Composting Weed Seeds

Want To Keep Your Compost Weed-Free? Compost can be a gardener’s best friend. It can deliver dramatic improvements in soil quality and nutrients to nourish your plants throughout the growing Organic weed management techniques described in detail. Does composting destroy weed seeds? Not as well as you might think. Today we take a look at why weed seeds still get into your garden after composting.

Want To Keep Your Compost Weed-Free?

Compost can be a gardener’s best friend. It can deliver dramatic improvements in soil quality and nutrients to nourish your plants throughout the growing season. But unless you use the right techniques, you may also find yourself with a bumper crop of weeds.

“Composting is a biological process that decomposes leaves, lawn clippings and other organic materials until they look like rich soil,” said Dr. Gary Wade, extension horticulturist with the University of Georgia. “But many of the rich ingredients in the mix can also be a source of weed seeds.”

When lawns are mowed, for example, seeds can be collected with the clippings. Seeds can also survive in leaf debris or on mature weeds that you pull from your garden.

Pay attention to time and temperature

If you want to keep weed seeds from sprouting and flourishing in your flower beds or garden, you’ll need to ensure your compost gets really warm. With the proper temperature, the volume is reduced, odors dissipate and those pesky weed seeds die.

Within a week, temperatures in a properly constructed compost pile will reach 130 degrees Fahrenheit. That quickly kills many seeds and stabilizes the composted material. But it takes 30 days of exposure to temperatures of 145 degrees or more to kill seeds from tougher weed species. To achieve that temperature, you’ll need the right mix of materials and moisture. (See composting tips below.)

Among the weed seeds that need high temperatures to decompose are common groundsel (Senecio vulgaris), bird’s-eye speedwell (Veronica persica), round-leaved mallow (Malva pusilla), common lambsquarters (Chenopodium album), spiny sowthistle (Sonchus asper), ladysthumb (Polygonum persicaria), wild buckwheat (Polygonum convolvulus), field bindweed (Convolvulus arvensis) and broadleaf dock (Rumex obtusifolius).

How do you know whether you’ve achieved the right temperature? Though compost thermometers are ideal, you can also simply reach into the pile. If it is uncomfortably hot to the touch, you’ve probably achieved the temperatures you need.

Turn your compost pile

For maximum seed destruction, it’s also important to turn your compost. That’s because the exposed surface and localized internal cool spots give weed seeds an opportunity to survive. By turning and mixing, you can minimize the problem.

Turning also can help you determine when your compost is ready to use. If the pile fails to reheat after turning, it most likely has reached a biologically stable state. It should have a fine texture and little or no odor.

Remember, though, that effective weed control doesn’t end with proper compost preparation.

“The same nutrients that benefit bedding plants and crops will also benefit any existing weeds and give them a real boost,” said Lee Van Wychen, science policy director for the Weed Science Society of America. “So it’s important to make ongoing weed management a priority on composted beds and gardens.”

SIDEBAR:

Ten Composting Tips for Home Gardeners

Here is a ten-step formula for building an effective compost pile – based on tips from the U.S. Department of Agriculture and the University of Georgia’s College of Agricultural and Environmental Sciences Cooperative Extension Service:

