Can You Compost Weeds for the Garden?

Weeds are a common nuisance in any garden, but did you know that you can turn them into a valuable resource through composting? Composting weeds not only diverts them from the landfill but also helps to create nutrients for your garden. In this blog post, we’ll explore the best practices for composting weeds, including what types of weeds to compost, how to set up the compost, and how to use the resulting ‘garden gold’ on your plants.

What weeds can you compost?

When it comes to composting weeds, not all weeds are created equal. Some are perfectly safe to toss into your compost pile, while others can cause problems. Here are some guidelines for what weeds to compost.

Unless they have gone to seed, these weeds can be composted safely:

• Dandelions (Taraxacum officinale): Be sure to remove those puffball seed heads to prevent them from spreading.
• Clover (Trifolium spp.): This nitrogen-rich plant breaks down well in compost while adding helpful nutrients. It also makes a great lawn grass substitute!
• Chickweed (Stellaria media): Chickweed is a fast-growing, edible weed with small leaves and delicate stems that break down easily.
• Plantain (Plantago spp.): Chop or shred common plantain, with its broad leaves and distinctive seed heads, for faster decomposition.
• Nettle (Urtica dioica): Despite their stinging properties, nettles are excellent for composting. When prepared correctly, they’re edible too. Use gloves when handling.
• Crabgrass (Digitaria spp.): Make sure this common annual grass hasn’t gone to seed before adding it to the compost pile.
• Purslane (Portulaca oleracea): This succulent-like weed with small, fleshy leaves adds moisture and nutrients to the compost pile.
• Pigweed (Amaranthus spp.): Pigweed, also known as amaranth, can be composted as long as it hasn’t produced seeds. Its leaves and stems break down relatively quickly.
• Lamb’s quarters (Chenopodium album): Lamb’s quarters, with its serrated leaves and white powdery coating, can be composted easily, though lambsquarter seeds require high temperatures to break down.

What weeds shouldn't you compost?

Resist the urge to compost weeds that have gone to seed or have invasive roots. This is likely to increase the problem.

The following weeds are best disposed of in the trash or burned to prevent them from spreading:

• Bindweed (Convolvulus arvensis): Bindweed or morning glory is a problematic perennial weed with highly invasive root systems. Composting bindweed will cause the weed to sprout wherever you spread the compost.
• Japanese knotweed (Fallopia japonica): Japanese knotweed is an aggressive and invasive plant that can spread easily from even small plant fragments.
• Horsetail (Equisetum arvense): Horsetail, also known as mare’s tail, is a perennial weed with deep roots. Composting horsetail can spread it throughout your garden, as the spores are highly resilient.
• Couch grass (Elymus repens): Couch grass, also called quackgrass, is a persistent perennial grass that spreads through underground rhizomes.
• Canada thistle (Cirsium arvense): Canada thistle is a perennial weed with deep, extensive root systems and tenacious seeds.
• Giant hogweed (Heracleum mantegazzianum): Giant hogweed is a highly invasive plant with toxic sap that can cause severe skin irritation.
• Poison ivy (Toxicodendron radicans): Poison ivy contains urushiol, an oil that can cause allergic reactions in many people. Composting poison ivy can release the oil and lead to skin irritation or respiratory issues.
• Bittercress (Cardamine spp.): Bittercress is an annual weed that can produce large quantities of seeds that can potentially spread throughout your garden if composted.
• Nutgrass (Cyperus rotundus): Nutgrass, also known as purple nutsedge, is a perennial weed with underground tubers. Composting nutgrass can lead to the spread of tubers and new growth.

Compost these weeds with caution.

Weeds that have a high seed production rate, such as some forms of thistle, dock, and burdock, can be composted, but be sure to take precautions to prevent any seeds that may exist from germinating. Dry the weeds out thoroughly before adding them to the compost by laying them on a tarp in the sun or compost them separately in a separate compost bin.

How to compost weeds effectively

There’s a reason we caution you to avoid composting weeds that have gone to seed. Most home composts don’t reach the temperatures required to kill weed seeds as they break down. If you avoid weed seeds, composting weeds can be a straightforward process easily integrated into your existing routine. Here’s a step-by-step guide on how to compost seed-free weeds.

