Garden Tips: Weather might be cause of crabgrass outbreak

This has been a banner year for crabgrass in lawns, including mine. About a month ago, I noticed patches of this bright green annual grass sticking out like sore thumbs across the front lawn.

We had not had a big problem with crabgrass in the past, so we did not apply a pre-emergent herbicide in the spring to prevent its germination. However, even folks who did apply crabgrass preventers at the recommended time are finding this weedy grass in their lawns too.

Before we talk about what to do about it, let's review. There are actually two different species of crabgrass found in our area: smooth crabgrass (Digitaria ischaemum) and hairy crabgrass (Digitaria sanguinalis). Native to Eurasia, they do exceptionally well here, germinating from seeds in the spring, growing rapidly during the heat of summer, flowering, setting seed, and then dying out with frost in the fall. They produce copious amounts of seed capable of germinating the following spring or in future springs.

Smooth crabgrass is most common in area lawns. When first noticed, it is light green. As the plant grows, it turns a darker dull green and forms flattened clumps. Its seeds are produced from August to September on flower stalks with finger-like spikes, resembling the those of Bermuda grass.

So why such a bad outbreak this year? I suspect it is because of the weather. In lawns where crabgrass preventer was applied at the recommended time, the chemical control may have partially dissipated by the time the weather started to warm and the crabgrass germinated. Because of this year's long spring, some of these products may have needed to be re-applied to extend the period of control.

Our long, cool spring was almost immediately followed by extended hot weather. A warm-season grass, crabgrass loves the heat, but our cool-season lawn grasses virtually stop growing in hot weather. Plus, lawn grasses can be further stressed by poor cultural practices, such as mowing, watering and fertilization.

So what can we do now? Healthy dense turf is one of the best ways to combat crabgrass next year. Scalping the lawn (taking off more than 1/3 of the leaf blades at one time) or regularly mowing too close weakens turf. Mow your lawn no shorter than 2.5 inches, mowing frequently enough so you don't scalp the lawn.

Water the lawn deeply when you irrigate. Avoid frequent daily light irrigations, which encourages shallow-rooted grass, the development of thatch, and promotes crabgrass germination and growth.

Fertilize your lawn at the right times to promote dense, healthy turf. Apply 4 pounds of nitrogen per 1000 square feet every year, applying one pound on Sept. 1, Nov. 1, May 1 and June 15. On sandy soils, consider using fertilizers with a percentage of nitrogen in a slow-release form.

If you have a crabgrass problem now, apply a crabgrass preventer next spring when the soil temperature (at a depth of one inch) is greater than 55 degrees consistently for a week. That is usually about the time when the yellow flowering forsythia has been in bloom for several weeks. It is also important to apply the preventer evenly across the lawn, taking care not skip areas.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

This story was originally published August 28, 2014 at 12:00 AM with the headline "Garden Tips: Weather might be cause of crabgrass outbreak ."