TechNotes: Weeds - Winter Annuals

Many growers underestimate the potential of certain weeds to prosper during the winter months.  Several weeds, commonly referred to as winter annuals, continue to be problematic for growers on crops being produced or over-wintered during the coolest months of the year.

Typically, winter annuals are weeds that germinate in the summer or early fall, overwinter as seedlings, and flower during the spring and summer months.  However, these weeds are often observed at various stages of development throughout the year. Bittercress, common chickweed, common groundsel, deadnettle, henbit, yellow rocket, and wild mustard are examples of weeds commonly referred to as winter annuals.

The first effective weed management strategy is to prevent weeds from entering the production facility. Maintaining a 50 foot weed-free barrier around each production site will prevent many weed seeds from blowing onto the crops. Inspect incoming starting materials for the presence of small weeds that could possibly manifest to a much larger problem over time.

Once production has started, it is important to remain weed-free throughout the production cycle.  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.

In certain situations, weeds can be effectively controlled using post- and/or preemergent herbicides.  Herbicides can be much more cost effective than hand weeding.  Unfortunately applying herbicides is often not feasible, due to crop tolerances to these products (injury observed following applications) or they are not labeled for application in production facilities (covered structures).

Postemergent herbicides can be used, is some cases, to control existing weeds in containerized crops.  Postemergent herbicides are most commonly used to control existing weeds in non-crop areas such as along driveways or the perimeters of production sites. 

Several products (diquat, pelargonic acid, glyphosate, and glufosinate-ammonium) are labeled for applications within enclosed structures.  Most postemergent herbicides 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 to ensure spray drift does not move over the crops being produced. 

Preemergent herbicides essentially control weed seeds as they germinate and up to a few days after germination.  These products form a chemical barrier (up to 1-inch thick) over the surface of the growing medium after they are applied (when applied properly). As weed seeds germinate and grow within the chemical barrier, their growth is either inhibited or the seedlings are killed.

Pre-emergent herbicides 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 in greenhouses (covered/enclosed production houses).  Growers commonly apply pre-emergent herbicides (isoxaben, oryzalin, pendimethlan, and prodiamine) to crops in outside production sites or several weeks before moving them into enclosed facilities while they are dormant or just 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 postemergent herbicides where appropriate.  ALWAYS read product labels and apply herbicides according to the labeled instructions. Failure to apply herbicides properly could lead to significant injury and crop losses.
 
-Paul Pilon, Perennial Solutions Consulting