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.

Fortunately, there are other solutions.

A Big, Hot Pile

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.

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.