Crabgrass invasion

Crabgrass growing in a bare patch at the edge of a lawn

Crabgrass is an annual strain of wild grass (a new plant will grow from seed each year).  Seeds travel on the wind and are transported by animals. Once in the ground, the seeds lie dormant until conditions are right for germination. Crabgrass seeds have been known to remain in soil for up to 15 years and normally blow down the street resting near curbs and driveways until the seed has the right amount of soil contact, heat and water.

Note the overlapping blades in the upper left

There are two varieties of crabgrass commonly found in Ontario. Large, or hairy crabgrass (Digitaria sanguinalis) and small, or smooth crabgrass (Digitaria ischaemum). Both are recognisable by their broad, pointed leaves that grow from a common stem.

Crabgrass germinates when soil temperatures average 12.8°c consistently. Typically in the Greater Toronto area we encounter most crabgrass growth from July through September. Because of the need of warmth, crabgrass typically first appears along the edges of driveways and other paved areas.

There is not currently any effective, selective treatment for actively growing crabgrass. Because it is a type of grass, it is difficult to develop a treatment product that does not also effect desirable varieties.

However, there are several things that can be done to reduce the amount of crabgrass you get in the future.

As an annual, crabgrass dies at the end of the year. Before it dies each plant can drop over 150,000 seeds, which can lie dormant for years. So getting rid of crabgrass is all about preventing as much seed from growing on the lawn as possible.

Removing crabgrass and preventing new seeds

  • Manually pull as much crabgrass as possible before seeds mature
  • Crabgrass growing on hard surfaces like interlock or between stones can be destroyed with horticultural vinegar
  • Mow frequently and bag all clippings to capture seeds. Raking prostrate crabgrass tillers before mowing to stand them up will improve results

Reduce germination of crabgrass seeds

  • Rake out dead plants along the edge of driveways early in spring to prepare for seeding with desirable grasses varieties
  • Thicken the lawn with core aeration and over-seeding to reduce thin or bare areas to crowd out the crabgrass
  • Let the lawn grow to two and a half or three inches tall to shade the soil and reduce the amount of sunlight and warmth that reaches dormant crabgrass seeds
  • Water deeply, once a week, to strengthen desired grass and minimize surface moisture that crabgrass seeds need to germinate
  • If you are not planning to seed the lawn, apply corn gluten meal (it is a decent fertilizer also)

Corn gluten meal can be used to create a barrier on the surface of the grass which helps to limit seed germination. If you are considering this treatment, be aware of false claims, know its limitations and be patient. Corn gluten meal can prevent up to 40% of crabgrass germination but will have the same effect on desirable grass seeds, and will take up to three years for protein levels to become established in the soil.

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