How To Know if Your Compost Is Weed Free

Composting ensures that you recycle and utilize important resources for a healthy garden. However, weeds can get in the way, as they slow down the process of composting. That’s why you need to take steps to ensure no weeds are in your compost pile. 

You can know if your compost is weed free by carefully monitoring what goes into the pile and drying out any weeds you choose to put in. Also, it is important to turn the compost pile regularly to ensure that weeds reach a high enough temperature to die before they spread. 

Let’s talk about how you can eliminate and manage weeds that can grow in your compost pile.

1. Dry Out Any Weeds Before Adding To Compost

Drying weeds out is the best way to ensure that weeds don’t spread through your compost pile. You can do this by leaving them out in the sun. This sounds simple enough, but it might help to understand the exact rationale behind this and how to do it properly. 

The reason you dry out weeds is to kill their seeds and ensure they don’t spread. For that reason, you shouldn’t leave your weeds near or on top of grass and soil the seeds could stick to. Instead, choose an area that gets plenty of sunlight and is reasonably far from the soil. 

Some people do this in their driveway or along their sidewalk. But if that’s not an option for you, you can try putting the weeds on top of a trash bag. Provided you place it under the sun or some other heat source, the bag should heat enough to kill the weeds while keeping them from touching any soil nearby. 

The ideal heat for killing weeds via this method is about 140°F (60°C). You’ll need to leave your weeds in the sun for a few days, and put some kind of cover over them to protect them from the rain and prevent them from germinating. After a few days, the weeds should have dried out enough not to spread through your compost pile. 


One method of drying out weeds is by mulching them. Essentially, you cover the weeds such that they won’t be able to access the things they need to germinate their seeds.

You can use just about any type of dry mulch for killing your weeds. If you use wet mulch that’s rich in nutrients, you may end up giving the weeds a chance to survive instead of killing them. 

Generally, you create mulch by gathering your materials, crushing them as finely as possible (with a wood chipper, if necessary), and mixing them together in a way that there’s barely any space between the chippings.

There are several ways to use mulch to kill weeds, but here are the general steps you can follow.

  1. On the weeded area, put at least a couple of layers of newspaper.
  2. Put a weight (like a stone or concrete block) to keep the layers of newspaper in place. Make sure the corners of the newspaper are weighted; otherwise, any gaps will allow the seeds to reach for the moisture and sunlight they need.
  3. Put a layer of dry mulch over the weight. 
  4. Leave all of the above to dry for a few days. 

2. Turn the Compost Pile Regularly

When you turn your compost pile regularly, you introduce oxygen into the pile and raise its temperature at the same time. Not only will this break down your materials and make them suitable for composting, but it’ll also raise the pile’s temperature enough to kill any weed seeds.

3. Use Weed Killers To Ensure They Are Dead

If possible, you’ll want to use all-natural methods to get rid of weeds in your compost pile. Aside from mulch, you can use substances for killing weeds that you can — fortunately — source from your kitchen cupboard. 

Natural Weed Killers

  • Vinegar (with a little bit of dish soap). The acid in vinegar is strong enough to kill weeds. If you mix it with dish soap, vinegar can be even more effective for killing weeds. Some people add salt, though I believe the vinegar and soap solution is potent enough.
  • Vodka. Vodka will not only kill your stress (at least, temporarily) but also potentially kill your weeds as well. Like vinegar, you may want to mix it with a little bit of dish soap for it to work its best. 
  • Boiling water. As I’ve mentioned earlier, extremely high temperatures can kill off plants like weeds. The beauty of this method is that you don’t have to do any mixing. Just boil your water in a kettle (or a similar heater), and pour it over the offending weeds.

I should note that a couple of these substances (i.e. vinegar and vodka) may kill your other plants as well. Therefore, you should be careful to only spray them onto your weeds and keep your other greenery out of the way. 

Chemical Weed Killers

If you have a serious weed problem and the natural methods of weeding seem impractical, you can consider using chemical weed killers.

When you choose weed killers, you need to check their ingredients list to make sure they’re as toxin-free as possible. For example, ingredients like aminopyralid, clopyralid, and picloram are safe for use in your gardens and around livestock. 

If you choose to go the herbicide route for killing weeds, you must follow the directions carefully. Otherwise, the herbicide might end up killing your good plants as well. 

For an organic chemical weed killer, I recommend one that contains acetic acid (which, interestingly, is also an ingredient in vinegar) and can eliminate weeds within a day or so. Even though it’s quite potent, it’s also eco-friendly, so you don’t have to worry about any unwanted side effects on your plants.

4. Monitor What Goes in the Pile

If you compost large amounts of material, this may be easier said than done. But if you don’t mind putting in a little more effort to keep your compost weed-free, you need to more closely monitor what goes into your compost pile. Alternatively, you can remove the weeds from your composting material before they make it into the compost pile. 

When Should You Avoid Composting Weeds Altogether? 

It is best to avoid composting weeds altogether if you are not sure that the compost pile can reach a high enough temperature to kill the weed’s seeds. If you cannot consistently monitor the pile’s temperature and give it enough time, you should avoid composting weeds. 

To ensure the pile is hot enough, you can either feel the pile yourself or use a compost thermometer to do the job. A thermometer is especially useful if you’re not an experienced gardener who can tell the temperature of a compost pile just by touch.

There are beginner-friendly compost thermometers available that measure the temperature of your compost in Fahrenheit, and it also has useful indicators like “Steady,” “Active,” and “Hot.” Ideally, your compost should be within the “Hot” range to kill the weeds in it.

If you have trouble determining whether the composting process is complete, check out my other article. I’ll give some helpful tips to ensure that your compost pile is ready to go: How to Test Your Compost to Know If It’s Ready

Final Thoughts

Overall, there are many ways to ensure that your compost pile is weed-free. The most important part of composting weeds is to ensure that no weeds (or seeds) in the compost pile survive. Do this by ensuring the compost pile meets the right conditions before using the compost on your soil. 

You can also:

  • Dry out the weeds.
  • Turn the compost pile regularly.
  • Use natural or chemical weed killers (with the necessary precautions).

To learn more about whether composting will kill weed seeds, you could check out my other article here: Will Composting Kill Weed Seeds?

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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