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Because You Asked

Can I Compost Weeds Without Them Re-Growing?

By Recyclebank |
To compost weeds effectively you have to breakdown the process. The key is heat. 


Dear Recyclebank: One thing I would like to be able to compost is weeds. I pull so many when gardening, but I worry if I compost them, they will spring back to life. Is there a way to compost them without reviving them? –E. Mijares

Dear E. Mijares: There is nothing worse than having your best efforts backfire! To avoid that fate when composting weeds, you have to breakdown the process. The key ingredient is heat. Organic matter, including seeds, will decompose if subjected to enough heat for enough time. If weed seeds get above 140 degrees, then they will be sterilized (unable to grow), and you won’t have to worry about future weed growth — at least not coming from your compost batch. There are a couple ways to achieve a weed-compost and avoid zombie weeds coming back to haunt you.

The first way is to solarize the seeds. Solarizing is a relatively quick decomposition process. You can simply seal the weeds in a black plastic bag (which will absorb and retain heat), and then place the bag in the sun to cook. The minimum amount of time will depend on the type of weed you’re dealing with and on the intensity of the sun. Three-to-four days in direct sunlight will do the trick in the majority of cases, but if you want to be extra sure, it doesn’t hurt to leave them in the bag for a longer period of time. After the weeds have been scorched, you can add them to your compost pile. Be sure to reuse the plastic bag in future weed scorches.

The second way is to make sure your compost pile is healthy and thriving. A compost pile that is thriving should reach temperatures above 140 degrees, effectively purifying your future compost humus from any traces of weeds. The key here is to get your compost system tuned for optimal performance. If you want to get technical, you can buy a compost thermometer to ensure you’ve reached the right temperature.

There are four things to be aware of when working toward a strong compost pile:

1. Moisture content. Water your pile when you water your garden. The aim is to never let it dry out, but also to never let it become fully soaked. It should be a similar dampness to a wet towel after it’s been wrung out.

2. Oxygen availability. Mix up your pile regularly — a good time is after you water it. This brings the inside portions out to the atmosphere so the microbe populations can breathe and flourish, and do what they do best: Decompose organic matter.

3. The pH balance. The acid-alkaline balance on the pH scale is also a driver in the health of your compost pile. You can get science-y and use a digital pH meter (like a food thermometer for soil) to check your compost’s pH, or you can use the built-in one you already have in your nose! If the pile stinks, that means it’s out of balance. If that’s the case, add lime to shift the balance toward alkalinity. If you’re measuring with pH paper, a mature compost pile should have a pH between six and eight.

4. Carbon-Nitrogen balance. Keeping these two vital nutrients in the right ratio will boost the activity of the microbes in your compost pile, and drive the temperature up. An easy way to achieve this is to add equal amounts of brown, dry matter (carbon), and green, moist matter (nitrogen). The availability of brown and green matter will vary with the seasons making the spring and fall optimal times for composting. Also, it’s possible to keep the weed waste in a separate pile until it turns brown, if it’s midsummer and no other brown matter sources are available.

If all of these parameters are in balance and your pile is healthy, it will release steam when you turn it. Repeat the process. Scorching weeds first and then keeping a healthy compost pile should give you a strong insurance plan against weed seeds in your compost. As you continue to replenish your garden with healthy, weed-free soil year after year, you may see fewer weeds appear.

SOURCES: U.S. Department of Agriculture, University of California Agriculture & Natural Resources, Texas A&M AgriLife

Do you compost weeds without using plastic bags? What’s your secret? Let us know in the comments below.
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  • John C. 3 years ago
    My local garden supply outlet made me aware that some weeds will not be destroyed at 140°.
  • Arianna B. 3 years ago
    Thank you so much. I really needed this info. I've found such unreliable info on this in the past.
  • Gina L. 3 years ago
    So much of killing weeds, composting and just keeping a great yard depends upon where you live. If in the South you can kill a weed in a day. The moister and cooler areas need the 3 day plan. Just make sure you get the entire root wherever you live.
  • Floyd G. 3 years ago
    Good information on composting weeds. I tend to throw weeds in our garbage can.
  • Andrea S. 3 years ago
    I like the solarizing idea a lot. To this point I've just been using the "pile it on and forget about it" approach. Would solarizing also kill any diseases (bacterial, fungal) that might be on the weeds, too?
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