April showers bring May flowers. BUT, March showers just seem to bring WEEDS! We’ve being getting lots of questions about putting those weeds into a compost pile. There is not a simple yes or no answer to this query. Weeds have many nutrients so they are a great addition to the compost pile, but you do not want compost filled with viable weed seeds.
The best time to take care of weeds is when they are small and have not established deep roots or seed heads (easier to pull, too, especially if ground is still moist). However if the weeds have invasive running roots, you might want to consider additional action. The most effective way to kill them is drying them in the sun; the desiccated roots are no longer viable. When the weeds are brown and dried they can be composted as a brown; they are no longer considered a green.
If your weeds have started to form their seeds, you do not want to put them in a passive/static pile since you will end up using your finished compost to further propagate the weeds (never a good idea!) If you are committed to making a hot pile and know that you can maintain “cooking” temperatures (above 131˚ F) for a few weeks, the weed seeds will be destroyed with high temps.
Another method to deal with weeds is to solarize them in a clear or black plastic bag (or between to sheets of plastic sealed with bricks or stones). This can get slimy, but the weed seeds are destroyed in this anaerobic, high temperature environment. You can also drown them! Plants need air, so by covering the weeds with rainwater for a few weeks (cover container so you don’t create a mosquito breeding ground), all the living vegetative materials will die– including the running roots and the seeds. The liquid can be quite smelly but it can be used to feed plants since it is full of soluble nutrients, or the whole bucket can go into your compost pile.