However, for most of us, it isn’t as easy as that. Unfortunately, most home compost bins do not get hot enough to destroy all seeds and roots. Smaller and slower acting compost piles may take up to two years to kill weeds. This does not fit in with most people’s annual rotations of their compost piles, so this method is out for many gardeners.
Remember: We will never eradicate weeds, they blow about, birds drop them and they can sleep in the soil for years then soil disturbance will set them off.
If you have a large quantity of weeds to get rid of then you can simply bag them in compostable bags (ideally paper yard waste bags). Find a convenient place to ‘hide’ these bags; such as behind your garden shed. Cover with black plastic, or carpet (something to exclude the light). Leave for 2 to 3 years.
How to Kill Weeds by Rotting
What does that mean?
This should be long enough to kill even the toughest weeds. You can then use the resulting compost on your garden.
If your compost pile gets plenty of heat and is steaming to the touch, then weeds can compost in as little as 6 to 8 months. To get your compost piles hot enough requires a perfect mix of browns and greens, regular working and (ideally) a sunny location. The heat from your compost should damage and destroy roots and seeds so that your weeds will not propagate from the compost.
Learn more about how to compost with the bokashi method. Note that this link refers mainly to composting food waste, but the same methods can be applied to “bokashi’ing” weeds in your garden.
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.
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.
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.
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.
This can actually be rather charming when the volunteers are tiny impatiens seedlings, tomato plants, or even pumpkins that volunteer because last Halloween/s jack o’ lanterns were added to the compost heap. It’s far less charming when the volunteer plants are hundreds of dandelions or tiny sprigs of bindweed or crabgrass that get into the garden via the compost you spread.
Can I compost my weeds? This is a typical question for beginner composters. The question is important because when adding compost to your garden beds you run the risk of introducing unwanted plants. The secret is to ensure that no viable weed seeds or roots survive the process.
This doesn’t mean you can’t compost weeds. As plants in the compost pile decompose, it gets pretty hot. If the temperature is high enough, weed seeds will die and won’t be a problem later. The pile may not get hot enough though, and also there may be cooler pockets that harbor weeds.
Can You Compost Weeds?
The problem of having weeds in the compost bin is the possibility of getting weeds in your garden. Seeds and roots that survive composting can crop up in your beds. Sometimes this means getting a little volunteer tomato plant or a zucchini. When the volunteers are weeds, it’s a lot less fun. You’ve just added to your weeding chore.
Compost from weeds can be perfectly safe and full of good nutrients. The secret is hot composting – ensuring the compost gets hot enough to kill any seeds and roots. Here’s how to do it:
Another way to put weeds to use without risking getting their viable seeds into the compost is to make them into a liquid fertilizer. Soak weeds in water for about a month and then strain. The resulting liquid can be diluted and used to safely add nutrients to beds.