by Connie Oswald Stofko
Stop weeds before they even pop out of the ground–that’s what pre-emergent herbicides do. Pre-emergent herbicides act on seeds at the germination stage.
Corn gluten meal is an organic pre-emergent herbicide. There are synthetic pre-emergent herbicides, too.
These can work well on grass seed and broad-leaf seeds. However, like every herbicide, they don’t kill every type of weed. They don’t kill existing weeds. And they don’t work on plants with tap roots, such as dandelions.
They may kill seeds that you want, so don’t use a pre-emergent herbicide within a foot or two of an area where you want seeds to grow.
These pre-emergent herbicides “don’t distinguish between weeds and cultivated plants,” said said John Farfaglia, extension educator with Cornell Cooperative Extension in Niagara County.
Using pre-emergent herbicides isn’t an issue for transplants, he noted. You can transplant a new perennial into an area where you have spread these products.
Timing is critical when using pre-emergent herbicides.
Timing is critical
The optimal time to use pre-emergent herbicides is early in the spring, just before the forsythias bloom or while they are in bloom, Farfaglia said. This is usually at the beginning of April; sometimes earlier and sometimes later. (The forsythias are blooming now in my neighborhood.)
Once the plants pop out of the soil, it’s too late to use pre-emergent herbicides.
Some weeds emerge in early autumn, he said. For those weeds, apply the pre-emergent herbicides in the beginning of September.
Corn gluten meal
Corn gluten meal isn’t the same as corn meal that you have in your pantry; corn meal won’t help with weeds.
Corn gluten meal is derived from corn and is mostly used as a livestock feed ingredient, according to this profile from the New York State Integrated Pest Management Program and Cornell Cooperative Extension.
As a pesticide, corn gluten meal has been shown to inhibit germination of both grassy and broad-leaf weeds. See more specific information on weeds that it controls in the section on herbicidal activity here.
No incidents of adverse incidents to human health, non-target species or the environment have been reported.
Synthetic pre-emergent fertilizers
One synthetic pre-emergent fertilizer is Preen. See a list of weeds that Preen controls here.
Its active ingredient is trifluralin. Trifluralin doesn’t pose adverse effects to humans or the environment, according this fact sheet from the Environmental Protection Agency.
It is practically non-toxic to birds and mammals. However, it can be toxic to fish and other aquatic animals, so you have to make sure it doesn’t get into ponds, creeks or other waterways.
Trifluralin generally is of low acute toxicity, but has been classified as a possible human carcinogen, according to this fact sheet. The main concern is for people who work with it a lot. They can reduce their risk by wearing protective clothing.
The home gardener should also read and follow package directions, Farfaglia said.
“Does Preen worry me if it’s used properly?” Farfaglia said. “No.
“Whether you’re using an organic or synthetic product, the directions for use are important.”