Tuesday, March 11, 2014

Stop crab grass before it starts

          Spring has sprung with a vengeance.  Foxtails are already putting up seeds stalks.  Sprouting crab grass cannot be far behind.  It’s time to mulch your lawn.

 Sprouting crab grass in bare soil.  The leaves are not only fat, but fuzzy.

          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.

The fuzziness of young crab grass leaves shows better in this young plant.

          Even though it is early March, lawns are growing and being mowed.  Lawn grasses can readily grow through an inch 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 it into the compost where it is patchy.  Dad’s lawn is mainly fine fescue, a rhizomatous grass, and white Dutch clover, both of which fill in when given sufficient organic matter, nutrients and neutral soil.  I also spread blood meal and ashes, bringing up the nitrogen, iron, and Ph to help kill the moss and feed the grass.

This young mature crab grass is growing in what looks like well-mowed Bermuda grass before its bloom.

          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. 
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, 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.


Green, mature crab grass with dry, seeded foxtails and cheat in dry ground.  Crab grass, being a perennial, is not a fire hazard like the other two as it stays green and growing until frost.

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 Kengyu garden scissors, 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).  It kills the plant, but fertilizes the seed in the soil; it comes back greener every time until you spray so much, so frequently that you over-fertilize.