  1. Select the right site. You’ll need a level location with good drainage. While a compost bin or wire mesh structure can be helpful, it isn’t a requirement. You can start your pile directly on the ground.
  2. Build from the bottom up. Use a coarse layer of woody material as your base. You’ll have better air circulation and will create a chimney effect that will help to heat your compost pile.
  3. Add organic waste. You’ll need an eight-inch to 10-inch layer of leaves, grass, plant trimmings or similar material. One caution: Don’t use grass clippings if your lawn has been recently treated with herbicides; instead, leave the clippings on the lawn for at least three mowings. Also, check the herbicide label to make certain there is no restriction on using treated lawn clippings in compost.
  4. Add a microbial source. Top the organic layer with a one-inch layer of soil or completed compost. Doing so delivers the microbes needed for decomposition and reduces the leaching of mineral nutrients during the composting process. Note: If layers aren’t your thing, you can simply mix each of your composting ingredients into a single heap.
  5. Add nitrogen. You can use a nitrogen fertilizer such as ammonium sulfate (one-third cup for every 25 square feet of surface area), green grass clippings or even lake plants. Manure is also a good source of nitrogen. But if you decide to use it in your compost, contact your county extension service for advice on safe handling practices.
  6. Top it off. Repeat the layering process as needed, and then top your pile with five to eight inches of straw or hay.
  7. Water each layer. You want your compost pile to be moist, but not soggy. Also, scoop out a section in the center to collect rainwater, which serves as an ongoing moisture source.
  8. Give it air. Punch holes in the sides of your compost pile for aeration.
  9. Turn it over. Periodically flip your compost to move material from the center to the outside and material on the outside to the center. That will keep the temperatures even throughout.
  10. Pull them out. If despite your best efforts weed seeds find a cool spot and sprout, make certain you pull them before they go to seed.
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Adding compost to your planting bed can dramatically improve soil quality and deliver nutrients that will nourish your plants throughout the growing season. Photo courtesy of Dr. Gary Wade, University of Georgia.

If you choose, you can use a compost bin or wire mesh to keep your compost contained. Photo courtesy of Dr. Gary Wade, University of Georgia.

Add a layer of finished compost two to three inches thick on your planting bed and incorporate it to a 12-inch depth. Photo courtesy of Dr. Gary Wade, University of Georgia.

Seeds in compost

Many materials used for making compost are contaminated with weed seeds. Late cut hay will certainly contain weed seeds. Straw can be examined for fruiting stalks of weeds. All manure other than poultry manure should be considered contaminated unless you have tested it. Horse manure and manure from other animals that have access to weedy pastures or pastures along roadsides are most likely to be contaminated with weed seeds.

In general, it is easier to use weed free materials to make compost than it is to try to kill weed seeds during the composting process. The problem with adding weedy compost to your garden is rarely that you will be immediately overwhelmed with weeds: usually the weed seed density of garden soil is higher than that of manure or poorly made compost. Rather, the problem is that you may introduce some new pernicious weed species that will cause management problems for years to come.

You can test manure for weed seeds by mixing several quarts of manure taken from various parts of the pile with potting mix in a 1:1 ratio and spreading it in flats. Keep the flats warm during the day and cool but not cold at night. For example, run the test inside in the winter, outside in the summer and in a cold frame during the spring or fall. Water the flats regularly, and observe any weed seedlings that emerge over the following two to three weeks. This test will usually show if weed seeds are present, but it may not accurately predict their density since some seeds may be dormant.

To kill weed seeds during the composting process, the pile should reach 140? F for at least two weeks. Some of the more resistant species may not be killed even by this treatment. For a small compost pile achieving a high sustained temperature may require insulating the pile (e.g., with loose, straw over plastic). If the weather is warm and sunny, the heat generated by biological activity can be supplemented with solar energy by covering the pile with clear plastic. Since the outside of the pile is unlikely to attain the required temperature for a sustained period in any case, thoroughly mixing the pile several times will probably be necessary. For a very small pile (e.g., < 1 cubic yard) attaining a high enough temperature for a long enough time to kill most weed seeds may be impossible.

Does Composting 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 one of my compost piles a few years ago. Granted, it wasn’t a regularly turned pile, meaning that they probably missed the hottest part of the compost… but how many of you have turned your compost regularly and still had little tomatoes or weeds pop up in 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:

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“The key word is properly.”

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

Why Our Backyard Compost Doesn’t Kill Weed Seeds

A typical backyard compost heap isn’t insulated or turned often enough to maintain heat and rotate all the viable seeds in the compost through the hot center of the pile.

