Composting Weed Seeds

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. 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

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:

“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?

See also  I Grew Up From A Seed Tough As A Weed

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.

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.

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.

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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.