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Will Compost Kill Weeds? Tips for Composting Weeds

Composting is an incredibly organic and effective way to reduce waste and help your garden flourish the way you want it to. Any scraps you have after cooking, your coffee grounds, dead or dying plants, and in some cases, even weeds can help other plants grow!

While composting can kill weeds and turn them into organic food for other plants, weeds can survive in some situations and different composting routines. You need to know exactly how to compost to kill weeds before expecting it to just happen on its own.

Throwing a bunch of food and plant scraps into a bin may technically lead to a compost pile; it can actually be the perfect place for weed seedlings to germinate if you aren’t aware of the circumstances needed to kill the seeds.

How To Kill Weeds In Homemade Compost

There are different types of composting depending on what you need it for. However, most composting types fall into hot or cold composting, both of which have pros and cons, but only one kills weeds.

Hot Composting

It takes heat and moisture to allow seeds to germinate, so in some instances, compost can be the perfect place for weeds to grow. However, if you hot compost correctly and control your temperature and consistently monitor it, you should be able to get it hot enough to kill the weeds.

The compost needs to be a minimum of 145℉ to kill the seeds; however, it needs to be this temperature throughout the entire compost bin to kill all of the seeds. Otherwise, you will have sections of the compost that are actually germinating the seeds instead.

If you want to ensure that your hot composting is killing weeds and effectively breaking down everything, you want to do these three things:

  • Turn your compost pile frequently
  • Give it time to breakdown organic matter
  • Weed your garden before adding compost

(Source: Epic Gardening, Survival Gardner)

Cool Composting

If hot composting seems like too much of a task for you, there is an alternate option. You could do cool composting, which is a lot less demanding. With cool composting, you will be taking older compost from the bottom to use in your garden while adding new compost to the top.

This allows you to skip the temperature checks, as well as the constant turning. If you want to do cool compost while ensuring your weeds are killed effectively, you need to make sure you don’t put any pernicious weeds into your compost pile.

Certain weeds require a lot of heat to kill, and if your compost pile doesn’t get to that temperature, you will end up germinating them. You can always prebake the weeds before adding them to ensure no more growth.

A few ways to do that are:

  • Baking them in the oven for 5-10 minutes
  • Laying them in a black trash bag or on a metal sheet pan in the sun for a few hours
  • Put them in the microwave for a few minutes until crispy.

Once the weeds are killed, you can add them to your compost without fear that they will grow.

(Source: The Spruce, Survival Gardner)

Does Compost Keep Weeds Down?

Compost will help keep weeds down, as long as you do it the correct way. To ensure that your weeds are kept from germinating and growing, you need to keep watch on a few things with your compost.

P.H. Testing

The P.H. of your compost pile needs to be anywhere from 6-8 to effectively break down the organic matter, as well as killing seeds. If the PH is off at all, you risk the chance of growing bacteria and microorganisms that could hurt your soil and plants.

While you can buy a PH test at most lawn and garden places, the easiest way to tell if your PH is correct is usually by smell. Your compost should smell earthy, like fresh soil or mulch. If your compost smells like rotten eggs or sour, your PH is off.

Temperature Checks

Of course, you also need to be keeping tabs on the temperature of your compost pile. You want the compost at around 145℉ to ensure that all seedlings are killed. If you have specific desires for only one type of weed to be killed, your temperatures could be lower, but not by much.

(Source: Survival Gardner, My Garden)

Correct Carbon/Nitrogen Ratio

Your compost pile needs a good ratio of energy and nutrients, otherwise known as the carbon and nitrogen ratio. You can keep this ratio where it needs to be by alternating layers of brown dead organic matter and green organic matter.

(Source: My Garden, Epic Gardening)

Oxygen Content

Your compost pile needs oxygen to break down your organic matter effectively. Consistently turn your compost while breaking down clumps and mixing the dry sections with the moist sections.

Moisture Content

Your compost needs to have enough moisture to keep the good microorganisms alive so they can kill the weed seedlings. However, you don’t want too much water because it can remove nutrients and microorganisms from your compost, throwing off your ratios.

(Source: My Garden, Epic Gardening)

Which Weeds Should Not Be Composted

While every plant and compost pile is different, there are a few hard and fast rules for what weeds you should not put into your compost pile. These are:

  • Seed heads – unless your compost is hot
  • Any weed that reproduces from its roots
  • Any weed that may have fungus or be diseased

You can always take your chances, but these are things that could easily take over your garden if your compost isn’t hot enough to kill them off.

(Source: Epic Gardening, Fine Gardening)

Final Thoughts

Composting is a great way to use your organic matter without just throwing it away. Not only does composting reduce waste, but it also gives new life to your plants. However, there may be some things you can’t compost.

Unless you are going to check PH levels and temperatures of your compost pile every day, it is not recommended that you put weeds, or roots of weeds, into your compost pile. They can germinate and grow back into your garden, ruining your plants.

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