Weeds are the bane of many gardeners’ existence. No matter how diligent you are with weeding your garden and landscaping beds, it seems like these pesky plants keep popping up everywhere! As annoying as weeds can be, did you know that you can put them to good use by composting them?
Composting weeds allows you to turn these unwanted plants into a beneficial soil amendment for your garden. However, there are some important things to keep in mind when adding weeds to your compost pile. Certain weeds like poison ivy should never be composted, and others may need extra precautions to make sure no viable seeds make it back into your garden soil.
An Introduction to Composting Weeds
Weeds are prime candidates for composting because they are fast-growing plants that are loaded with nutrients like nitrogen, phosphorus, and potassium. By composting weeds instead of throwing them in the trash, you can recycle those nutrients back into your garden. This reduces waste and enhances your soil at the same time!
However, some weeds spread rapidly via seeds, runners, or rhizomes, so care must be taken to avoid introducing viable plant parts back into the garden. The good news is that the microbial activity during composting generates enough heat to destroy most weed seeds and roots if conditions are optimal.
Composting weeds successfully depends on:
- The type of weeds you are composting
- Using the right composting methods
- Achieving sufficiently high temperatures
- Allowing the compost to age properly before using
Follow the guidelines below to get the most out of composting your garden weeds.
Weeds You Should Not Compost
While the majority of weeds can be added to the compost pile, there are a few types that should always be avoided:
Invasive or Noxious Weeds
Plants that have been designated as invasive or noxious weeds in your region should never be composted. Even after going through the composting process, some seeds from extremely hardy invasive weeds may still remain viable.
Examples of invasive plants that should not be composted include Japanese knotweed, kudzu, purple loosestrife, and wild morning glory. Check with your local agricultural extension office for a list of regulated noxious weeds in your area. Dispose of these weeds with municipal yard waste collection if available.
Weeds with Chemical Residues
Do not compost any weeds that have been treated with chemical herbicides or other pesticides. Chemical residues can remain stable through the composting process and harm plants when the finished compost is applied. Only add weeds that have been pulled or removed manually without chemical use.
Poisonous Plants
Weeds that are poisonous to humans and animals should never be composted. This includes poison ivy, poison oak, poison hemlock, and toxic seeds or berries from plants like nightshade or castor bean. Errant seeds from these poisonous plants can also germinate after composting. Dispose of poisonous plants with yard waste pickup or by burning.
By steering clear of invasive, chemically-treated, and poisonous weeds, you can avoid potential risks from composting the wrong materials. When in doubt if a weed is safe for composting, leave it out!
Weeding and Gathering Plant Material
Before you can compost weeds, you first need to pull or gather your weed plants. Here are some tips for collecting ample weed material for composting:
- Time weeding to coincide with compost pile building. Pull weeds before they go to seed and set some aside for compost.
- Use a hula hoe, dandelion weeder, or other weeding tools to easily harvest weeds with roots intact. The roots are valuable for adding carbon to the compost.
- Collect yard waste like fallen leaves and trimmed branches to layer with freshly-pulled weeds. This provides additional carbon and airflow.
- Mow down tall weeds or invasive plants before seeds develop. Leave clippings in a pile to pre-compost.
- Consider using a compost crock or weed compost bag to pre-compost small amounts of weeds as they are harvested.
- Use five gallon buckets to transport large harvests of weeds to your main compost pile in batches.
By stockpiling weeds as they become available, you’ll have a steady supply of green material to balance out your compost.
Choosing a Compost Method Based on Weed Type
Not all weeding composting methods are equally effective at killing weed seeds and plant parts. The composting approach you use should be based on the type of weeds you are working with.
Annuals, Biennials, and Non-persistent Perennials
Weeds like chickweed, dandelions, purslane, and wild amaranth are relatively easy to compost. Their seeds and root bits are killed off quickly in active compost piles. No special precautions are needed.
Compost these weeds using:
- Hot, fast composting
- Cold composting
- Sheet mulching
- Compost piles or bins
- Slow composting in an enclosed bin or tumbler
Turn compost regularly to speed decomposition and kill weed remnants. Finished compost can be used without fear of reintroducing weeds.
