Try as we might, it seems that no weed management program controls 100% of the weeds in a field.
Inadequate or excessive rainfall can limit the success of preemergence herbicides. Less-than-ideal soil moisture can result in unsatisfactory in-season cultivation. Even when we have effective herbicide options, late emerging weeds often cannot be treated within the labeled pre-harvest interval. As a result, time-consuming and expensive hand-weeding is often used as a last resort.
Roguing is the act of removing unwanted plants from a field. To maximize the benefit, focus roguing efforts on the following types of weeds, keeping in mind that some weeds may fit into more than one category:
- Suspected herbicide-resistant weeds. If weeds once controlled by a herbicide application no longer die, they may be herbicide-resistant. Heavy reliance on herbicides with the same mode of action (or Group number) over time tends to select for herbicide-resistant individuals. The resistant weeds that survive a herbicide application, pass on this resistance to their offspring. Over time more of the weed population is herbicide-resistant. One way to slow a herbicide-resistant weed population is to rouge out resistant individual weeds before they set seed. A growing list of herbicide-resistant weeds can be found at weedscience.org. Currently 13 different weed species have been reported to be herbicide-resistant in Indiana, including glyphosate-resistant marestail (Figure 1).
Figure 1. Suspected glyphosate-resistant marestail continues to grow following a glyphosate application.
- Young, perennial weeds. Perennial weeds live for years. Once established, they are often very difficult to kill because they establish large, deep tap roots or extensive, spreading root systems. But seedlings of perennial plants are often easy to control with tillage, herbicides, and hand-removal. Figure 2 shows pokeweed seedlings emerging from the soil where a pokeweed fruit fell the previous year. At this stage of growth, they can easily be controlled. Roguing young, perennial weeds will be more useful in no-till production systems and in perennial cropping systems.
Figure 2. Pokeweed seedling
- Difficult-to-control weeds. Some weeds have few effective, registered herbicides in vegetable crops. They may be immune to tillage or germinate late in the season after soil-applied herbicide activity is gone and after postemergence herbicides have been applied. Perennial sedges fit into this “difficult-to-control” category. Herbicides registered for use in vegetable crops and capable of providing some level of nutsedge control include Dual Magnum® and Sandea®. Even though both can provide excellent control, they often fail to provide complete nutsedge control. In cases like this, roguing out survivors may be an effective control option on limited acreage.
Figure 3. Yellow nutsedge emergence through a hole in plastic mulch.
- Weeds with high seed production. Some weeds are notorious for the number of seeds they can produce under optimal growing conditions. Many pigweed and waterhemp species, for example, can produce hundreds-of-thousands of seeds per plant. To put this into perspective, if only one pigweed plant is allowed to survive on 1 acre of land and it produces 100,000 seeds, it would have enough seed to produce 2 pigweed plants per square foot over the entire 1 acre field. This time of year many weeds have already started to produce viable seed. In this case, it is best to dispose of rogued weeds outside the field to avoid depositing weed seeds onto and into the soil.
Figure 4. The author stands next to a Palmer amaranth plant in a sweet potato field, circa 2008.
Additional Resources:
To learn more about weed control options in Midwest vegetables, consult the Midwest Vegetable Handbook online at mwveguide.org