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killing weed seeds

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.

Controlling Weeds That Have Gone to Seed

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Controlling Weeds That Have Gone to Seed

Q. I’m looking for advice on how to get an area of my garden back under control without using chemicals . I failed to keep up with the weeding in a section of my vegetable garden that is about fifteen by 30 feet. It had crabgrass, clover and a few other weeds that went to seed. I ripped them all out as best I could, but can’t get all the roots out—and I can see that a lot of seed has scattered on the surface of the soil. I wonder if covering the area with impermeable black plastic until next May would kill everything? Or maybe it makes things worse by keeping the area warm enough for the weeds to survive the winter?

—Kat in Leesburg Virginia

A. Let’s start with the black plastic idea.

Wishful thinking. Weeds need no help surviving winter; and only clear plastic stretched tight over a perfectly prepared area for an entire summer can kill weed seeds. It’s called ‘soil solarization’, and this description of the proper technique is not my opinion; it’s the only way that diligent researchers got it to work. (Here’s the details in a very popular previous Q of the Week .)

Her best bet now is to use a ‘flame weeder’—a hand-held, propane-powered garden torch that shoots a small flame out the business end of a long shepherd’s hook-like wand—to slowly toast one small section of the infested area at a time, preferably on a hot day when that soil is bone dry. It’s much less work, you get to stay upright while you deactivate those weed seeds, and every little ‘pop’ you hear is an emotionally satisfying sign that you’re doing it right.

(I’ve always been emotionally satisfied by my flame weeders. [Mom said it was OK to be easy; just not cheap.])

So–what about those roots still in the ground?

It sounds like Kat had a typical round of frantic but useless pulling. I suggest she let those roots re-sprout, and when they’re tall enough, soak the soil thoroughly and pull gently at their base. (Be sure to compost those pulled roots with lots of soil still attached; they’re full of nutrients, and they’re the perfect ‘green material’ to mix with shredded fall leaves. And the attached soil contains lots of microbial life to get that compost cooking!)

In general, this is the best way to handle weeds of almost any size: wait until right after a heavy rain or soak the soil for hours—really saturate it—and then pull ‘low and slow’; get down to where the plant meets the soil and pull slooowly and gently. That’s how you get the roots out the first time.

‘Angry yanking’ may seem emotionally satisfying, but it doesn’t get the job done. And it tends to be the panicked tactic of choice for large areas that seem insurmountable. Better to slow down and spread the job out over a week or so. An hour a day of doing it right in small area after small area gets rid of the weeds without you needing a corner man.

Now: why did I say ‘weeds of almost any size’?

Because, as Kat discovered, yanking weeds that have already gone to seed spreads the seeds. When weeds are seedy, don’t pull right away. Instead use that flame weeder to incinerate the tops of the plants. Then run the flame up and down the sides of the plants; wherever you see seed pods. The torch will toast those seeds, and you can then pull out the plants without planting next year’s crop of weeds. Bonus: Many weed seeds look like Munchkin fireworks when they pop!

(Yes, the words ‘easily amused’ do come to mind; saves me a fortune on my cable bill.)

Anyway; next season, delay planting these areas until any missed weed seeds have germinated and been growing for two weeks. Then carefully and methodically slice them off at the soil line with a hoe that has a flat super-sharp blade, like a diamond hoe—creating what’s known as a ‘stale seed bed’. And whatever you do, don’t till the soil; tilling plants weed seeds!

An alternative tactic would be to use the natural pre-emergent herbicide, corn gluten meal . Just as you would do with cool-season lawns, applying corn gluten meal at the rate of twenty pounds per thousand square feet in the Spring (when forsythia and redbud just begin to bloom or when the soil temp reaches 55 degrees F. measured four inches down) would prevent a lot of dormant weed seeds from sprouting. Heck—because those seeds are on the surface, ‘CGM’ might provide astounding control. (Be sure to apply as per package directions.)

Now—that would also put a lot of Nitrogen into the soil, so I would then use that area to grow a non-fruiting, but Nitrogen-hungry crop like sweet corn , field corn, popcorn, salad greens, potatoes, onions or other things I’m not thinking of right now.

Corn of any kind would be especially ideal; ‘maize’ loves a Nitrogen-rich soil, and 15 by 30 would be a perfect size to seed a big patch and get lots of nice full ears. (I personally vote for popcorn—super-fun to grow!

) You just have to wait six weeks after applying the CGM or pre-sprout the corn seeds, which I recommend anyway. (You’ll find those pre-sprouting details under C for corn [maybe] and under P for peas [definitely].)

In addition, the way she describes this area, we have to assume that it’s a flat earth garden; a design—or lack of design—that guarantees maximum weed woes. You get a lot more eatin’ with a lot less weedin’ from raised beds . Four by eight is the ideal size for each bed, with two-foot-wide walking lanes in between. Because you build the growing area up to a foot above the soil line, grassy weeds like the clover and crabgrass she specifically names can’t migrate in from the outside. And you can just mow the actual lanes to keep their weeds under control—no pulling.

And the cool air of fall is perfect for getting outside and doing some raised bed building! And if you build those beds now, they’ll be ready to plants in the Spring. Hint; hint….

Final note: But if Kat does take the ‘corn gluten meal followed by some kind of corn’ advice, she should build those raised beds in other areas of her garden. Corn is one of the few crops that does better in flat earth than raised beds. Sounds like a plan!

Mike offers advice on reclaiming areas that have been taken over by weeds, in particular weeds that have set seed.