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.
Composting Weeds in the HOTBIN
Can I Compost Weeds in the HOTBIN?
Yes. Even pesky perennial weeds can be composted in a HOTBIN. Here are the precautions and considerations to take into account.
What is a Weed?
Technically a weed can be anything in your garden that is growing in the ‘wrong place’. Do bear in mind though that one mans junk is another mans treasure and it’s the same for weeds. What one person thinks is a weed can be a precious flower to another.
Those weeds we don’t want can usually be characterised as such because they are pinching space, light and food from those plants we want to keep. A familiar site in domestic gardens as they pop up between flagstones; they are even more of an issue on allotments.
Types of Weeds
Weeds can be annual, biennial or perennial. The latter being the most problematic from dandelion, buttercup, nettles, docks and thistles to invasive types such as couch grass, bindweed/convolvulus, horsetail and ground elder.
Horsetail or Mare’s Tail (Equisetum Arvense) is an invasive, deep-rooted perennial weed which forms dense carpets of foliage forcing out less vigorous plants in beds and borders.
Easily recognised by its upright, fir tree-like shoots that appear in summer, Horsetail has light brown stems in the Spring which appear with a cone-like spore producing structure. In summer, sterile green shoots develop into fir tree-like plants.
Horsetail is an invasive and tough weed where rhizomes bury deep below the surface allowing them to enter other gardens beneath the surface. So if you are planning to compost this plant you need to check that the seeds/bits are only added to the top of an already hot pile (above 40°c). DO NOT fork in or turn the contents of the HOTBIN as seeds will fall down to cooler base.
Composting a weed – The Importance of Destroying Seeds and Rhizomes
Can I compost weeds?
As far as composting goes, weeds aren’t any more difficult to compost than other plants so they shouldn’t be wasted. If you are following general weed advice and removing them whilst they are young, the weeds will breakdown and make excellent compost quickly.
So why do some people not want to compost them?
The main complication surrounding composting weeds is to do with the seeds and the rhizomes, both of which can cause chaos if not destroyed effectively.
Proving to be problematic seeds can survive in soil and cold compost heaps laying dormant for many years. This can include seeds from melons and tomatoes plus weeds like couch grass and dandelion which when left in a cold compost heap are planted in nature’s best growing medium – humus/compost.
If they are not destroyed properly the weeds can end up being spread over flower and vegetable beds in final compost causing mayhem to your plot.
Rhizomes are a feature of perennial weeds, similar to strawberry stolons, however the rhizome represents the main plant stem, whereas stolons are “off shoots” from an existing stem. Rhizomes are subterranean roots; growing underground helps the weed survive through the winter and is responsible for the aggressive nature at which they spread. To destroy them you need to do more than cut them back to ground level, the roots need to be thoroughly dug out.
- Reading up on weeds and make sure you don’t have any in your garden that require specific care. For example Japanese Knotweed which is a controlled weed species is particularly troublesome; we recommend following detailed advice for controlling it.
- Catch weeds early before they go to seed and remove all rhizomes/roots.
- Add to a self-insulating compost pile – the HOTBIN compost bin. Sustained heat of 40°C+ is required to kill weeds and seeds.
- After hot composting invasive weeds like Horsetail you may wish to give your compost a germination test. Leave the compost in an open maturation pile for a few months to check there is no re-sprout – f it does, gently tease out all roots and rhizomes again and zap it through the hot compost again.
Do I need to destroy the weeds before adding to the HOTBIN?
Some sites advise destroying the weeds before adding to compost bins. Heat is the successful factor in the successful destruction of weeds and seeds if you want to compost them. Even the peskiest perennial weed cannot survive sustained hot composting temperatures of 60°c.
Advice | Destroying Weeds Through Sustained Heat in the HOTBIN
- Ensure you are HOT composting between 40-60°c. Never add seeds to a HOTBIN that’s not up to temperature, otherwise seeds will be spread in final compost.
- Add weeds and seed heads into the top of the bin, the hottest part and place in the middle – do not fork the mix in as they could then fall to the cooler layers or down the sides and may survive.
What About Weed Killer?
Domestic weed killers are biodegradable so adding grass or weeds with weed killer on shouldn’t be a problem, however please check individual product instructions.
Test Your Final Compost
To reassure yourself that the weeds are gone especially with invasive ones you may decide to do a test. Simply plant up some small pots with your final compost and water to see if anything germinates, if not, hot composting has successfully destroyed them.
Bear in mind that at the end of the day it’s difficult to get rid of all weed seeds – birds will kindly drop them, the wind will carry them and they can even lay dormant in the soil for years… and then there are the rhizomes!
Compost various weeds from dandelions and buttercups to horsetail and ground elder in the HOTBIN. Just make sure you are hot composting first to ensure the weed seeds are killed off.