Crabgrass season is off to an early start this year. If the seeds are present in the soil, the only requirement in our area is three days in a row of temperatures 50 -55 degrees for those seeds to germinate. While we are enjoying a little warmth and the bloom of daffodils, forsythia and nut trees, this weed, really an annual, is invading our lawns, vegetable patches, and garden landscapes.
Two types exist: smooth and large. Smooth is a summer annual, spread by seeds and culm nodes on top of bare soil. You can mow it down to ¼ inch and it will still set seed waiting to surprise you given the right conditions. This is the type most commonly found in lawns.
Large crabgrass also spreads by seed and by nodes lying on top of bare soil that root. It's called “large” because it can grow to 2 feet if not mowed. According to UC IPM, one large crabgrass plant can produce upwards of 150,000 seeds, and the seeds from both large and small crabgrass can live at least 3 years in the soil. In other words, take care of the problem weed now, or you will be taking care of it all summer.
The best strategy is to have good strong turfgrass to prevent crabgrass from taking root in the first place. Fertilize your lawn when it is growing. Over-seed your lawn as needed. Don't over-water your lawn. Deep, infrequent irrigation is better than short, daily watering. Mulch garden beds. Soil solarization can be used as a long-term strategy. Many gardeners use pre-emergents, but they have to be applied before the seeds germinate (which they likely already have) and the pre-emergents can prevent the fresh lawn seed from germinating. It takes about 4-5 months for the pre-emergents to break down.
One option that has been touted is corn gluten meal, CGM for short. It is a by-product of corn milling and it has about 9-10% nitrogen, so it's a fertilizer, too. But research studies from California, Washington State University and Iowa State have shown that CGM is not effective outside of greenhouse conditions for crabgrass control and extermination. Weather conditions have to be perfect to allow the CGM to dry up the emerging weeds, and CGM is expensive. The recommended amount is 20-40 pounds per 1000 square feet and then it needs to be applied twice a year. And if you are trying to grow organically, you would need to research if the CGM is milled from genetically modified corn.
Finally, post-emergent products are available, but the crabgrass has to be small (1-3 leaf stage), otherwise, the product can damage the good turf around it. More information about specific pre-emergents and post-emergents for homeowners and professional landscapers can be found on the UC Pest Notes #7456 and “The Myth of Weed-Killing Gluten: ‘Cornmeal gluten is an effective organic herbicide'”, by Dr. Linda Chalker-Scott.
