Getting rid of weed seeds in compost

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.

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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.

Does A Compost Pile Destroy Weed Seeds?

Does a compost pile destroy weed seeds? Or more specifically, does YOUR compost pile destroy weed seeds?

We are regularly assured by composting experts that hot composting destroys seeds … yet I have some pumpkins that beg to differ.

Those pumpkins grew as volunteers from a compost pile a few years ago. Granted, it wasn’t a regularly turned compost pile, meaning they probably missed the hottest part of the heap, but how many of you turn your compost regularly? And I’m going to bet that you still had little tomatoes or weeds pop up in it. Come on, admit it!

My bet is ALL of you.

Here’s an example of “hot composting kills weed seeds” advice from Aggie Horticulture :

“The composting process also naturally kills weed seeds. Properly managed, a compost pile should easily reach 140°F, which breaks down all organic matter, including weed seeds.”

They recognize the difficulty, though, as the next line reads:

“The keyword is properly.”

My bet is that few gardeners reach that lofty, “proper” status.

Why Our Backyard Compost Pile Doesn’t Kill Weed Seeds

A typical backyard compost pile isn’t insulated or turned often enough to maintain heat. Those viable seeds in the compost don’t get rotated through the hot center of the pile.

Yes, the heat generated by thermophilic bacteria (an organism living at hot temperatures) is high enough to destroy weed seeds, but getting every bit of your compostable materials hot enough to kill the seeds takes very good compost management.

My old compost pile didn’t do it. It was built from reclaimed landscape logs with too many gaps to get everything hot. Plus, turning it was a pain.

I imagine if you owned a cement truck and packed the barrel of it with a proper mix of carboniferous and nitrogenous materials. Then you rotated it every day or so, and perhaps insulated the inside with foam. You could get that compost to heat up perfectly.

I’m joking. A bit.

My composting methods have gone from complicated to simple over the years. I’ve realized creating perfect compost doesn’t really matter.

Nature doesn’t create perfectly sifted, totally rotted, brown humus. No! She throws logs and leaves on the ground. There’s always some finished material and some fresh material, fungi eating at this, and some insect boring away at that.

But Let Me Back Up. What Prompted Today’s Post?

This Viewer Asked a Question

There was a comment that prompted today’s great big post on weed seeds in a compost pile. Four words that led to 1,145 words (give or take):

Martha asked this question on this anaerobic compost tea video I posted:

My answer was:

“Good question. I try to avoid throwing plants with mature seeds into the tea. They never seem to get completely die in a hot compost pile, either. Even though we hear all the time that “hot composting kills weed seeds!” It’s probably true for the ones in the middle of the pile, but I’m always getting volunteer tomatoes, wheat from straw, weeds, and pumpkins popping up even from a hot compost pile. My guess is that this tea method will rot down most of the seeds, if it sits long enough … but not all of them.”

It takes a lot of faith in your compost to deliberately throw in weedy materials, no matter how you’re composting.

If you have spiny pigweed going to seed in your food forest, do you really think you’ll be able to throw that in your compost bin and use the resulting compost in your spring gardens without spiny pigweed popping up?

Do you want to take that risk?

I Hear You, “But I Compost the Right Way!”

That’s fine—I appreciate the “thermometer and sifter” brigade.

To those about to compost, I salute you!

I am totally sure that I could destroy weed seeds by hot composting if I thought it out properly. However, my interest is more in gardening than in the processes that lead up to it. Making a “perfect” looking compost pile, or compost for that matter, isn’t as important to me as growing corn, pumpkins, beans, yams, and fruit trees. I also don’t like spending money to make perfect systems.

If you enjoy it, that’s fantastic. I love the smell, look and taste (well, maybe not taste) of finished compost. I made some nice-looking stuff myself this year and just sifted it the other day.

I made that compost with almost no work, though. No thermometers, no turning, no measuring ratios of carbon/nitrogen to get that 25/1 mix. No, I just threw it all on the ground in one of my garden beds.

And—oh YES—LOTS of seeds came up in it! Enough to start my new fruit tree nursery.

I view this as a feature, not a bug. Sometimes I just let compost piles turn into garden beds since there are so many volunteer edibles coming up.

But What About Killing Weed Seeds.

Right—that’s what you all want to know, right? How CAN you compost those pesky weedy plants?

My favorite method is to keep them out of the compost pile and gardens altogether.

In my former food forest, I would chop down weeds and throw them on the ground around my fruit trees and other shrubs. If they self-seeded and came back, I’d chop them down again.

Unlike delicate annual garden plants such as lettuce and cabbage, trees and shrubs don’t need to be perfectly weeded in order to produce. I just knocked down the weeds again and again. Every time I did, guess what?