Vigorous or Persistent Perennials
Weeds like bindweed, Japanese knotweed, horseradish, and quackgrass have spreading roots that resprout readily. Take extra steps to remove viability before composting.
Recommended pre-treatment before composting:
- Solarization under black plastic
- Rotting in water or an anaerobic environment
- Hot water immersion
- Smothering under mulch or tarps until dead
After pre-treating, add perennial weed remains to an active hot compost pile. The high microbial activity will continue destroying any bits trying to regrow.
Weeds Gone to Seed
Mature weeds that have already produced seed heads require extra processing before composting. Otherwise, seeds may survive and end up back in your garden!
Options for pre-treating seeded weeds:
- Enclose in tied burlap bags and leave in hot sun
- Soak in water for 7-10 days until mushy
- Dry out thoroughly before composting
- Compost separately in covered, aerated bins
Hot, fast composting will finish breaking down weed seeds so they are unviable. Screen finished compost to remove any remaining seed casings.
By matching your composting method to the type of weeds being processed, you can avoid reintroducing viable weed parts back into your garden soil.
4 Effective Systems for Hot Composting Weeds
The most foolproof way to compost weeds is using hot composting methods. Maintaining high internal temperatures between 130-150°F generated by microbial activity will reliably kill off any weed seeds, roots, or regrowth.
Aim for the following conditions to successfully hot compost weeds:
- Target carbon-to-nitrogen ratio (C:N) between 25:1 and 30:1
- Moisture content around 50-60%
- Aeration and oxygenation of the pile
- Frequent turning or mixing
Here are some of the best hot composting systems to consider for weed control:
Turned Piles
This traditional hot composting approach involves building free-standing piles at least 3′ x 3′ x 3′ in size. Mix weeds, greens, browns, and amendments in layers as it is constructed. Use a pitchfork to turn and mix the pile every 2-3 days for fast decomposition. Turned piles consistently reach 140°F+ interior temperatures to destroy weed seeds and roots.
Aerated Static Piles
For static compost piles that don’t get turned, aeration tubes, chimneys, or buried wood can provide oxygen to the microbial community. The increased airflow drives up temperatures. Make piles at least 4′ x 4′ x 4′ tall. Monitor internal temperatures and turn or mix if they drop below 130°F.
Enclosed Composting Bins
Bins made from wood, wire mesh, or plastic are designed to contain and optimize compost piles. The enclosure helps insulate piles. Lids and doors make turning and mixing easy. Maintain high temperatures by monitoring with a compost thermometer and adjusting moisture and aeration as needed.
Insulated Tumblers
Drum composters and rotating tumblers allow easy mixing and aeration by rolling or spinning. Insulated, enclosed designs help retain heat. Use 10-14 day cycles of rolling and resting to hit peak temperatures. Screen finished compost to catch any stray seeds. Timely emptying ensures space for new weedy additions.
The key is actively managing the compost pile conditions to sustain high temperatures over time. This reliable heat treatment eliminates the risks of composting weeds.
5 Tips for Effective Cold Composting of Weeds
Cold composting relies on slower microbial breakdown without much heat generation. While it takes longer, you can still compost weeds at lower temperatures by following a few guidelines:
1. Use Deep Bins or Piles
Compost depth is key to holding in moisture and excluding light which helps destroy seeds. Piles less than 3′ tall stay too cool. Build up piles in batches as you harvest weeds.
2. Limit Turning and Mixing
Turn cold compost piles only 2-3 times per year. Excessive handling exposes seeds to light and air which can help them survive.
3. Monitor Moisture
Alternating wet and dry cycles helps break plant material down. Use a compost thermometer to check inner moisture and aim for 40-60%.
4. Allow Long Composting Time
Cold composting can take 6-12 months to finish. Make sure piles are curing for at least 3 months after building.
5. Screen Before Using
Sift finished cold compost through 1/2″ mesh to catch any remaining seeds before using around plants.
Even with lower temperatures, given enough time most weed seeds decay fully in deep, undisturbed cold compost piles.
