How to Kill Weed Seeds in Compost Ideally, you wouldn’t add weeds that are in seed or even in the late part of their blooming cycle to the compost pile. Thus you can avoid the problem of their Composting weeds can make some gardeners nervous, thinking that they are going to end up spreading weeds all around their garden whenever they spread compost. 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 Kill Weed Seeds in Compost
Ideally, you wouldn’t add weeds that are in seed or even in the late part of their blooming cycle to the compost pile. Thus you can avoid the problem of their seeds germinating in the garden when you later use the compost you produced. But sometimes, you have little choice: perhaps the most easily available compostable material (horse manure, hay, etc.) contains seeds or else the endless sorting of weeds according to their “seediness” would just be too complicated. Or, like me, you just feel that everything organic should be composted.
Fortunately, there are other solutions.
A Big, Hot Pile
A compost pile that gives off water vapor is working hard to kill weed seeds. Source: Anatomy of Living, http://www.youtube.com
In general, the bigger the compost pile, the more heat it produces … and heat kills seeds, even weed seeds.
After a week at 130 ° F (55 ° C)*, most weed seeds will be dead, but it takes a month at 145° F (63 ° C) or more to kill the most resistant ones. Curiously, most common weeds actually produce seeds that are fairly easy to kill and they’ll die at relatively low temperatures. That’s the case with dandelions (Taraxacum officinale) and Canada thistle (Cirsium arvense), for example.
*Note that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!
Heat-resistant weed seeds requiring treatment at 45° F (63 ° C) include:
- Bird’s-eye speedwell (Veronica persica)
- Broadleaf dock (Rumex obtusifolius)
- Common groundsel (Senecio vulgaris)
- Common lambsquarters (Chenopodium album)
- Field bindweed (Convolvulus arvensis)
- Ladysthumb (Polygonum persicaria, now Periscaria maculosa)
- Round-leaved mallow (Malva pusilla)
- Spiny sowthistle (Sonchus asper)
- Wild buckwheat (Polygonum convolvulus, now Fallopia convolvulus)
To find out if your compost pile heats up enough to kill weed seeds, simply insert a compost thermometer into it and note the temperature. If you don’t have a compost thermometer, try sinking your hand into the pile. If it’s so hot you to feel uncomfortable, it’s heating up enough.
Do not forget to return the pile regularly, not only because that helps to oxygenate it and thus stimulates microbial life, leading to and maintaining higher temperatures, but also so the ingredients on the outside of the pile, where it’s cooler, can also get their full heat treatment.
Note too it may be necessary to water your compost pile from time to time. Compost heats most efficiently when it is neither dry nor wet, but moderately moist.
When the Pile Is Not Heating Up Enough
The compost bins commonly sold generally can’t hold enough material to ensure high temperatures. If you’re using one, you’ll have to resort to other methods if you want to kill weed seeds in your compost.
Bury compost to prevent weed seeds from germinating. Source: thelegitimatenews.com
It’s important to understand is that weed seeds* can only germinate when exposed to light. If you are concerned that your compost might contain viable weed seeds, simply bury it when you use it, covering it with soil or, if you apply it to the surface, cover the compost with mulch. Problem solved!
*Warning: unlike annual and perennial weed seeds, a few tree seeds, especially nuts, will germinate when covered with soil or mulch.
You can also kill the seeds at the end of a composting cycle by solarization. To do this, spread the compost on a very sunny surface and cover it with a sheet of transparent plastic, holding the plastic in place with rocks or bricks. That will quickly create a greenhouse effect and very high temperatures. Even if there is some germination at first, the heat underneath the plastic will be such that it will soon kill both the seedlings and any remaining seeds, leaving you with weed-free compost you can use as you want.
With these methods in mind, you can dare to add weeds at any stage of their life to your compost pile.
5 Foolproof Ways to Compost Weeds
Composting weeds can make some gardeners nervous, thinking that they are going to end up spreading the weed all around their garden whenever they spread compost. Troublesome weeds such as couch grass, nettles, buttercups and ground elder, have large root systems. Their root systems are what makes them such prolific growers and so hard to eradicate from your garden. These roots absorb huge amounts of valuable nutrients from the soil.
What does that mean?
Don’t throw them away! Your garden can benefit from all of those extra nutrients. Leaves aren’t the only thing you should be composting from your yard!
