It takes as many growing degree days for forsythia shrubs to burst into bloom as it does for crabgrass seeds to germinate. If you don’t want crabgrass in your lawn, those blossoms mark the perfect time to apply a crabgrass pre-emergent.
The paths in my garden are full of crabgrass. To get back into the house I have to drive my electric scooter across the lawn, undoubtedly spreading crabgrass seeds, but there’s no crabgrass in my lawn. Why? Because for the past several years I’ve applied or had applied corn gluten meal during the forsythia blossom time.
Corn gluten meal (CGM) is a byproduct of corn starch milling (which in turn manufactures high fructose corn syrup), and for years has been used for animal feed and pre-emergent seed control. The high fructose corn syrup connection is probably responsible for part of the growing cost of CGM, since that syrup has been blamed for a lot of human obesity in this country, and people are trying to consume less of it. The supply of CGM has probably been correspondingly reduced.
In the early 1990s, horticultural researchers at Iowa State University discovered the pre-emergent capability of this product of the corn wet-milling process, and the university began to obtain patents on CGM. Now, if you buy CGM as a pre-emergent, Iowa State University gets royalties. If you buy CGM sold as animal feed, it’s cheaper because Iowa State doesn’t get a cut. It’s all the same product.
One of the reasons I prefer to use CGM to control crabgrass is that it’s also a fertilizer, about 10 percent nitrogen. That’s enough nitrogen for our lawn to last through the summer. Too much nitrogen, especially in hot weather, results in thatch. CGM, deriving from a food source, is reliably safe for humans and pets, I think.
How does CGM work?
Some say it dries out the seed before it can germinate, but I disagree. I believe the explanation that I saw documented years ago that it destroys support roots as they try to establish a seedling. My opinion is based on the fact that CGM does not prevent dandelions or mallow, both tap-rooted “weeds” whose seeds could have dried out. I knew it didn’t prevent tap-rooted plants because I occasionally use CGM in my vegetable garden for fertilizer, and recall my dismay when I discovered I had spread CGM where I had previously planted carrot seeds. Carrots came up, undeterred by CGM.
I also found sources years ago that the more years you use CGM, the more effective it is. Nearly all weed seeds we want to control are in the top half inch of soil (many require light to germinate), and the annual spread of CGM builds up in that top half inch. I’m sure the effective pre-emergent (or support root killing) effect of previous applications has diminished, but there may be a bit left.
There are other pre-emergent materials available too, and some post-emergent applications you could use. Study the label, for the label instructions are the law. Some herbicides are not selective, and can leave your lawn with ugly dead islands or kill the entire lawn if you use the wrong herbicide as a former neighbor did. Her landlord made her replace the lawn. I dislike the selective herbicide 2,4-D for weed control because it disrupts human endocrines and because it’s really a growth accelerator of plants. Apply it to rid your lawn of dandelions, and it will race to flower and set seeds so you have even more dandelions. The Natural Resources Defense Council (NRDC) calls it “the most dangerous pesticide you’ve never heard of “ for its effect on estrogen, androgen and the thyroid gland. The Environmental Protection Agency (EPA) has warily refused to condemn it, claiming the evidence is not conclusive. It is a major money-maker for chemical companies.
A botanist once advised subscribers to a garden list to mow lawns at a height of 3 inches, to prevent weeds. Mowing at that height does do a pretty good job of shading the soil so that weed seeds seldom germinate. Some “yardeners” recommend mowing shorter the first few mowings to get sunlight to grass roots quickly, but I don’t think that helps. It does give weed seeds a boost, though. If you can’t get grass to grow under your trees, you’re not alone. Grass does need sunlight. You can leave the ground bare or use a groundcover such as lamium, although some lamiums can become invasive thugs. Other ground covers for shade include Japanese pachysandra or periwinkles.
Send garden questions to firstname.lastname@example.org or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.