How to Rapid Compost Weeds in Place
Don’t have an established compost pile yet? No problem! You can quickly compost weeds right where they are growing using a few different techniques:
Solarization
Lay sheets of clear plastic over densely-growing weeds. The heat and lack of light kills plants and seeds in 4-6 weeks.
Sheet Mulching
Smother weeds by layering cardboard, compost, and mulch. The piles heat up as weeds decompose and enrich the soil.
Occultation
Cover weeds with organic materials like wood chips, leaves, or straw. The blocking of light kills existing growth.
Herbicide Treatment
Spray fast-acting, non-residual herbicides on weeds. Leave dead plants to decompose and release nutrients.
Shallow Tilling
Lightly work weeds into the top 2-3 inches of soil using a hoe or tiller. This disturbs their growing cycle.
For rapid on-site composting, combine occultation with mulches or plastic sheeting to speed decomposition.
What NOT to Include When Composting Weeds
Avoid contaminating your weed compost pile with these materials:
- Invasive or noxious weeds
- Poisonous plants
- Weeds with herbicide residue
- Diseased weed plants
- Weeds with developed seed heads
- Pernicious roots like bindweed or quackgrass
- Berries and seeds from trees/shrubs
- Wood ash or sawdust from treated lumber
- Meat, fats, or dairy products
- Pet or human waste
Stick to uncomposted vegetation to keep your weed compost clean and safe for gardening use. Monitor for signs of regrowth and remove suspicious material.
Troubleshooting: Problems and Solutions
Composting weeds doesn’t always go smoothly. Here are some common issues and how to fix them:
Problem: Weed seeds regrowing in finished compost
Solution: Hot compost piles may have cooled off too soon. Screen finished compost through 1/4″ mesh before using in garden beds.
Problem: Persistent weed roots resprouting in compost
Solution: Pre-treat vigorous weeds before composting. Increase turning frequency and monitor pile temperatures.
Problem: Unpleasant odors coming from the compost pile
Solution: Improve aeration. Add coarse material like straw or wood chips to allow airflow. Turn and remix the pile.
Problem: Slow decomposition in the compost pile
Solution: Adjust the carbon-to-nitrogen ratio by adding nitrogen-rich greens. Optimize moisture levels around 50-60% and turn or aerate pile.
Problem: Weed seeds germinating beneath plastic sheet mulch
Solution: Use UV-stabilized plastic sheeting to block light better. Leave sheeting in place for at least 2 months before removing.
Troubleshoot compost piles by carefully observing what works and doesn’t. Adjust your methods until you get consistent weed-free results.
Alternative Uses for Weeds Besides Composting
If you decide composting certain weeds is too much hassle, here are some other ways to utilize them:
- Gather flowers, seeds, and berries from edible weeds to use in cooking, teas, and herbal remedies. Common edible weeds include dandelion, chickweed, plantain, clover, wild violet, lambsquarters, and purslane.
- Use tough, fibrous weeds like jute and kenaf for making baskets and other woven crafts. Extract plant fibers by retting and pounding.
- Ferment weed tops to produce liquid feeds for plants. Comfrey, horsetail, nettles, and borage are ideal for making nutrient-rich weed tea fertilizers.
- Float chopped aquatic weeds like duckweed and water hyacinth in ponds to shade out algae growth and filter nutrients.
- Dry and store weed seeds from useful dynamic accumulator plants. These include lambsquarters, ragweed, and yellow dock. Save seeds to sprout mineral-rich greens.
- Use prickly weeds trimmed from pastures and fields as a barrier hedge to deter livestock from escaping fenced areas and support local wildlife habitat.
Get creative and try repurposing your weedy bounty. What’s a nuisance in your garden can often become a useful resource!
Conclusion
The takeaway is that composting weeds is doable and beneficial but requires a bit more care than other organic materials. Understanding weed types, pre-treating properly, and using hot composting methods is key to avoiding regrowth. With close management of temperature, moisture, and processing times, you can destroy weed seeds, roots, and leaves to produce safe, nutritious compost.
Composting weeds allows you to close the growing cycle. Rather than sending pulled plants to the landfill, their nutrients get recycled back into your garden. This organic matter improves moisture retention, drainage, and soil structure over the long term. Harnessing the power of composting is a sustainable weed management strategy for any gardener.