Controlling Weeds - Winter Annuals

From BASF: The Chemical Company Website

By Paul Pilon

Do you get caught off guard each year at the amount of growth that certain weeds put on during the coolest months of the year? 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. Additionally, the outside temperatures in Southern locations, are conducive for the development of these cool season weeds during the late fall and through early spring.

Weeds commonly called winter annuals typically germinate in the late summer or early fall, overwinter as seedlings, and flower during the spring of the following year.  However, in many greenhouses and nurseries, the environmental conditions are suitable for continuous development of these weeds throughout the winter months.  There are often various stages of development present at any time and with many weed species, they often occur throughout the year and are not only confined to the coolest months of the year.

Annual bluegrass, bittercress, common chickweed, common groundsel, deadnettle, henbit, mustard species, Sheppard's' purse, speedwell, yellow rocket, and wild mustard are examples of weeds commonly referred to as winter annuals.  However, many of these weed species also will be present during other times of the year.

There are several approaches to successfully managing winter annuals.  Some of the most effective strategies are those that reduce the introduction of weeds or weed seeds from entering the production facilities.  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 pre-emergence 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).

Post-emergence herbicides can be used, in some cases, to control existing weeds in containerized crops.  Post-emergence herbicides are most commonly used to control existing weeds in non-crop areas such as along driveways or the perimeters of production sites.  Avoid applying post-emergence herbicides over crops unless the product label allows for applications to crops; the improper application of  post-emergence herbicides can cause significant crop injury or even plant death in some instances. 

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

Pre-emergence 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-emergence herbicides are commonly used in non-crop areas; in many instances, they can be applied on containerized plants in OUTSIDE production sites.  Currently, there are NO pre-emergent products labeled for application to crops being grown in greenhouse structures (covered/enclosed production houses).  Growers commonly apply pre-emergence herbicides (isoxaben, oryzalin, pendimethlan, and prodiamine) to crops in outside production sites, several weeks before moving plants 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 post-emergence 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.

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.