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Composting weeds can make some gardeners nervous, thinking that they are going to end up spreading the weed all around their garden whenever they spread fresh compost. Troublesome weeds, such as couch grass, nettles, buttercups, and ground elder, have large root systems.
Their root systems are what makes them such prolific growers and so hard to eradicate from your garden. These roots absorb huge amounts of valuable nutrients from the soil.
What does that mean?
Don’t throw them away! Your garden can benefit from all of those extra nutrients, and even weeds can be composted. You could be composting leaves and much more from your yard!
By following these few simple tips, you can compost weed matter without the risk of spreading them throughout your garden.
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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.
5 Methods For Composting Your Weeds
1. Using the Hot Compost Method
If your compost heap gets plenty of heat and is steaming to the touch, then you can try composting weeds in as little as 6 to 8 months. Getting your compost piles hot enough requires a perfect mix of browns and greens, regular working, and (ideally) a sunny location. The hot composting should damage and destroy roots and seed heads so that your weeds will not propagate from the compost – but only if you get it hot enough!
You can use a compost thermometer to check the temperature of your compost heap.
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 seeds and weeds themselves. This does not fit in with most people’s annual rotations of their compost piles, so the hot composting method is out for many gardeners.
2. Rotting (Making Weed Soup)
If your compost heap doesn’t heat up enough — don’t worry, most domestic-sized compost tumblers don’t — then you need to use a different approach. You need to make….weed soup!
How to Kill Weeds by Rotting
- Put the roots of your perennial weeds into a bucket.
- Fill the bucket with water and weigh down the weeds with a brick or stone. You want to ensure the roots remain below the surface of the water.
- Cover the bucket to keep out light, prevent evaporation, and prevent rain from making the bucket overflow.
- Leave for approximately 1 month. That should be long enough to drown even perennial weeds.
- The drowned roots can now be added to your compost pile and composted as normal.
3. Desiccating
During hot summers, you can take advantage of the sun’s heat to desiccate your weeds. Lay the roots of your perennial weeds on concrete or corrugated iron (keep them off the soil). Allow the summer sun to dry them out for 2 to 3 weeks. The roots should then be baked hard and are safe to add to your compost pile.
For this method, be aware that it will NOT kill off weed seeds!
4. Bagging (Good for Large Quantities)
If you have a large quantity of weeds to get rid of, then you can 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 a black plastic bag or carpet (something to exclude the light). Leave them until the weeds begin to break down.
This should be long enough to kill even the toughest weeds. You can then use the resulting compost on your garden beds. This method does not always kill off all weed seeds, so be forewarned if you’re adding weeds that have already formed seed heads.
5. Put in a Bokashi Composter
Bokashi composting is a popular method for composting food waste. Bokashi composting can also be used effectively to compost weed matter. The bokashi composting process uses microbes to ferment organic material. This fermentation process creates an acidic environment that will kill weed seeds and the roots of your weeds.
To compost weeds in a bokashi system:
- Add the roots and seeds from your weeds to your bokashi composter and sprinkle on the bokashi bran. The bran is inoculated with specially selected bokashi microbes which will get to work fermenting and killing your weeds.
- Layer your weeds with the bran. Add approximately 1 inch of weeds and sprinkle with 1-2 heaping tablespoons of bokashi bran.
- Repeat until your compost bin is full. Compact the weeds well; you’ll be amazed how much you can pack into one bin.
- Seal the lid and leave the compost bin to ferment for at least 2-4 weeks. Drain the liquid bokashi leachate every 2-3 days.
- After fermentation is complete, you can add the bokashi bucket contents to your compost pile or bury them straight in your garden.
Learn more about how to practice the bokashi composting method. Note that this link refers mainly to composting food waste, but the same methods can be applied to “bokashi’ing” or composting weeds in your garden.
Frequently Asked Questions
Q: Is it OK to compost weeds?
A: Yes. As long as the seeds and root remnants are effectively killed, you won’t risk spreading them around your garden as you spread compost. Depending on the type, a few weeds in a cool compost pile shouldn’t be an issue.
Q: What weeds should not be composted?
A: Avoid composting invasive weeds, as they tend to be invasive due to the optimal environment that supports their growth. Instead, throw these weeds away.
Q: What is the best way to compost weeds?
A: You can hot compost them to kill off weed seeds and roots.
Q: Can you put weeds into a compost pile?
A: You can, but you’ll have to wait for the weeds to break down to use the compost. Some weeds in a cool compost heap can take up to two years.
Q: What to do with weeds after pulling?
A: You can use one of the 5 methods above to compost your weeds, or you can avoid the risk and throw them away.
Q: Can you put crabgrass in compost?
A: You can, but I wouldn’t recommend it. Instead of composting this invasive species, throw it away after pulling.
Q: Are there any plants you shouldn’t compost?
A: Diseased plants, invasives (like crabgrass and Bermuda grass), and those that have been treated with fungicides or herbicides should be thrown away rather than composted.
Q: What breaks down fastest in compost?
A: High-nitrogen garden plants and plant material break down most quickly. Grass clippings are one example of a high-nitrogen plant.