1. Prepare the compost bin or pile: Ensure that your composting area is well-drained and easily accessible. If you’re using a compost bin, make sure it has good ventilation. Layering the bottom with twigs or straw can promote airflow and add carbon into the composting mix.
2. Add your weeds to the compost: As you pull weeds from your garden, separate them into categories: those suitable for composting, those you can compost with caution, and those that should be discarded. Place weeds from the first category into the compost pile. It’s best to chop or shred the weeds into smaller pieces to speed up the decomposition process and layer with carbon rich materials. Lay any weeds that can be composted with caution on a tarp in the sun or place in a designated weed composting bin.
3. Add a handful of finished compost: To inoculate your pile, throw some garden soil or finished compost into the mix. This will help transfer the necessary microbes into your new pile to kickstart decomposition.
4. Maintain a balanced compost: Remember to include more brown (carbon-rich) materials than green (nitrogen-rich) materials. Weeds are generally considered “green” materials, so mix them with dry leaves, straw, or shredded newspaper. This balance helps the compost decompose efficiently.
5. Monitor the moisture levels: Keep an eye on the moisture levels in your compost pile. It should be damp, like a wrung-out sponge. If the compost becomes too dry, add water. If it becomes overly wet, mix in dry brown materials to improve aeration.
6. Turn the compost: To accelerate decomposition and ensure even breakdown, turn the compost pile every few weeks. This helps to provide oxygen to the microorganisms responsible for breaking down the organic matter. It also helps mix any weed seeds that might exist back into the hottest part of the pile. Using a compost tumbler can help with this process.

Related: Start Composting in a Tumbler

Is it possible to compost weeds that have gone to seed?

While composting weeds is a great way to repurpose garden debris, you’ll need to take some extra steps if you decide to compost weeds with seeds. Here are a few things to keep in mind.

Prevent weed seeds from germinating

Some weeds produce a large number of seeds that can survive the composting process and sprout in your garden when you use the compost. If you do include weeds with seeds, ensure your compost pile reaches a high enough temperature (140 F for most non-invasive plants) to kill the seeds. Plants like lambsquarters and broadleaf dock will need a month at 145 F before their weed seeds are truly broken down. Consider using a separate compost bin specifically for weeds with seeds, so you can keep them separate from your other compost.

The extension office at Texas A&M University recommends the following conditions for creating a compost pile or bin that heats up effectively enough to kill weeds:

• A ratio of carbon to nitrogen from 25:1 and 40:1.
• Compost that is 40 to 60 percent moisture by weight and 5% or more oxygen content.
• Compost with a pH level ranging from 6 to 8.

Larger piles heat up more effectively than smaller ones. Use a compost thermometer to be sure you reach the right temperatures and turn your pile periodically, rechecking every few days. If you are unable to create a hot enough pile, it’s best to avoid composting weeds that have gone to seed.

Don’t forget: avoid perennial and invasive weeds

It’s so important, we’ll say it again: don’t compost invasive species. These weeds, such as bindweed or Japanese knotweed, can quickly take root and spread in your garden, even from small plant fragments. Once in place, it’s nearly impossible to get them out. It’s best to dispose of these types of weeds in your regular waste or consult with your local waste management authority for appropriate disposal methods.

Using your compost

Once your composting process is finished and your weeds have decomposed, it’s time to put it to use in your garden. But remember, don’t rush or you risk spreading partially composted weeds all over your garden. Weeds need a combination of high temperatures and an adequate duration to compost effectively. Here’s what you need to know.

Determine compost readiness

Before using the compost, make sure it’s fully decomposed and ready for application. The compost should have a dark, crumbly texture, and any visible pieces of weeds or plant matter should be well broken down. This process typically takes several months to a year, depending on the materials composted, the temperature, and the composting method used. If your compost has larger pieces remaining, leave it for a longer duration or screen through a fine composting sifter.

Feed your soil

Incorporate the compost into your garden soil to improve its fertility and structure. Spread a layer of compost over the garden beds and gently work it into the topsoil using a garden fork or tiller. This will help enrich the soil with organic matter, providing essential nutrients for your plants’ growth.

Help control weeds

Ironically, using composted weeds in your garden can indirectly help control weed growth. A healthy, well-nourished soil promotes strong plant growth, which can outcompete weeds. Apply a layer of compost as a mulch around your plants to suppress weed germination and growth. Be mindful not to apply thick layers of compost that can smother your plants or cause water runoff issues.

Related: How to Use Finished Compost

Composting weeds, one at a time

By incorporating compost into your garden, you’re improving soil health, enhancing plant growth, and indirectly discouraging weed proliferation. Enjoy the benefits of your composting efforts by creating a thriving garden environment while reducing waste at the same time.