers under benches to prevent weeds from growing under the benches and reduce algae growth. Herbicides are registered for use in and around greenhouses, although caution should be exercised when using herbicides in greenhouses. A pre-emergent herbicide can be applied before the weeds emerge, while a post-emergent herbicide can be applied after the weeds appear. However, take care to avoid accidental plant damage (phytotoxicity) when using herbicides, especially those with systemic (when applied as a spray) and post-emergent activity, by making applications when greenhouses are empty. Always read label directions before mixing and loading. Large weeds (6 inches tall) should be physically removed by hand, taking care to remove both the above-ground portion and the roots. Weed-free zones or areas around the outer perimeter of greenhouses (10 to 30 feet) can reduce insect migration, such as winged adults of western flower thrips (Frankliniella occidentalis), through openings, which reduces the potential incidence of disease transmission.


Algae are an ideal breeding substrate for fungus gnats and coastal flies; then algae from sofas and floors must be reduced or eliminated. The practices of not over-watering and over-fertilizing plants and using well-drained growing media will certainly help to avoid algae problems. In addition, algae reduction can be accomplished by using commercially available sanitizers such as those containing the following active ingredients: hydrogen peroxide, hydrogen dioxide, and quaternary ammonium chloride salts. Another practice is routine high-pressure washing of the growing medium from benches and walkways, which will reduce algae build-up and prevent you from dealing with insect infestations.


Plant residues, such as leaves, flowers and growing medium residues, provide shelter for certain insect and/or mite infestations. Insects and even mites can migrate to fresh plant material if plant debris dries out. For example, reports have shown that plant matter and growing medium waste placed in unsealed waste containers can be a source of insect pests. If plant material dries out, adults can migrate to the main crop. Therefore, always dispose of waste in waste containers with tight-fitting lids. Also, any leftover growing medium provides places for adult fungus gnats (Bradysia spp.) to lay eggs and western thrips to pupate. Use a broom or shop vacuum to remove plant or growing medium debris. In addition, old stock plants or plants left over at the end of the growing season should be removed as they can be a potential source of insect and mite infestations. Ancient stock plants can also serve as a reservoir for the viruses transmitted by insects such as western flower thrips. Another important hygiene practice is to immediately remove plants that are heavily infested by insects or mites from the greenhouse.

Source: Raymond Cloyd, Kansas State University

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