Crabgrass is one of the most ubiquitous weeds of lawns and landscapes. Native to Europe and Eurasia, crabgrass was introduced to the United States in the mid-nineteenth century as a possible forage crop. Since then, crabgrass has spread far and wide throughout the continent, growing just about anywhere with disturbed soil, from agricultural lands to lawns and gardens. It does especially well in sunny lawns and is most competitive in hot and dry weather when turf grasses are under stress and beginning to go dormant. The wide, yellowish-green leaf blades of crabgrass stand in stark contrast with the dark green cool-season grasses – a look that is intolerable to many lawn enthusiasts. Fortunately, crabgrass is merely an aesthetic issue in most situations.
Crabgrass is a summer annual species, which means it completes its entire lifecycle in a single growing season. It reproduces by seeds and “tillering,” or, in other words, segmented shoots that grow from the parent plant and root at the nodes. It is estimated that a single plant can produce dozens of tillers and as many as 150,000 seeds. Seeds begin to develop when the days become shorter in mid to late summer. Mature crabgrass plants are killed by the first hard frost, but not before they have produced and dropped seed. Seeds lie dormant through the winter and begin to germinate in the spring when the soil temperature reaches 55°F at a depth of 1-2 inches for about a week, and continue germinating through the late summer.
Effectively managing crabgrass takes patience, and it is important to have realistic expectations for getting an infestation under control. Complete elimination typically takes multiple years due to the copious number of seeds that plants can produce and the ability of seeds to lie dormant for a few years before germinating.
Establishing a dense and healthy lawn is the best way to keep crabgrass and other weeds at bay. Herbicides can be useful tools, but they won’t entirely keep the crabgrass away unless the following cultural management techniques are observed:
- The presence of crabgrass can be an indication that there are underlying issues with the lawn, so getting a soil test is a good place to start. Adjusting the soil pH, adding appropriate nutrients and fixing drainage issues can help achieve this.
- Perhaps the most effective cultural practice is proper mowing. Mowing the lawn to a height of three or more inches not only improves the health and resiliency of the grass itself, but taller grass shades and cools the soil, discouraging weed seeds from germinating.
- Irrigation is often necessary to maintain a dense lawn in the hot summer months and during dry spells. Watering deeply is far more important than watering frequently, because it encourages grass to grow deep roots and makes plants more drought resistant. Cool-season grasses perform best when they receive at least an inch of water a week, applied in either one irrigation session, or split over two. Check to make sure water is reaching the depth of the deepest roots (approximately 3-4 inches down).
- New lawns should be seeded in late summer or early fall. Crabgrass that germinates in the new seedbed will be killed by frost, giving cool -season grasses a competitive advantage in the spring. Over-seeding in the fall can be helpful to restore lawns that are thinning. Any bare soil in the lawn is likely to start sprouting new weeds.
- Only fertilize the lawn in the spring and fall when cool-season grasses are actively growing. Fertilizing in the heat of the summer may actually benefit weeds more than the turf grasses that have entered dormancy.
It’s usually not possible to completely eliminate crabgrass without using an herbicide, but chemicals should only be applied as a last resort. Pre-emergent herbicides that inhibit growth are the best tool available to home gardeners as long as they are applied at the proper time. Prime crabgrass germination occurs when the soil temperature is above 55℉ at a depth of 1-2 inches for at least four or five days. This is typically around the time that forsythia is in full bloom. Pre-emergence herbicides should be applied around this same time or even slightly prior to peak bloom. They need to be on the ground before germination takes place in order to work properly. If the application window is missed, the only remaining options are to hand-pull the crabgrass or to apply a post-emergent herbicide, the latter of which is not nearly as effective as pre-emergents. If using an herbicide, make sure to read and follow all instructions on the product label.
Some gardeners swear by the use of corn gluten as an organic pre-emergent herbicide, but university research to support its effectiveness is inconclusive. Research data show mixed results, and some studies have found that corn gluten does not work especially well against crabgrass. Corn gluten typically contains about ten percent nitrogen, and this fertilizer may help green up lawns and result in dense growth that out-competes weeds. Yet, it may contribute to ground and surface water contamination depending on the application rate and any other fertilizer products used. For weed control, the established recommended rate is 20 pounds per 1,000 square feet. At this rate, about 2 pounds of actual nitrogen are added to the lawn. However, New Hampshire’s turf fertilizer law allows no more than 0.9 pounds of actual nitrogen to be applied per 1,000 square feet per application. Thus, applying corn gluten at a rate that may suppress weeds puts the applicator on the other side of the lawn and may cause environmental harm. Corn gluten can certainly be used as an organic nitrogen fertilizer as long as no more than 0.9 pounds nitrogen per 1,000 square feet are applied at one time, not exceeding 3.25 pounds of nitrogen in a year.
For those who aren’t particularly concerned about the composition of their lawn as long as it is green, crabgrass actually does a pretty good job of covering the soil. While eradicating crabgrass can take some time, it is possible to get ahead of it by practicing good cultural methods and exercising a little bit of patience.
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