Asked September 28, 2019, 5:30 PM EDT

We have public rose gardens in Ladd's Addition which we maintain largely with volunteers. The West garden was one garden which was not treated with glysophate and many of the beds are overrun with crabgrass. My understanding is that crabgrass is an annual, and it seems it may be most efficient just to remove the seed heads and leave the roots in the ground to provide their organic matter, if indeed they are going to be killed by frost and only the seeds will sprout in spring. Could you share your insights into the best way for us to weed crab grass without spraying poison?

Multnomah County Oregon

1 Response

This response came from other States Master Gardener Programs but it contains reasonable suggestions.If you want to avoid the use of chemicals in pre-emergent herbicides, corn gluten is often cited as being an effective alternative. However, there does not seem to be consensus on this point. See the following:

The crabgrass seed germinates when the soil temp reaches 60 degrees F for 3-5 days. It sets seed in July and dies with the first frost in Fall. Seed can lie dominate in the ground for several years before germination sp any control will be on going not just a single year of work. A lot of time and money is spent by homeowners attempting to eliminate crabgrass. There are a lot of products which are very effective reducing its presents in any year; however most are herbicides. Like most weeds, crabgrass is an indicator of other problems with the growing conditions in the lawn. The conditions have to be corrected to get long term crabgrass control. These include cultural, chemical and mechanical control. You are looking for other than the chemical control.
Cultural Control: create a dense, healthy turf, this will reduce the crabgrass and prevent it from germinating. MOW at 2.5 to 3 inches, mowing below this level increases crabgrass germination. Mow frequently so as not to remove more than 1/3 of the blade. Water deeply and infrequently. Daily light watering promotes shallow rooted, non-drought turf, and encourages crabgrass. Water to depth of rooting. Fertilize Get a soil test available from your local Extenson office and follow their recommendations
Preemergence Control: Corn gluten is a preemergent non chemical herbicide (even used as livestock feed) which is applied before the seed germinates. Around this area when the forsythia is starting to bloom is a good time for application. Once the product is applied, the ground must not be disturbed by raking or other means as it will disturb the barrier. Another problem may be the extensive rains we had this year which deluted the effectiveness of the product. Do not use corn gluten on new seedlings or before seeding an area as the product works just as well on any seed.
MECHANICAL CONTROL: When plants are small, they can be removed by pulling; however, extensive patches make this unrealistic. Mowing before the plant goes to seed will help reduce the seed bank.