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getting rid of weed seeds in compost

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

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!

Why Our Backyard Compost Pile Doesn’t Kill Weed Seeds

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.

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.

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.”

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.

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

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

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

Bury compost to prevent weed seeds from germinating. Source:

When the Pile Is Not Heating Up Enough

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.

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.