Jun 16, 2021When and how to use pesticides in the greenhouse
Vegetable growers often use greenhouses or hoop houses to start transplants for field production or for full-season protected culture. Certain types of pests and diseases can be reduced in these controlled settings, but the occasional outbreak may require treatment from a pesticide. Vegetable transplants can sometimes benefit from preventative applications of a fungicide before they reach the field.
Many foliar-applied pesticides have longer residuals in certain greenhouse settings. Usually, pesticides degrade with exposure to sunlight. Greenhouses that use UV-blocking materials remove a large spectrum of light between 10 and 400 nm that we cannot see with our own eyes, but contributes to pesticide degradation. Therefore, sidewalls and coverings that block UV light increase residual activity of pesticides.
Glass and acrylic sheeting and untreated polyethylene films allow the most amount of light across the entire spectrum to penetrate to the crop canopy. Fiberglass, polycarbonate and rigid PVC sheeting, as well as PVC and treated polyethylene films can either partially-block or fully-block UV light.
Greenhouse label language
The label is law. Label language will indicate whether a certain pesticide application is allowed in a greenhouse, and a restriction statement is usually found in the “Directions for Use” section. Very often, greenhouse applications are only allowed on certain crops or crop stages.
Some labels contain different rates and recommendations for the same crop inside and outside of a greenhouse. For example, streptomycin is an antibiotic that is only allowed on tomato transplants in the greenhouse as an effective control for bacterial diseases, and is not allowed for use on outdoor tomatoes at all.
Occasionally, a label will not indicate greenhouse restrictions, but also will not provide special instructions for greenhouse use. When the label is silent on greenhouse use, it is classified as an implied use, and can be used as long as the target crop is on the label.
How does Michigan define a greenhouse?
The Michigan Department of Agriculture and Rural Development (MDARD) defers to the Worker Protection Standard (WPS) for its definition of a greenhouse. The most recent update to the WPS has termed it “enclosed space production,” and defines it as “production of an agricultural plant indoors or in a structure or space that is covered in whole or in part by any nonporous covering and that is large enough to permit a person to enter”.
So, if a pesticide label does not allow its use in an “enclosed space production” area, then you cannot use it in a poly film hoop house even when the sidewalls rolled up and end walls are open. The Environmental Protection Agency (EPA) anticipates most greenhouses, hoop houses, high tunnels and similar structures will fall within the definition of enclosed space production, but a final determination will be made on a case-by-case basis applying the parameters of the definition to each situation.
Some operations will use “shade cloth” during certain production or market phases. Shade cloth used within a greenhouse would be subject to the “enclosed space production” procedures. Where shade cloth is the sole “covering,” the employer will need to determine if the particular material is porous or nonporous.
In addition, there are “porous” versions of Polyethylene (PE), Polypropylene (PP), Polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE) and Ethyl Vinyl Acetate (EVA). As there has not been guidance issued on these materials, consider the traditional greenhouse structures to be “enclosed space production” areas.
Worker pesticide safety and greenhouse applications
Greenhouse pesticide applications require compliance with restricted-entry intervals (REIs) and spray notification regulations under the WPS. The following scenarios are modified from the one-page reference document, “Pesticides in Enclosed Agricultural Spaces,” and identifies the entry restrictions when applying pesticides for enclosed space production to ensure workers and other persons are not exposed to the pesticide(s) being applied. The restrictions depend on the types of pesticide(s) or application method used.
When using a fumigant, the entire enclosed space and structures/areas that cannot be sealed off are restricted until ventilation criteria* is met or REI expires, whichever is longer. If a fumigant had an REI of 48 hours, and the grower chose to ventilate for 24 hours without ventilation, they must still wait another 24 hours because the REI is 48 hours total.
When applying fine droplets that DO NOT require a respirator, the entire enclosed space is restricted until ventilation criteria* is met or the REI expires, whichever is longer. If this is not possible, use larger droplets. For a product with an REI of 4 hours, sprayed in an enclosed space with a fine nozzle the grower would need to ventilate the entire space by any of those methods listed above, and whichever takes longer is the entry time. If a grower chose to wait 24-hours without ventilation, then that REI is effectively 24 hours.
When applying fine droplets (293 microns or smaller) that REQUIRE a respirator, the entire enclosed space is restricted until ventilation criteria* is met. No one is allowed into the entire space until it is vented. The “treated area” is restricted until ventilation criteria is met or REI expires, whichever is longer. For greenhouses, a “treated area” can be as small as one plant.
When applying medium droplets or larger (greater than 294 microns) MORE THAN 12 inches away from growing media, the treated area plus 25 feet in all directions within the enclosed space is restricted until the application is complete. The treated area is restricted until the the REI expires. Ventilation is not required. For greenhouses, a “treated area” can be as small as one plant.
When applying medium droplets or larger LESS THAN 12 inches away from growing media, the treated area is restricted until the REI expires. Ventilation is not required.
Ventilation must continue until one of the following conditions is met:
- Ten air exchanges are completed.
- Two hours of ventilation using fans or other mechanical ventilating systems.
- Four hours of ventilation using vents, windows or other passive ventilation.
- Eleven hours with no ventilation followed by one hour of mechanical ventilation.
- Eleven hours with no ventilation followed by two hours of passive ventilation.
- Twenty-four hours with no ventilation.
If the REI of the product is greater than four hours in an enclosed space or 48 hours in an outdoor space, a warning sign must be posted for all applications. If the REI is less than or equal to four hours in an enclosed space or 48 hours in an outdoor space, workers can be notified with an oral warning or a posted sign. Employers must also post application locations when a label requires “dual notification” regardless of the stated REI.
Vegetable pesticides that can be used in the greenhouse
So, what commonly used pesticides labeled for vegetable use can be used in the greenhouse? Below is a list in order by trade name. If the label is open to all greenhouse uses, we indicate “yes.” If the label specifically disallows greenhouse uses, we indicate “no,” or cautions against its use, we indicate “avoid.”
If the label makes specific uses allowable in certain crops or crop stages, we put “certain crops only, see label.” In the cases where a label implies a use by not including any greenhouse language, we indicate “silent.” Always double-check the label that comes with your specific product and formulation.