Preemergence Crabgrass Control Strategies

By Jeffrey Borger

For many turf managers, spring is the end of a long hibernation away from the outdoors. You most likely notice the aftermath of the winter in some of the turf you maintain — tree limbs, gravel from plowing roadways, and leaves that were never collected before the onset of cold weather.

You also know that crabgrass germinates in the spring. But it can be controlled with a range of differing preemergence chemistries. The following are some preemergence crabgrass control strategies.

Crabgrass plant’s developmental stages

Knowing the particulars about how the crabgrass plant germinates, develops, and expires is essential. Crabgrass germinates in the spring (Figure 1). The experienced turf manager knows, on average, when this happens in their respective regions. One should never rely solely on the calendar to determine the germination patterns each year. Nature has a phenological indicator to assist in predicting crabgrass germination. A phenological indicator can be defined as any recurring biological phenomena such as one plant growth stage that happens at a similar time another seasonal action is occurring.

Fig 1. Newly germinated crabgrass in the spring of the year.

Fig 2. Forsythia is in full bloom.  The yellow petal drop has not started.

Fig 3. Forsythia yellow pedal drop has begun.  About this time crabgrass will start to germinate.

Figure 4. A visual representation of the preemergence herbicide concentrations in the top 1/4 inch of the soil profile following an application in the spring of the year.
All photos and graphics provided by Jeffrey Borger

For example, where I live, the Mid-Atlantic region, when the forsythia plant drops its yellow flower petals can be a phenological indicator of the start of the germination of crabgrass. This timing can still prove to be a bit ambiguous. When exactly in the petal drop process will the crabgrass germination happen? Refer to Figures 2 and 3 to see some stages of forsythia petal drop.

To more accurately predict crabgrass germination in the spring, soil temperature and moisture should be monitored and recorded. Crabgrass germination happens when the soil temperature at a 1/4-inch depth maintains a temperature of 50 to 55 degrees Fahrenheit for three consecutive days and nights, and moisture is present.

Crabgrass germination begins sometime from late March to mid-May in our region. Generally, the majority of the crabgrass will stop germinating about mid-July. The crabgrass plant will mature and be eliminated from the sward at the first heavy frost. Sometimes, a late heavy frost in the spring can also eliminate newly germinated crabgrass populations. However, following a heavy frost in the spring, crabgrass will continue to germinate and still be a problem if not attended to in some fashion.

Formulate a management strategy

A strategy to control crabgrass using preemergence material can seem simple. The difficult part of most strategies is the implementation phase. There are many things that can be controlled by the turf manager and equally as many that cannot. Although the list of things that cannot be controlled is long, the most unpredictable is the weather. Remember, formulate the management strategy and be prepared to change it often.

There are really two distinct preemergence crabgrass control strategies — apply preemergence materials only once or apply them twice. There are several additional things to consider. If the materials will only be applied once, the rate and the longevity of them must be evaluated. For example, one product may have a five-week residual when applied at a certain rate and a different material could have an eight-week residual. The cost of the materials and the longevity could be deciding factors. Conversely, the amount of labor to make more than one application of any material may top the list of deciding factors. These issues must be adjusted to accommodate each turf manager’s specific requirements.

Figure 4 represents the application of preemergence materials and the expected length of time crabgrass will be controlled. This graph shows how things are supposed to work. The X axis represents time and the Y axis represents the concentration of preemergence herbicide in the soil where the crabgrass seed will be germinating. For example in the Mid-Atlantic region, typically crabgrass germination starts sometime in late April and continues until about mid-July. This is about 3 months (or 12 weeks or 90 days). The preemergence material must have adequate concentration in the soil profile in order to prevent the crabgrass seed from germinating. This is represented by the yellow line on the graph. Any level of concentration of preemergence material above the yellow line will prevent germination, and any level below will not. The red line on the graph represents the concentration level of a preemergence material applied once. The blue lines represent “split applications” (two applications) of materials. As indicated on the graph, when time passes, the concentration of preemergence material decreases in the soil profile but in both cases have lasted 90 days or more. This should control the majority of crabgrass germination.

Product labels will have information about the residual (longevity) of the product and application rates. If the preemergence control of crabgrass has not been attainable, reevaluate your strategy. Review the following questions: Has the correct rate of product been applied at the correct time of the year? When was the last time you calibrated you application equipment? Does the product have enough residual to last for the season using the single application methodology? Should split applications of product be incorporated into the program? Was the product watered in per the label instructions? These are some of the key questions to consider. Remember you can make a plan — but plan to change it frequently to adapt to nature.

Jeffrey Borger is an instructor, Turfgrass Weed Management, in the College of Ag Sciences at Penn State’s main campus in University Park, Pa.

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