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how to kill weed seeds in soil

How to Use Soil Solarization to Kill Weeds

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Soil solarization is a preventive, organic method of killing weeds before weed seeds even sprout.   But the advice below is also meant for homeowners wishing to start a garden with a clean slate, reclaiming a patch of land where weeds have taken over, in such a way as to reduce to a minimum the hassle of future ​weed control. Want to transform a piece of land that has “gone to pot” into usable space? Then the method explained below may be the solution to your problems.

Preparation

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.​

First hack down the tall vegetation with a sickle, power trimmer, etc. But before doing so, make sure you know how to identify poison ivy, poison sumac, etc.

If there are shrubs and trees present, cut them down with an ax or chainsaw. The ground needs to be smooth before you begin soil solarization (since you will be spreading plastic over it), so you will also have to remove the stumps left behind. If you are looking for a cheap way, use a tool called a “mattock.” Dig and chop your way with the mattock under the root-ball to access and remove the taproot. Warning: this is hard work and may be feasible only for smaller stumps.

Run a mower over the land to reduce the weeds’ height further. Now that all the weeds are as short as possible and the stumps have been removed, rent a large tiller to uproot all the weeds. Since this plot of ground is uncultivated soil, you will need a tiller that has some power: Do not undertake this task with a small garden cultivator! Allow the tiller’s tines to dig deep enough into the ground to loosen the weeds, so they can be removed—roots and all, if possible.

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.”

Killing Weeds Through Soil Solarization

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.

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.

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).

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.”

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.

Speaking of mulch, applying a layer of it over your landscape fabric is the final step in this project. Do not pile up mulch heavily around newly planted trees or shrubs; it invites diseases. When old mulch decomposes it needs to be removed and replaced with new mulch.

Soil solarization is an organic method of killing weeds before they even sprout. Use this method of weed control when starting from scratch.

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.