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.
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.
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.
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.
For hot composting to fully kill all weed seeds and roots, follow these tips:
4.) Another strategy is to dry your weeds in the sun. This won’t kill seeds, but is good for weeds with roots still attached which could potentially take root in your compost pile. When you are certain that no life is left in the roots, the weeds may be composted.
The main parts of the plant to cause concern are the weed’s seeds, and its roots. Either can potentially be spread through your compost pile and wreak havoc in your garden later on. In addition, there are weeds, such as ground ivy, that can root from any part of the plant. These are especially easy to spread.
Composting weeds incorrectly can create the risk of having those weeds spread when the compost is applied to your garden.
What to Do with the Weeds that Do Sprout in the Garden
Digging brings weed seeds to the surface, causing them to sprout. So not only does no-dig gardening save me from having to spend time and effort digging, it helps to keep weeds down in my garden.
A 2 – 3″ layer of mulch does a great job of keeping weeds at bay by blocking the sunlight that the weeds need to sprout. We use both hay and wood chips as mulch in our garden.
Those in the raised beds get far fewer weeds for several reasons. The boundary created by the raised beds keeps my feet (and those of visitors) on the path and out of the garden. Which brings me to my next strategy – not stepping on my garden soil.
Throwing weeds in the landfill means that you are throwing precious nutrients away. Instead, learn to compost them the proper way so that those nutrients can be returned to your garden soil.
Rule 2: If weeds have already gone to seed, compost them in a hot compost pile. Temperatures of 55 to 60 C (130 to 140 F) will kill most weed seeds.
To be sure there are no seeds remaining, you can solarize the compost. But don’t get over 70 C (160 F) as that will harm the good soil life. Or you can put the near finished moist compost in a bucket or bag, let the seeds sprout, and then toss them around to mangle the sprouts. Soil life is happy, seed spouts are crushed.
Rule 1: Compost weeds before they go to seed.
You could start one compost heap or bin for all the weedy, seedy waste. Keep another pile or bin strictly for the more weed free material such as leaves, sawdust, grass clippings, coffee grounds, etc. In your weedy bin, as decomposition progresses, turning the pile will mash weed sprouts. If you can attain temperatures of 55 C (130 F) for three days, most seeds will be destroyed without losing the beneficial microbes.
Rule 5: Noxious weeds (buttercup anyone?) can be solarized. Stuff them in a plastic bag with a little water in the sun until they turn into slime. Manure and hay are wonderful compost additives but often bring a curious assortment of weed seeds. But don’t give up on these precious resources. There are various ways to conquer the incoming weeds.