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remove weed seeds

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.

“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.”

Weed management toolbox

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:

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.

“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.”

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Most spacing recommendations, however, are based on the assumption that adjoining plants will barely touch when they reach mature size, so stick with the guidelines when working with plants that are prone to foliar diseases, such as bee balms (Monarda didyma and cvs., USDA Hardiness Zones 4–9) and phloxes (Phlox paniculata and cvs., Zones 4–8).

I’m hoping to get my garden under control. I was thinking about laying down some weed killer, then use some landscaping fabric. Would some mulch installation be good on top of the fabric?

More on controlling weeds

Put drought on your side by depriving weeds of water. Placing drip or soaker hoses beneath mulch efficiently irrigates plants while leaving nearby weeds thirsty. In most climates, depriving weeds of water reduces weed-seed germination by 50 to 70 percent. Watch out, though, for the appearance of deeply rooted perennial weeds, such as bindweed and nutsedge, in areas that are kept moist. They can take off in a flash when given the benefits of drip irrigation.

In lawns, minimize soil disturbance by using a sharp knife with a narrow blade to slice through the roots of dandelions and other lawn weeds to sever their feed source rather than digging them out. Keep in mind that weed seeds can remain dormant for a long, long time.

Mulch benefits plants by keeping the soil cool and moist and depriving weeds of light. Organic mulches, in particular, can actually host crickets and carabid beetles, which seek out and devour thousands of weed seeds.

Some light passes through chunky mulches, and often you will discover—too late—that the mulch you used was laced with weed seeds. It’s important to replenish the mulch as needed to keep it about 2 inches deep (more than 3 inches deep can deprive soil of oxygen). In any case, you can set weeds way back by covering the soil’s surface with a light-blocking sheet of cardboard, newspaper, or biode­gradable fabric and then spreading prettier mulch over it.