Does Composting Kill Weed Seeds?

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Composting is an amazing method for converting biomass into a nutrient-rich medium that nourishes your plants, protects them from disease, and ensures a bountiful harvest. For home gardeners, one of the most abundant sources of biomass comes from weeds that are regularly removed from the garden. For many gardeners using weeds in their compost is a huge concern as many pest plants also produce hundreds if not thousands of seeds that could then contaminate your compost. Thankfully, with proper composting methods seeds of all sorts are broken down and no longer viable to germinate. As we’ll explain the secret to eliminating a weedy seed bank from your compost is making a hot compost pile!

Weeds and Their Seeds

A large majority of weeds reproduce and spread through the production of large quantities of seeds. These seeds are extremely resilient and are designed to stay dormant in the soil for decades waiting until the perfect time to germinate. In your garden, weeds can proliferate extremely quickly and become a nuisance to your other plants. Ideally, gardeners will remove these weeds before they produce more seeds that will continue causing problems in your garden.

Weeds must be killed in a HOT compost

Since seeds are extremely resilient the only way to properly eliminate them from compost is through biologically active hot compost. The biology that takes compost temperatures up to 170 degrees Fahrenheit is what is necessary to properly digest and break down seeds. It is important to ensure your entire pile is properly decomposed before applying it to your garden because it could create the perfect environment to spread weeds! Most weed seeds are easily killed in a hot compost but some of the tougher ones could take up to 3-4 weeks at temperatures of 135 degrees!

Weeds that Require High Heat Treatment

Below is a list of weeds that require adequate attention for the extermination of seeds. These should be heat treated particularly well because of their strength and resistance. If you have large quantities of these weeds you may consider using the solarization method described later in this article. This is just a short list so be aware there are countless other weeds that have hardy seeds!

  • Broadleaf dock (Rumex obtusifolius)
  • Yellow dock (Rumex crispus)
  • Common groundsel (Senecio vulgaris)
  • Common lambs quarters (Chenopodium album)
  • Black Mustard (Brassica nigra)
  • Field bindweed (Convolvulus arvensis)
  • Ladysthumb (Persicaria maculosa)
  • Flea Bean (Erigeron sp.)
  • Spiny sowthistle (Sonchus asper)
  • Milk Thistles (Cirsium sp.)
  • Velvet Leaf (Abutilon theophrasti)
  • Wild buckwheat (Fallopia convolvulus)

How to Make a HOT Compost to Kill Weed Seeds

1. Gather all your weeds together in a large pile. Remove any first or excess soil from plant roots if possible.

2. Weeds are generally anywhere from 25:1 to 35:1 Carbon to Nitrogen which is just about what you want for composting. Adding 15=25% manure or food scraps will help the pile heat up just in case there isn’t enough nitrogen.

3. In arid climates or if your materials are dry you should try and keep your moisture level at field capacity. Field capacity is the moisture level where you can squeeze your material and only release a couple of drops of water. 

4. Add 1 cup of Molasses to 1 gallon of water and spread that onto your compost pile to help stimulate bacterial growth. This should be replaced with compost tea or some other microbial inoculant if you have that available. This step is not absolutely necessary but will help.

5. Flip regularly! Anywhere from once every 2-3 days to once a week minimum. The more you flip it the quicker it will decompose. Ensure that the compost is producing heat and steam from the center. When flipping your pile try to take all the material from the outside and make it the center of your new pile.

6.Wait until your compost cools down and is finely decomposed. This should take 3-5 weeks if properly done. 

7.Put your compost through a compost screen to separate larger undecomposed materials or apply it directly in your garden.

8.Store compost out of the sun and protected from the rain.

Things to Consider for Composting Weeds

  • Ideally, you will pull weeds before they go to seed! This ensures seeds don’t spread in your garden and have less of a risk to contaminate your compost.
  • If possible cut weeds at the root base instead of pulled from the soil. This reduces soil disturbance which can promote the growth of new weeds and avoids excess soil in your compost.
  • Plants that easily resprout from vegetative growth should also be treated through a hot composting process.
  • If your pile has trouble heating up it may be lacking nitrogen. Add nitrogen in the form of manure or food scraps. The other possibility is that it has excess moisture, in this case let it dry up or add extra dry materials.
  • You can test your compost for weed seeds by placing it in a pot and watering it. If weeds germinate then you know it is not ready for your garden!
  • If you already have a hot and active pile you could strategically place your weeds in the center of the pile when you flip it. This will ensure they get hot and catch up in decomposition to the rest of the pile.
  • The bigger the compost pile the hotter! This means it will be more effective at killing weeds and their seeds!

Solarization is a Guaranteed Way to Kill Weeds

If you want to guarantee you kill all the weed seeds in your material you can use the process of solarization. This process uses the power of the sun and clear plastic to heat your material to temperatures up to almost 200 degrees! You do this by placing a clear plastic above your material and place it in a place it can be blasted by the sun. In areas with cloudy weather or cooler temperatures, you can try placing a black plastic beneath your clear plastic to capture more of the sun’s energy! Use a thermometer to confirm your temperatures are reaching above 140 degrees and leave material for 3-4 weeks. Another easy way to achieve this is by placing your weeds in a black plastic bag in the sun! This will not only heat them but can promote fermentation that will predigest your weeds!

Composting Weeds Killed By Herbicides

We do not recommend composting weeds that have been killed using synthetic herbicides. These may contaminate your compost and result in potential health consequences for your soil, plants, and final consumers. If you decide to compost weeds killed by herbicides then we recommend only using this compost for ornamental non-edible plants.

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