Yes, the heat generated by thermophilic bacteria in a hot compost pile is high enough to destroy seeds, but getting every bit of your compostable materials hot enough to kill the seeds takes very good compost management.

My old 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 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 as I’ve realized creating perfect compost doesn’t really matter.

Nature doesn’t create perfectly sifted, totally rotten-down brown humus. No, she throws logs and leaves on the ground. There’s always some finished material and some fresh material, some 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 compost. Four words that led to 1145 words:

Martha asked this question on this anaerobic compost tea video I posted back in the summer:

“Good question. I try to avoid throwing plants with mature seeds into the tea. They never seem to get completely killed in hot compost piles, either, though, 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 hot piles. 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-fu 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 then use the resulting compost in your spring gardens?

Do you want to take that risk?

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. My interest, however, is more in gardening than in the processes that lead up to it. Making “perfect” looking compost 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 piles and gardens altogether.

In my former food forest I would just 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, and every time I did, guess what?

Those fallen weeds rotted into humus.

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Nature does this all the time. The winter freezes come once a year and toast all the weeds, letting them fall down and rot into the soil, improving it. The Bible instructed the children of Israel to let their land go completely fallow one year out of seven. Weeds regenerate the soil, as I’ve written before.

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 and they’ll go crazy in your eggplants. Beneath a layer of mulch, however, they’ll eventually rot away safely.

That’s my two cents on composting destroying weed seeds. Yes, it can – but most of us aren’t doing it “properly,” so don’t trust too much in the magic of compost to pile-drive your pesky pigweed problems.

Personally, I prefer cold composting anyhow as I believe it keeps more of the good stuff in the pile instead of steaming away into the air. Nature almost always cold composts, and while that process takes longer I think it’s a simpler and gentler method.

Then again, I may just be lazy.

And go get my book Compost Everything if you really want to transform your thinking on the wonders of composting. It’s even available as an audiobook, read by me.

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15 comments

I’m also on the cold compost bandwagon. I was reading about soil solarization today using plastic mulch after I saw Huw doing it, and learned it kills at least half the microbes, and that was only heating the soil above 45C. Don’t tell Huw.
My regular cold compost loves to grow pumpkins, corn and weed seeds, however what I have found interesting is my experimental vermicompost pots.
I made a flow-through vermicompost bin out of large plant pots stacked on top of one another with the base of the top pot fitting into the top of the bottom pot. I then cut the bottom out of the top pot and added some holes or slits with a knife for air to both. Then I fill from the top with compost, worms from the garden and kitchen scraps to get it started. I raise the pots to collect the leachate and use a pot tray as a lid. I keep adding scraps, and it keeps wanting more, eventually worms keep falling out to make a worm soup leachate. When it’s eventually full, I remove from the pot at the bottom and because the pots have tapered walls they hold the contents and worms in place. I throw all my weed seeds in there and I haven’t seen any make it through yet.
The only problem is that I get this thick mud-like vermicompost out of it, but when I add it to the top of pot plants and beds and then cover with mulch they love it. The stuff is the bomb. The worm soup leachate goes on all my seedlings and makes them go boom too. I noticed that when I put my used guinea pig oaten hay on top the mycelium go nuts and it’s covered in threads in no time.
The other day I saw a permie throw his biochar in a cement mixer with rocks to micronise it, as this increases the surface area and CEC further.
I’m thinking of adding some of that to it next, to see if that makes it less like mud.

I like that cement mixer idea.

Also, I feel you on soil solarization – yet at the same time, a lot of that microbiology can regenerate very quickly. I’m on the fence.

I used to be on the fence too. The solarization studies I read used gene sequencing to measure the diversity. And while I agree that microbiology can regenerate quickly, diversity may not. One study mentioned that after only 4 weeks ofsolarization, that native arbuscular mycorrhizae took 6 weeks to even begin recolonizing plants and never reached the same level as the control, likely because of competition. Most of the solarization studies I came across were about killing plant pathogens for industry and while they achieved that they also often hurt yield.