Those fallen weeds rotted into humus.

Nature does this all the time.

The winter freezes come once-a-year and kill all the weeds. They fall to the ground and rot into the soil, which improves it.

If you want to use weeds to feed your gardens, you’ll have much better luck in a no-till system where you throw a pile of seedy weeds on the ground. Then, cover them up with mulch … and then, DON’T TILL!

If you till, you’ll bring those seeds up to the light and warmth. They’ll go crazy in your eggplants. However, beneath a layer of mulch, they’ll eventually rot away safely.

That’s my two cents on composting and destroying weed seeds. Yes, a compost pile can destroy weed seeds … BUT … and it’s a big but … most of us aren’t doing it “properly.”

Don’t trust too much in the magic of compost to pile-drive your pesky pigweed problems.

Personally, I prefer cold composting anyhow! I believe it keeps more of the good stuff in the pile instead of steaming it away into the air. Nature almost always cold composts! While that process takes longer, I think it’s a simpler and gentler method. I wrote an entire book on composting (Compost Everything: The Good Guide to Extreme Composting) and many of the methods in that book are cold compost approaches.

How to Kill Weed Seeds in Compost

Ideally, you wouldn’t add weeds that are in seed or even in the late part of their blooming cycle to the compost pile. Thus you can avoid the problem of their seeds germinating in the garden when you later use the compost you produced. But sometimes, you have little choice: perhaps the most easily available compostable material (horse manure, hay, etc.) contains seeds or else the endless sorting of weeds according to their “seediness” would just be too complicated. Or, like me, you just feel that everything organic should be composted.

Fortunately, there are other solutions.

A Big, Hot Pile

A compost pile that gives off water vapor is working hard to kill weed seeds. Source: Anatomy of Living, http://www.youtube.com

In general, the bigger the compost pile, the more heat it produces … and heat kills seeds, even weed seeds.

After a week at 130 ° F (55 ° C)*, most weed seeds will be dead, but it takes a month at 145° F (63 ° C) or more to kill the most resistant ones. Curiously, most common weeds actually produce seeds that are fairly easy to kill and they’ll die at relatively low temperatures. That’s the case with dandelions (Taraxacum officinale) and Canada thistle (Cirsium arvense), for example.

*Note that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!

Heat-resistant weed seeds requiring treatment at 45° F (63 ° C) include:

  • Bird’s-eye speedwell (Veronica persica)
  • Broadleaf dock (Rumex obtusifolius)
  • Common groundsel (Senecio vulgaris)
  • Common lambsquarters (Chenopodium album)
  • Field bindweed (Convolvulus arvensis)
  • Ladysthumb (Polygonum persicaria, now Periscaria maculosa)
  • Round-leaved mallow (Malva pusilla)
  • Spiny sowthistle (Sonchus asper)
  • Wild buckwheat (Polygonum convolvulus, now Fallopia convolvulus)

To find out if your compost pile heats up enough to kill weed seeds, simply insert a compost thermometer into it and note the temperature. If you don’t have a compost thermometer, try sinking your hand into the pile. If it’s so hot you to feel uncomfortable, it’s heating up enough.

Do not forget to return the pile regularly, not only because that helps to oxygenate it and thus stimulates microbial life, leading to and maintaining higher temperatures, but also so the ingredients on the outside of the pile, where it’s cooler, can also get their full heat treatment.

Note too it may be necessary to water your compost pile from time to time. Compost heats most efficiently when it is neither dry nor wet, but moderately moist.

When the Pile Is Not Heating Up Enough

The compost bins commonly sold generally can’t hold enough material to ensure high temperatures. If you’re using one, you’ll have to resort to other methods if you want to kill weed seeds in your compost.

Bury compost to prevent weed seeds from germinating. Source: thelegitimatenews.com

It’s important to understand is that weed seeds* can only germinate when exposed to light. If you are concerned that your compost might contain viable weed seeds, simply bury it when you use it, covering it with soil or, if you apply it to the surface, cover the compost with mulch. Problem solved!

*Warning: unlike annual and perennial weed seeds, a few tree seeds, especially nuts, will germinate when covered with soil or mulch.

You can also kill the seeds at the end of a composting cycle by solarization. To do this, spread the compost on a very sunny surface and cover it with a sheet of transparent plastic, holding the plastic in place with rocks or bricks. That will quickly create a greenhouse effect and very high temperatures. Even if there is some germination at first, the heat underneath the plastic will be such that it will soon kill both the seedlings and any remaining seeds, leaving you with weed-free compost you can use as you want.

With these methods in mind, you can dare to add weeds at any stage of their life to your compost pile.