Rule 3: If you are not hot composting, kill the weeds before going to the bin by drying them in the sun. Put them on a piece of cardboard or newspaper so they do not spring back to life. A few days of dry sun and the weeds are toast. The cardboard or newspaper will start decomposing and you can toss that in the pile too.
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 1: Compost weeds before they go to seed.
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.
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.
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
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.
The classic method of composting—the method purists would call the “right” way—is known as hot composting. This simply means that you turn the pile regularly and allowing it to really heat up to 145 degrees Fahrenheit or more. A properly maintained hot compost pile will kill weed seeds, as well as many other pathogens, so you can compost weeds without having to worry about them popping up in your garden beds.