“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.”
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.”
Why Our Backyard Compost Pile Doesn’t Kill Weed Seeds
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.
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.
We are regularly assured by composting experts that hot composting destroys seeds … yet I have some pumpkins that beg to differ.
A gardener who experiences such an explosion of volunteer weeds may well swear off composting altogether, or at least stop adding weed material to the compost pile. To be clear, there is no reason to stop composting weeds. With a slight adjustment to the composting process, you can ensure that weeds and their seeds will be killed completely and won’t be resurrected where you least want them.
How do you know if your compost is getting hot enough to kill all weeds? A variety of compost thermometers are available that can gauge the temperature of your pile. Experienced gardeners may simply thrust a hand into the pile. If it feels uncomfortably warm to the touch, it likely is warm enough to kill all seeds and roots in the pile.
How Weeds Survive
Compost is a great way to recycle organic material in your garden. All those spent flower blossoms, fall leaves, dead plants, grass clippings—even non-meat kitchen scraps—can be transformed into a great soil amendment and nutritious mulch, simply by throwing them into a heap and allowing the refuse to decompose naturally.
Done correctly, composting creates a sterile organic material that does nothing but good things for your garden and the plants in it. However, nearly every gardener who practices composting has occasionally experienced “volunteer” plants sprouting up in the garden where the compost has been spread.
In an ideal compost heap, the temperatures generated by the breakdown of plant material can get quite warm, and if temperatures exceed 145 degrees Fahrenheit, pretty much all seeds and roots will be killed. However, if the temperatures do not get warm enough—or if a portion of the compost heap does not experience sufficiently high temperatures—seeds or perennial roots can survive the composting process. When these seeds or bits of root later reach your garden inside the compost, they can—and usually do—quickly germinate or take root again.