Weed Management

In Maryland, populations of common ragweed have developed resistance to three herbicide groups, Group 9 (glyphosate), Group 2 (FirstRate, Synchrony), and Group 14 (Flexstar, Valor). These herbicide-resistant populations limit options for effective postemergence control. Even though alternative herbicides are available, control can be inconsistent when applied to common ragweed plants exceeding 4” tall (Figure 1). 

Herbicide resistant common ragweed (Ambrosia artemisiifolia L.) is prevalent on the Lower Eastern Shore of Maryland and other areas in the region. In 2019, common ragweed populations on the Eastern Shore of Maryland and Delaware were found to have two or three-way mode-of-action resistance to herbicides. They were resistant to glyphosate (group 9), cloransulam (Firstrate; group 2 “ALS inhibitors”), and/or fomesafen (Reflex; group 14 “PPO Inhibitors”). Early-season management of common ragweed is strongly dependent upon reducing seed emergence and controlling ragweed populations prior to soybean planting. However, due to the limited herbicides with efficacy against common ragweed, it is even more critical to utilize alternative and multiple means of control.

Mowing is a relatively inexpensive mechanical weed management option that imposes minimal disturbances to the soil. Several types of commercial mowers including rotary, flail, reciprocating sicklebar and reel can be used to suppress weed growth. Still, mowing is generally not thought of as a formable integrated weed management (IWM) tool as it is not congenial to most cropping systems or all land types. For instance, having a smooth soil surface free of rocks or other obstructions is a necessity for mowing operations, and if mowing close to the ground, the soil surface should be even. Some have declared that mowing is primarily used to limit seed production and restrict unsightly weed growth in un-tilled herbaceous and woody perennial crops.

Pros and cons of vinegar as an alternative to glyphosate

Poison Hemlock (Conium maculatum, Fig. 1), is a member of the plant family Apiaceae, which contains a few important crops such as carrots, celery, and parsnips. This weed is a tall, invasive, and highly poisonous weed that is sometimes mistaken for one of its crop relatives. It is also commonly mistaken for Queen Anne's Lace (Daucus carota).

In last month’s issue of Agronomy News, I discussed considerations for glyphosate-based burndown programs. This month I wanted to share some of my results using similar programs to manage a rye (Fig. 1) and hairy vetch (Fig. 2) cover crop. Treatments were applied on April 11, and consisted of glyphosate, glufosinate, paraquat, glyphosate + 2,4-D, glyphosate + dicamba, glyphosate + glufosinate, and glyphosate + paraquat. Currently, this is what I am seeing:

Starting clean or weed-free is the key to a good weed control program, especially when noxious weeds, such as Palmer amaranth are present. While conventional growers can use soil-active herbicides to manage these weeds, control is more complicated in organic systems. Flame weeding is a non-chemical tactic that has been shown to control several grass and broadleaf weed species.

Controlling Italian ryegrass, the potential causes of control failures following glyphosate applications, and useful tips for managing the species.

Garlic Mustard Identification and Management in the Landscape

Understanding Preemergence Herbicides for Common Weeds in Vegetables