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Soil Solarization: An Easy Method for Killing Weeds

Suppressing Weed Seeds and Killing Weeds That Have "Taken Over"


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 with a clean slate, landscaping a yard where weeds have taken over, in such a way as to reduce to a minimum the hassle of future weed control. There's lots of work involved, since soil solarization entails getting to the root of the problem, underground. And we won't be taking the shortcut of using herbicides, so that means a bit more work. But if you don't mind getting your hands dirty, then let's roll up our sleeves and begin stopping our 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. Consult my pictures of poison ivy and poison sumac pictures to bring yourself up to speed.

If there are trees, cut them down with an ax or chain saw. If the stumps are too large to remove at this time, cut them as flush to the ground as possible and spray-paint them red (we'll come back to those stumps momentarily). Run a mower over the land to reduce the weeds' height further. The red on the stumps will alert you of their presence, so you don't accidentally hit them with the mower.

Now to get back to the leftover tree stumps. I allude to a few removal options in "Stump Removal". If you're looking for a cheap way, I suggest you use a tool called a "mattock." Dig and chop your way with the mattock under the rootball to access and remove the taproot. Warning: this is hard work!

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'll need a tiller that has some power: don't 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 you've 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. As a final preparation for soil solarization, our organic weed control method, take a garden hose and moisten the area that you've just raked.

Killing Weeds Organically Through Soil Solarization

Perhaps you're 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'll 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's 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 I had you do above was done diligently enough, there will be no sharp objects sticking up to puncture the plastic. "The plastic can be clear construction grade plastic and vary in thickness from 1 to 6 mils," writes Carl Strausbaugh, University of Idaho. In the Northern Hemisphere, the best time for soil solarization is June and July, when the sun's at its peak. Keep the sheet tightly stretched out over the area for 4-6 weeks. 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.

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 airborn seeds? Sure, you'll be applying mulch. But airborn weed seeds can wend their way through mulch particles. If they find dirt, then they're "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'd better keep new weeds pulled, faithfully. Vigorous roots pushing downwards can stress landscape fabric and break through. On the bright side, these weeds should be relatively easy to pull, since mulch is a lot looser than dirt, and weed roots won't become impossibly entrenched.

Speaking of mulch, applying a layer of it over your landscape fabric is the final step in this project. Don't 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. For more, see my article on Garden Mulch.

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