Diligent sanitation is the key to successful weed control inside greenhouses. Weed control also requires special precautions and the use of herbicides that have little or no chance to vaporize. Consequently, only a few herbicides are registered for use inside entirely enclosed greenhouses. Always turn off ventilation fans while applying postemergence herbicides, and avoid contact with desirable vegetation.
clethodim (Envoy Plus)
Rate 0.09 to 0.24 lb ai/A (12 -32 fl oz) with max of 64 fl oz/season)
Time Apply postemergence to actively growing annual or perennial grasses as listed on label.
Remarks Consider environmental and plant growth conditions that affect leaf uptake (see label).
Caution Do not exceed 68 fl oz/A per season.
Site of action Group 1: acetyl CoA carboxylase (ACCase) inhibitor
Chemical family Cyclohexanedione
Rate 0.5 to 1 quart/100 gal water
Time Apply with nonionic surfactant any time weeds are small and actively growing.
Remarks Avoid spray contact with desirable ornamental plants. Acts as a contact herbicide; absorbs energy produced by photosynthesis, forming peroxides that disrupt living cells.
Caution A moderately toxic herbicide that requires protective face shields, respirators, and clothing for handling and application. Do not ingest or inhale spray mist.
Site of action Group 22: photosystem I electron diversion
Chemical family Bipyridilium
glyphosate (numerous products)
Rate Consult labels
Time Apply to actively growing weeds when desired vegetation is out of greenhouse and air circulation fans are turned off.
Remarks Consult label for specific rates, application times, and species controlled. Honcho requires adding a nonionic surfactant; Expedite Application Equipment contains a specific formulation using small-droplet technology.
Caution Do not exceed 10.6 lb ai/A per year.
Site of action Group 9: inhibits EPSP synthase
Chemical family None generally accepted.
oryzalin (Surflan AS)
Open greenhouse-type structures only
Rate 2 to 4 lb ai/A (2 to 4 quarts/A Surflan AS)
Time Prior to weed emergence.
Remarks Inhibits cell division or mitosis, primarily in roots.
Caution Do not apply in enclosed greenhouses or enclosed shade-house type structures.
Site of action Group 3: microtubule assembly inhibitor
Chemical family Dinitroaniline
pelargonic acid (Scythe)
Rate Broadcast: 3 to 10 gal product/100 gal water. Spot spray: 1 to 10% solution (1 to 13 fl oz/gal) to cover 400 sq ft
Time Apply to thoroughly wet weed foliage in and around greenhouse, under benches, or preemergence in seedling beds.
Remarks Avoid contact with desirable plants. Disrupts cell membranes, causing leakage and symptoms of leaf burn.
Site of action Group 26: unknown
Chemical family Carboxylic acid