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how to kill weed seed in soil

Cover crops can be a tool for weed seedbank management. “Cover crops have had a long history in the Midwest, and can make a comeback should market forces and weed management realities combine to make them necessary,” Davis says. “Adding cover crops to the weed management toolkit can reduce opportunities for weed seedlings to establish, and can also provide a critical delay in weed seedling emergence so that crop competition can further suppress weeds.”

Davis conducted that early research to evaluate a valuable option for growers who want to manage hard-to-control weed populations: Manage the weed seedbank to prevent weeds from getting started.

Davis says the most important element of a successful weed seedbank management strategy is to implement a diversified crop rotation so crops with contrasting life histories are grown in different phases of the rotation. “This prevents any one weed species from getting too comfortable, and makes it difficult for weeds to reproduce in the canopy of a crop with a different life history,” he says.

Weed management toolbox

As a graduate student, Adam Davis spent his Septembers crawling around on his hands and knees through crop fields trying to find and recover giant foxtail seeds for his research studies. He soon discovered that seed predators had already eaten the seeds on the soil surface. Mice, crickets, ground beetles and other organisms were doing a highly effective job at reducing the number of weed seeds.

“We focus on killing weed seedlings because we can see them, and we have products available to do this,” Davis says. “Certainly, we don’t want to give up managing weed seedlings, but if we reduce weed seedbank population densities, managing weed seedlings becomes much easier.”

The key to weed seedbank management is to reduce the ability of the seeds to germinate. Then growers either don’t have the weeds to control or they have fewer weeds to handle with other weed management tools. Here are Davis’s suggestions for managing a seedbank:

“I began to measure weed seed predation and saw that within two days, nearly all of the seeds I’d put out in the field [to measure the amount of weed seed predation] would be gone,” says Davis, an ecologist with USDA’s Agricultural Research Service at the University of Illinois. “Over the course of a growing season, I’ve seen weed seed predators eat between 40% and 90% of the seeds produced that year. It’s an important weed management benefit that we take for granted, but which really makes a difference.”

Of course, if you use an organic mulch (such as a bark mulch), it will eventually decompose anyhow, becoming fertile ground for weeds. What can you do? Well, you had better keep new weeds pulled, faithfully. Vigorous roots pushing downwards can stress landscape fabric and breakthrough. On the bright side, these weeds should be relatively easy to pull, since mulch is a lot looser than dirt, and weed roots will not become impossibly entrenched.

There is lots of work involved since soil solarization entails getting to the root of the problem, underground.   And we will not be taking the shortcut of using herbicides, so that means a bit more work. But if you do not mind getting your hands dirty, then let’s roll up our sleeves and begin stopping our weedy foes in their tracks.​

When you cut slits in the landscape fabric and install new plants, be careful that you don’t get dirt all over the landscape fabric. After all, why prepare a home for airborne seeds? Sure, you will be applying mulch. But airborne weed seeds can wend their way through mulch particles. If they find dirt, then they are “weeds waiting to happen.”

Preparation

Now use a steel rake on the area that you have just tilled, wielding it like a fine-toothed comb to remove the majority of the uprooted weeds. Next, rake the area again, this time with the object of evening out the soil as best you can and removing stones, twigs, etc. The final preparation for soil solarization will require the use of a garden hose. According to the University of Idaho Extension (UIE), you should moisten the area that you have just raked to “conduct and hold heat, to stimulate weed seed germination, and to prevent dormancy of below-ground vegetative plant parts.”

Now you truly have a “clean slate” with which to work. Remove the plastic and lay down landscape fabric. You should try to use one of the stronger types of landscape fabric if possible, just in case—in spite of your best efforts—any sharp objects remain in the ground (which would puncture the landscape fabric).

Cover the raked, moistened area with a clear polyethylene sheet. The edges of the sheet can be held down by cinder blocks to keep the plastic from blowing away. If the raking mentioned above was done diligently enough, there will be no sharp objects sticking up to puncture the plastic. The sheet of clear plastic can be anything from 1 to 6 mil. in thickness. In the Northern hemisphere, the best time for soil solarization is June and July, when the sun is at its peak. UIE recommends keeping the sheet of clear plastic tightly stretched out over the area for about 2 months. During that time, the sun will be killing weeds for you—”cooking” them before they have a chance to sprout. Plant pathogens will be killed, to boot.

Perhaps you are wondering at this point, “Why do I need soil solarization? Why can’t I just lay landscape fabric at this point, punch some holes in it, plant my new plants and then cover with mulch?” Well, the reason you can’t is that your job of killing weeds has only just begun. Weed seeds that you can’t even see are lurking beneath the surface, just waiting to sprout. If the weeds are vigorous enough, they will find a way back to the light (remember, the integrity of the landscape fabric will be compromised when you punch holes in it for your new plants). So you need to kill those seeds before you proceed with laying landscape fabric. And that is a job for soil solarization.