The only three guarantees in the life of a gardener are birth, death, and weeds. If you're a compulsive composter like me, you can't stand the idea of stuffing all that free green material into a plastic bag and sending it to a landfill. The problem is that if you compost weeds incorrectly you'll plant viable weeds the next time you spread finished compost in your garden.
So can you compost weeds? Absolutely. There are five ways you can compost weeds and be confident that the compost you produce will be weed-free.
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How to Compost Weeds
Selective Weed Composting
One way to avoid growing weeds in your compost is simply to be careful about which weeds you compost. It is important to consider both the species of weed and its structure and life cycle.
If you have a cool compost pile, you can compost annual weeds like lambs quarters or common nettle, as long as the seed-heads haven't formed yet. These weeds will not regrow.
On the other end of the spectrum are perennial weeds like wild violets. These should never be added to a cool compost pile without first using one of the complementary strategies listed below (specifically solarization, light deprivation, or drowning).
If you're just starting out as a composter, check out this introduction to composting for more information on creating a basic cool compost pile where you can compost your young annual weeds.
Hot compost
With thermophilic composting or "hot composting," you'll need careful compost pile management so that it gets hot (ideally around 140 degrees). A well-maintained hot compost pile usually releases steam as beneficial bacteria, insects, and other microorganisms turns your garbage to hummus.
A hot compost pile's rising temperature kills weeds seeds, insect eggs, pathogens, and other nasties). You can also compost any weeds using this method. However, some gardeners avoid adding some of the nastiest perennials, especially if they are not confident how hot their pile is.
Getting your compost pile hot enough requires adding the right mixture of green materials, brown materials, air, and water. For a detailed guide to heating up your compost pile, check out this guide.
Solarization
If you aren't confident your compost pile is hot enough, you can kill the weed seeds before you put the weeks in your pile. Solarization is sort of like pre-compost composting. You use intense sunlight to bake the weeds into oblivion. When the weeds are totally desiccated and on their way to decomposition, you can safely add them to your compost pile.
The key difference between solarizing and composting is that you do not put the weeds in a heap. You spread them thinly over a heat-absorbing surface, such as a black tarp or, ideally, a paved slab of cement or asphalt. This ensures that they all get extremely hot, as they would in a hot compost pile.
Note: Unlike a compost pile, which must be kept damp, you want your solarized weeds to be cooked extra crispy. This generally takes a few weeks in the hot summer months. Solarization doesn't work as well other times of year.
Drowning
Another way to kill your weed seeds and perennial weeds is by drowning them.
Here's how:
- Place the weeds in the bottom of a barrel and weigh them down with rocks.
- Fill the barrel with water.
- Leave this mixture for at least six weeks.
- It's a good idea to cover the barrel, either with a lid or with a fine mesh screen. This will prevent it from becoming a breeding ground for mosquitoes.
When the contents are good and decomposed, pour the barrel onto your compost pile. If you'd like you can strain the water off first and use it to water your garden plants. In addition to hydration, the liquid works as a mild fertilizer. Whatever pulp remains in the bottom of the barrel can be added to your compost pile.
Light deprivation
Whether you're dealing with live weeds growing in the earth or dying weeds you've pulled, light deprivation can be a great way to kill them dead.
Mulching Live Weeds
When your weeds are still growing in the ground, you can use a mulch or ground cover to create a complete light barrier. This could be inorganic (such as black plastic) or organic (such as cardboard).
A light barrier suppresses weeds substantially in the short term, but you can't take shortcuts if you want to do it right. For example, after laying cardboard (topped with straw and leaf mulch to weigh it down) in my new garden bed, I saw virtually no weed activity in the nine months it was covered, and most of the annual weeds died off permanently. However, when I removed the cardboard and planted vegetables, eventually the weed suppressant effect wore off and two perennial weeds (wild violets and grass) returned.
Note: In general, in order to kill persistent, in-ground perennial weeds via light deprivation alone, you will need to cover the area for two years. However, if you're willing to pull the weeds there are other light-deprivation strategies that work faster that you might consider.
Plastic Compost Bins
Compost bins made out of black plastic (such as this option) or compost tumblers (such as this one), are excellent if you're dealing with a small amount of weeds. These systems not only deprive the weeds of light, but they also help get your compost pile hotter than it would get in a pile. For instance, a tumbler allows you to aerate the compost inside, which will ensure it gets hot enough to kill the weeds quickly.
Bagging
Another method of light deprivation is to put your weeds and grass clippings in black garbage bags, close them, and leave them out in the sun for several months. Wait until the weeds have turned into unrecognizable slime before adding them to your compost pile. The combination of total darkness and heat should prevent the seeds from surviving.
This same principle applies not just to weeds but to finished compost. In other words, you could take cold compost that is finished or nearly finished, bag it in black plastic and leave it out in the heat (or put the compost inside a cool dark area like a barn and leave it for a year or two). After that you can be confident you won't spread any live seeds.
Sandwiching
If you want to use the green matter of your weeds, but don't have a compost pile (or don't want to put in the effort to get it hot), there's a strategy I've used that I call sandwiching, which might work for you.
I use cardboard as mulch to create paths between my garden beds. When I pull weeds from the beds, I simply drop them on top of the cardboard pathways. The weeds decompose and solarize on the pathways. When the pathways begin to get so decomposed that they get squishy, I simply add another layer of cardboard on top to make them nice to walk on again.
This strategy is ideal for the lazy gardener; you don't have to worry about your compost pile's temperature, or even bother carrying the weeds to your pile. The cardboard ensures there is inadequate light for the weeds to regrow, and the weeds and cardboard decompose together, building good hummus in the pathways.
The Real Dirt on Composing Weeds
While no method is 100% foolproof, any of these strategies should help you feel confident about composting weeds without the risk of re-introducing them to your garden later. However, the best plan for weed seeds or perennial weeds is to combine multiple methods, to make sure those weeds are good and dead.
