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:

My answer was:

“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.

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.

You decide.

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.

And it’s funny.