Tips for Composting Weeds
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Compost is a great way to recycle organic material in your garden. All those spent flower blossoms, fall leaves, dead plants, grass clippings—even non-meat kitchen scraps—can be transformed into a great soil amendment and nutritious mulch, simply by throwing them into a heap and allowing the refuse to decompose naturally.
Done correctly, composting creates a sterile organic material that does nothing but good things for your garden and the plants in it. However, nearly every gardener who practices composting has occasionally experienced “volunteer” plants sprouting up in the garden where the compost has been spread.
This can actually be rather charming when the volunteers are tiny impatiens seedlings, tomato plants, or even pumpkins that volunteer because last Halloween/s jack o’ lanterns were added to the compost heap. It’s far less charming when the volunteer plants are hundreds of dandelions or tiny sprigs of bindweed or crabgrass that get into the garden via the compost you spread.
A gardener who experiences such an explosion of volunteer weeds may well swear off composting altogether, or at least stop adding weed material to the compost pile. To be clear, there is no reason to stop composting weeds. With a slight adjustment to the composting process, you can ensure that weeds and their seeds will be killed completely and won’t be resurrected where you least want them.
How Weeds Survive
In an ideal compost heap, the temperatures generated by the breakdown of plant material can get quite warm, and if temperatures exceed 145 degrees Fahrenheit, pretty much all seeds and roots will be killed. However, if the temperatures do not get warm enough—or if a portion of the compost heap does not experience sufficiently high temperatures—seeds or perennial roots can survive the composting process. When these seeds or bits of root later reach your garden inside the compost, they can—and usually do—quickly germinate or take root again.
How do you know if your compost is getting hot enough to kill all weeds? A variety of compost thermometers are available that can gauge the temperature of your pile. Experienced gardeners may simply thrust a hand into the pile. If it feels uncomfortably warm to the touch, it likely is warm enough to kill all seeds and roots in the pile.
The classic method of composting—the method purists would call the “right” way—is known as hot composting. This simply means that you turn the pile regularly and allowing it to really heat up to 145 degrees Fahrenheit or more. A properly maintained hot compost pile will kill weed seeds, as well as many other pathogens, so you can compost weeds without having to worry about them popping up in your garden beds.
For hot composting to fully kill all weed seeds and roots, follow these tips:
- Turn the pile frequently. All compost heaps have localized cool spots that are slow to break down. By mixing the pile frequently, you ensure that all material is achieving the necessary heat to kill the seeds and roots.
- Give it time. Practiced correctly, hot composting involves processing a volume of material fully until it is fully decomposed. Don’t continue to add small amounts of additional material to the heap; start another heap while the first one breaks down completely. The compost is ready to spread when turning and mixing the pile no longer causes the compost to heat up.
- Weed the garden before adding compost. Fresh compost is laden with nutrients, and if there are weeds growing in your garden, adding compost will simply nourish the weeds along with your garden plants. Make sure your garden is well weeded before adding fresh compost to the soil.
So-called “cool composting” is a more informal style of composting. It is a passive method that doesn’t involve constant temperature monitoring and mixing. In cool composting, fresh material is constantly added to the top of the heap as the lower levels are breaking down into compost. In cool compost bins, gardeners periodically remove the prepared compost from the bottom of the pile as fresh material is constantly added to the top. Cool composting is an easier style, though it can take somewhat longer.
Here are some tips to keep a cool compost pile free of weeds:
Weeds can be safely added to a compost pile if you make sure temperatures are high enough to kill the seeds and roots.
How to Compost Weeds the Safe Way
When I look at weeds, do you know what I see? I see plants that have removed minerals from the soil in which they were growing. And because of that, I want to compost them so that those nutrients can be returned to my garden soil. But composting weeds presents some challenges. Here’s how to compost them the safe way.
The Concerns with Composting Weeds
Composting weeds incorrectly can create the risk of having those weeds spread when the compost is applied to your garden.
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The main parts of the plant to cause concern are the weed’s seeds, and its roots. Either can potentially be spread through your compost pile and wreak havoc in your garden later on. In addition, there are weeds, such as ground ivy, that can root from any part of the plant. These are especially easy to spread.
