From BASF: The Chemical Company Website
Several weeds, commonly referred to as winter annuals, continue to grow and often become problematic for growers on crops being produced or over-wintered during the coolest months of the year. Annual blue grass (Poa annua), common chickweed (Stellaria media), common groundsel (Senecio vulgaris), hairy bittercress (Cardamine hirsute), henbit (Lamium amplexicaule), horseweed (Conyza canadensis), purple deadnettle (Lamium purpureum), yellow rocket (Barbarea vulgaris), and wild mustard (Sinapis arvensis) are examples of weeds commonly referred to as winter annuals.
Winter annuals typically germinate in the late summer or early fall, overwinter as seedlings, and flower during the spring months. Inside greenhouse structures, these weeds may be active with various life stages occurring simultaneously throughout the winter months. Many of these weeds are commonly observed throughout the year including during the heat of the summer.
Preventing weeds from entering production facilities is the first weed management strategy growers should implement. One point of entry is on incoming starting materials; all plant purchases should be inspected for the presence of small weeds that could possibly manifest into much large problems over time. Where possible, maintain at least a 50 foot weed-free zone surrounding the entire production site to prevent weed seeds from blowing onto the crops.
To prevent weed population explosions, it is important to manage weeds on an on-going basis throughout production. Existing weeds in the current crops and around the production facility should be removed by hand before they flower and produce seeds. Unfortunately, there are few alternatives to costly hand weeding for removing existing weed populations in established, actively growing containers.
Besides hand weeding, weeds can be effectively controlled using post- and/or pre-emergent herbicides in certain situations. Herbicides are more cost effective than hand weeding. The use of herbicides on crops is often not feasible due to crop tolerances to these products (potential to injure some crops following application) or they are not labeled for application in some production facilities (enclosed structures).
Post-emergent herbicides can be used, in some cases, to control existing weeds on containerized crops (selective post-emergent herbicides), but are most commonly used to control existing weeds in non-crop areas such as along driveways or the perimeters of production sites.
Several non-selective, post emergent herbicides (diquat, pelargonic acid, glyphosate, and glufosinate-ammonium) are labeled for applications within enclosed structures. These products have very specific restrictions on their use within enclosed structures, often limiting applications to under benches, in walkways, and around the foundation of the greenhouse. Use great care when applying herbicides within greenhouses as even small amounts of spray drift can cause significant injury to greenhouse crops.
Pre-emergent herbicides can be used to prevent new weeds from becoming established. These herbicides essentially kill winter annuals as they germinate and up to a few days after the germination process has begun. Pre-emergent herbicides form a chemical barrier (up to 1-inch thick) over the surface of the growing medium or ground after they are applied (when applied properly). As weed seeds germinate and grow into the chemical barrier, their growth is either inhibited or the seedlings are killed.
Pre-emergent herbicides containing dimethenamid-P, isoxaben, oryzalin, pendimethlan, and prodiamine are commonly used in non-crop areas and in many instances can be used on containerized plants in OUTSIDE production sites. Currently, there are NO pre-emergent products labeled for application to crops being grown inside greenhouses (covered/enclosed production houses). Some growers apply these products to crops in outside production sites or several weeks before moving them into enclosed facilities while they are going dormant or prior to applying the protective coverings for the winter months.
Controlling winter annuals involves numerous strategies such as preventing them from entering the production sites, inspecting plant materials from outside sources, maintaining weed free areas around the crops, hand weeding existing weeds, and using pre- and post-emergent herbicides where appropriate.
ALWAYS refer to herbicide product labels for proper application guidelines and restrictions BEFORE making applications to or around crops. Failure to apply herbicides properly could lead to significant injury and crop losses.
The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned. ALWAYS READ PRODUCT LABELS AND USE THEM AS DIRECTED ON THE LABEL.