By following these few simple tips you can compost weeds without the risk of spreading them throughout your garden.
Listen to this post on the Epic Gardening Podcast
Remember: We will never eradicate weeds, they blow about, birds drop them and they can sleep in the soil for years then soil disturbance will set them off.
5 Ways to Compost Your Weeds
1. Using the Hot Compost Method
If your compost pile gets plenty of heat and is steaming to the touch, then weeds can compost in as little as 6 to 8 months. To get your compost piles hot enough requires a perfect mix of browns and greens, regular working and (ideally) a sunny location. The heat from your compost should damage and destroy roots and seeds so that your weeds will not propagate from the compost.
However, for most of us, it isn’t as easy as that. Unfortunately, most home compost bins do not get hot enough to destroy all seeds and roots. Smaller and slower acting compost piles may take up to two years to kill weeds. This does not fit in with most people’s annual rotations of their compost piles, so this method is out for many gardeners.
2. Rotting (Making Weed Soup)
If your compost pile doesn’t heat up enough — don’t worry, most domestic sized composters don’t — then you need to use a different approach. You need to make….weed soup!
How to Kill Weeds by Rotting
- Put the roots of your perennial weeds into a bucket.
- Fill the bucket with water and weigh down the weeds with a brick or stone. You want to ensure the roots remain below the surface of the water.
- Cover the bucket to keep out light, prevent evaporation and to stop rain making the bucket overflow.
- Leave for approximately 1 month. That should be long enough to drown even perennial weeds.
- The strained liquid makes a great liquid fertilizer. Dilute 5 parts water to 1 part weed liquid.
- The drowned roots can now be added to your compost pile and composted as normal.
During hot summers, you can take advantage of the sun’s heat to desiccate your weeds. Lay the roots of your perennial weeds on concrete or corrugated iron (keep them off the soil). Allow the summer sun to dry them out for 2 to 3 weeks. The roots should then be baked hard and are safe to add to your compost pile.
4. Bagging (Good for Large Quantities)
If you have a large quantity of weeds to get rid of then you can simply bag them in compostable bags (ideally paper yard waste bags). Find a convenient place to ‘hide’ these bags; such as behind your garden shed. Cover with black plastic, or carpet (something to exclude the light). Leave for 2 to 3 years.
This should be long enough to kill even the toughest weeds. You can then use the resulting compost on your garden.
5. Put in a Bokashi Composter
Bokashi composting is a popular method for composting food waste. Bokashi composting can also be used effectively to compost weeds. The bokashi composting process uses microbes to ferment organic material. This fermentation process creates an acidic environment which will kill seeds and roots of your weeds.
To compost weeds in a bokashi system:
- Add the roots and seeds from your weeds to your bokashi composter and sprinkle on the bokashi bran. The bran is inoculated with specially selected bokashi microbes which will get to work fermenting and killing your weeds.
- Layer your weeds with the bran. Add approximately 1 inch of weeds and then sprinkle with 1-2 heaping tablespoons of bokashi bran
- Repeat until your bin is full. Compact the weeds down really well; you’ll be amazed how much you can pack into one bin.
- Seal the lid and leave the bin to ferment for at least 2-4 weeks. Drain the liquid bokashi tea every 2-3 days and use as a liquid fertilizer. Make sure to dilute 1:100 before using as it is acidic. Alternatively, the bokashi tea can be used undiluted as a weedkiller!
- After fermentation is complete, you can add the bokashi bucket contents to your compost pile or bury them straight in your garden.
Learn more about how to compost with the bokashi method. Note that this link refers mainly to composting food waste, but the same methods can be applied to “bokashi’ing” weeds in your garden.
Disclaimer: Check your local regulations on disposing of certain invasive species (such as Japanese knotweed). Japanese Knotweed is a notifiable contaminated substance if removed from your garden. Both the weed and the surrounding earth should be removed.
Tips for Composting Weeds
Colleen Vanderlinden is an organic gardening expert and author of the book “Edible Gardening for the Midwest.” She has grown fruits and vegetables for over 12 years and professionally written for 15-plus years. To help move the organic gardening movement forward, she started an organic gardening website, “In the Garden Online,” in 2003 and launched the Mouse & Trowel Awards in 2007 to recognize gardening bloggers.
David Freund / Getty Images
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.