First Step: Preventing Weeds From Growing in the First Place
Preventing weeds from taking over your garden is the first step to a weed-free garden. Even in the best of cases, though, a few weeds get through and can be composted.
The three strategies that I use to prevent weeds are no-dig gardening, never stepping on my garden beds, and mulching.
While we get very few weeds in our garden beds, this bank in front of our house is another story.
No Dig Gardening
It is possible to garden without ever having to dig. The best way to do this is to build a raised bed and fill the bed lasagna-style. A lasagna garden is built by layering organic materials which eventually decompose into wonderful garden soil.
Digging brings weed seeds to the surface, causing them to sprout. So not only does no-dig gardening save me from having to spend time and effort digging, it helps to keep weeds down in my garden.
I have over 20 garden beds, and all but two of them have been built in the lasagna garden style. Some of them are contained in raised beds, but a few of them are not.
Those in the raised beds get far fewer weeds for several reasons. The boundary created by the raised beds keeps my feet (and those of visitors) on the path and out of the garden. Which brings me to my next strategy – not stepping on my garden soil.
Why Stepping on Your Garden is a Bad Idea
The roots of garden plants love loose soil which allows their roots to go deep for water and nutrients. Stepping on soil compacts it, reducing the space available in the soil for air and water.
In addition, many weeds love compacted soil. Our garden areas that are not contained in raised beds tend to get weeds along the edges where we walk. Since the paths are not always clearly defined, we tend to step into the beds near those edges, compacting the soil. Weeds always grow here.
Mulching Keeps Weeds Down
Mulching has so many benefits in a garden. It keeps the soil from drying out, reducing watering, and as it decomposes it helps to build healthy soil.
A 2 – 3″ layer of mulch does a great job of keeping weeds at bay by blocking the sunlight that the weeds need to sprout. We use both hay and wood chips as mulch in our garden.
What to Do with the Weeds that Do Sprout in the Garden
How to Compost Weeds the Proper Way
I like to call compost “black gold”. It is made up of decomposing organic matter which improves your soil, and keeps kitchen scraps and yard debris out of landfills.
All sorts of items are compostable from grass clippings and leaves, to egg shells and natural fibers. Find a list of items you can compost here. And yes, even weeds are compostable if you take a few precautions.
1.) First, make sure that the weeds you wish to compost haven’t gone to seed. Unless your compost heats up to 140°F, those seeds will survive and spread in your garden. My style of composting can be fairly passive; it just doesn’t get turned as often as I’d like. Therefore, I don’t add any weeds to my compost pile that have gone to seed because I know that it won’t always heat up to 140°F.
2.) If you do wish to compost weeds that have gone to seed, be sure to hot compost your weeds. That means that your compost pile must heat up to 140°F to kill any weed seeds. This temperature should be maintained for several weeks, turning the pile every time it goes below this temperature, which is generally every 4 – 5 days.
3.) An easier method for me to take care of weeds with seeds, or with roots attached, is to ferment them in water. Place the weeds in a 5 gallon pail, cover with water, and allow them to rot and ferment for several weeks before adding to your compost pile. The water can be added to the pile, too, or use it fertilize your plants. Keep in mind that this can get pretty smelly.
4.) Another strategy is to dry your weeds in the sun. This won’t kill seeds, but is good for weeds with roots still attached which could potentially take root in your compost pile. When you are certain that no life is left in the roots, the weeds may be composted.
5.) Lastly, you can place your weeds in a black plastic bag and allow this to heat up in direct sunlight for about a week. I prefer not to use plastic in my garden, so I’ve not used this strategy too often.
Throwing weeds in the landfill means that you are throwing precious nutrients away. Instead, learn to compost them the proper way so that those nutrients can be returned to your garden soil.
To learn how to build a garden that builds healthy soil, be sure to check out my eBook The Art of Gardening: Building Your Soil.
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Throwing weeds in the landfill means that you are throwing precious nutrients away. Instead, learn how to compost weeds the proper way so that those nutrients can be returned to your garden.