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How to kill weed seeds before they get started in your fields

How to kill weed seeds before they get started in your fields

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

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.

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:

  1. Reduce seed production by either competitive crop varieties or through agronomic practices.
  2. Kill newly formed seeds by harvesting and removing weed seeds from the field with modified harvest machinery. Modified combine harvesters have been developed for removing weed seeds, and a practice known as “spray topping” uses herbicide to kill developing seed.
  3. Look at diversifying crop rotations. Crops with different growing patterns and canopy structure can also increase seed predation.
  4. Delay primary tillage. To increase weed seed predation, delay primary tillage to give predators time to eat seeds on the soil surface. Avoid deep tillage so seeds are not protected deep in the soil profile.
  5. Encourage germination of weed seeds and plan on controlling the emergence flushes completely with a stale-seedbed approach. A stale seedbed can be made through either physical (field cultivation) or chemical means. “Field cultivation is usually more effective, since it not only kills the emerged weeds, but brings new weeds to the surface to be killed in a second pass a week or two later,” Davis says.

Cover crops comeback?

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 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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Adam Davis, an ecologist with USDA’s Agricultural Research Service at the University of Illinois, offers five ways to manage the weed seedbank.

How to Kill Weed Seeds in Soil [5 Easy Methods]

To kill weed seeds in soil you will have to apply one or more of the following methods:

  • Heat soil to temperatures high enough to kill weeds seeds
  • Force seeds to sprout and destroy growing weeds
  • Apply chemical or natural weed killers that prevent weeds from sprouting
  • Use flame weeding to destroy weeds and seeds at once
  • Layer mulch in garden areas to suppress weed sprouting and attract insects that eat seeds

With this arsenal of tricks for killing weed seeds before they sprout, you can stop the spread of weeds in both your lawn and garden.

Table of Contents

5 Tips for Killing Weed Seeds

Rather than spending time and effort battling weeds as they sprout, attack weeds at the source by killing weed seeds. Each of these methods destroys weed seeds, which ensures you won’t have to battle recurring weed invasions. Try these ways to get rid of weed seeds yourself.

Solarization

Soil solarization is a very powerful method for killing weed seeds. Weed seeds begin to die if soil temperatures surpass 108 degrees, with full seed death ensured by soil surface temperatures of 140 degrees or more. Solarization uses clear plastic tarps to trap heat at the soil surface, killing weed seeds within the tarped area. To solarize an area, follow these steps:

  • Clear the area of all vegetation through use of a hoe or other garden implement. Remove any woody stumps
  • Till the soil to further break up any weed root systems left behind.
  • Rake away all vegetation residue
  • Water the tilled and cleared soil with a garden hose until it is damp.
  • Lay a sheet of clear plastic over the area. Weigh it down tightly at the edges
  • Leave the plastic in place for at least two months.

Solarization is the best method to reclaim a weedy garden or other area. It is a “clean slate” for your soil, because seeds will be destroyed by the solar heat trapped beneath the plastic.

It is typically tough to implement solarization in large areas and is not usually suitable for use in lawns, where you may want to preserve grass or other plants. Pre-emergent weed killers and flame weeding are much better for use in lawns.

Till and Kill

Weed seeds can lie dormant in soil for decades and are only “activated” when brought to within an inch of the surface. One method to rid soil of dormant weed seeds is to force these dormant seeds to sprout, then attack them with a powerful natural or chemical weed killer. To do this:

  • In spring, till the affected area. Tilling brings dormant seeds to the surface
  • Water the area for 1–2 weeks with a sprinkler or soaker hose
  • When weeds begin to sprout, apply the weed killer of your choice

This is another “clean slate” method, where you force weed seeds to show themselves and then kill young weeds before they mature and cast seeds. Because of the invasive tilling step, it is not best used in areas with desirable grasses and plants.

Use Pre-Emergent Weed Killer

Pre-emergent weed killer stops weeds in their tracks. It works by attacking weed seeds just as they begin to germinate, killing them before they even poke above the surface. It’s a weed killer so good, the only sign it’s working is that there will be no new weeds at all.

Because weeds sprout early and pre-emergent weed killers only kill seeds, not mature plants, it’s crucial to apply pre-emergent early in spring. The best time to apply pre-emergent is in early April.

Although there are many high quality chemical pre-emergent weed killers on the market, you may want to use a natural approach. Corn Gluten Meal (or CGM) is a natural corn byproduct that works as a pre-emergent weed killer. Simply spread it in early spring, as you would with a chemical product. Corn Meal Gluten works to dry out weed seeds, preventing them from germinating.

Keep in mind that all chemical and natural pre-emergents do not discriminate. They will also prevent the sprouting of grass seeds and other plants, so use with caution and follow manufacturer guidelines.

Flame Weeding

How about a natural weeding solution that kills mature plants and destroys weed seeds? Flame weeding checks both of these boxes.

Fueled by propane, a flame weeder is a wand-like tool with a flame at the end. It is an easy-to-use, effective weeding tool for targeting weeds whether they appear in your lawn, garden, or among gravel and concrete. Simply torch the weeds where you see them.

The best part? Exposure to the heat of the flame will destroy weed seeds on the plant. You can even hear them pop like mini-popcorn as you torch the seeds. In comparison, weed killer sprays kill the plant but don’t do anything to stop it from dropping seeds as it dies.

While flame weeding won’t do much to destroy weed seeds dormant in the soil, it’s a great way to kill seed-bearing weeds in your lawn and garden without harming nearby grasses and plants.

Mulch

Bark mulch is one of the best weed seed killers to use in a weedy garden. Mulch not only deprives weeds of air and sunlight, preventing them from sprouting, it also attracts crickets and beetles that burrow in mulch and feed primarily on seeds.

By layering your garden with 3–4 inches of bark mulch or wood chips, you are both suppressing weeds and contributing to the death of weed seeds. It’s a fantastic all-in-one, natural solution for a garden invaded by weeds.

What Temperature Will Kill Seeds?

Weed seeds begin to die at temperatures of 108 degrees Fahrenheit, but to ensure death of all varieties, they should be exposed to temperatures above 130 degrees.

Solarizing soil in summer easily brings soil surface temperatures up to 140 degrees, with temperatures of 130 a few inches below the surface. This wipes out those hidden weed seeds.

The propane used to fuel most flame weeders burns at 3,500 degrees Fahrenheit, which is enough to destroy weed seeds on contact. Simply touch the flame to a crabgrass seed head or a dandelion bloom and the seeds will be killed in moments.

Can Weed Seeds Go in Compost?

Make sure to keep all weed seeds and mature weed seed heads out of your compost. If you allow weed seeds in, you’re simply creating a future weed crop by combining seeds with nutrient-rich soil.

Weed seeds are incredibly resilient to composting. They can remain dormant while other organic material decays. Then, when you spread your compost, those weeds will sprout vigorously.

Instead of composting, dispose of the tops of any mature weeds. If you must compost weeds, use only the root sections of weeds and make sure the roots are completely broken down before spreading the compost. There is still a risk to this method, as any soil clinging to the roots of weeds may have weed seeds in it. It’s often best to get rid of pulled weeds completely.

Kill Weed Seeds

Weed seeds are resistant to composting and weed killer sprays, which makes them difficult to manage. To kill weed seeds, either destroy them by exposing them to high temperatures (through solarization or flame weeding), destroy germinating seeds through use of pre-emergent control measures, or mulch a weedy area to suppress weed growth and invite seed-eating insects.

Learn how to kill weed seeds in soil to prevent weeds and other invasive grasses from continuing to sprout every year.