Weekly Weeder: Stop Crab Grass in your lawn

Baby crab grass in bare dirt

          Summer is here, and crab grass is sprouting in lawns, very thick in some.  It’s past time to mulch your lawn, but not too late to smother a lot of it.

Last year, I started mulching crab grass in Dad’s lawn when it started sprouting, which was a bit late for the amount of mulch I was using, an inch of compost.  Sprouting crab grass is very small, and can be hard to see before it gets big enough to grow out of the mulch.  Not that the effort was wasted; the compost still stopped a lot of crab grass, and the lawn is looking much healthier and greener than last spring, without moss under the tree.

Young crab grass, spreading from its crown

Lawn grasses can readily grow through an inch or more of compost and will be much happier for it.  If your grass is perennial rye, a clumping grass, it’s a good time to seed more rye into the compost where it is patchy.  Mulching leaves into the lawn over the fall and winter with the lawn mower will help smother small-seeded grasses like crab grass and Bermuda.

            Crab grass is a clumping relative of Bermuda grass that also roots along its stems, but it’s tender, and dies with the first frost.  It is a tender perennial, as it does not die from making seed, but only spreads and makes more seed until it freezes.  It roots deeply where it is well watered, and hardly roots at all where it is not watered well, living on dust and dew collected on its hairy leaves. 

This young, mature crab grass appears to be growing in well-mowed Bermuda.

Bermuda grows rhizomes that can travel under sidewalks and go 18” deep and goes dormant about 6 months of the year in Oregon, making it a lawn weed in this area, not good grass.   Bermuda and crab grass show their family relationship in the shape of their seed stalks, and the size of their seed, which is, thankfully, small and easily smothered with mulch.  So the same mulching that stops crab grass can also prevent Bermuda from germinating.

Ironically, in Arizona, where they use Bermuda as lawn grass, they mulch their lawns with steer manure every spring, which keeps them thick and green in the summer heat, and crab grass is rare.  Here in southern Oregon, I don’t recall seeing anyone else using steer manure or other compost on their lawns, and crab grass has spread all over town and down our country roads in the last 20 years.  Chemical fertilizers don’t smother weed seeds, and crab grass has no problem with dryness or low fertility.

Green crab grass with dry foxtails and cheat

Even if you mulch, it is unlikely that you will stop all the crab grass in your lawn, since the seed is everywhere.  Unlike annual grasses, the roots of crab grass are tough and wiry, and where they go deep, it is nearly impossible to pull after it flowers.  But it can be cut off its roots with garden scissors or a knife, and it won’t grow back, as it has no food in those wiry roots.

It doesn't pay to spray crab grass with Roundup (glyphosate salts), even in gravel and bare dirt.  It kills the plant, but fertilizes the seed in the soil; it comes back greener every time.

Revised June 2016, online at GardenGrantsPass.blogspot.com with photos. Follow Rycke or subscribe.

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Rycke Brown, Natural Gardener        541-955-9040      rycke@gardener.com