Composting Weed Seeds

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Can You Compost Weeds? (The Truth) You have a good idea of what should and shouldn’t go into your compost. But, it’s tricky knowing whether some types of waste belong there. Many gardening Weeds can be safely added to a compost pile if you make sure temperatures are high enough to kill the seeds and roots. Organic weed management techniques described in detail.

Can You Compost Weeds? (The Truth)

You have a good idea of what should and shouldn’t go into your compost.

But, it’s tricky knowing whether some types of waste belong there.

Many gardening experts warn against composting weeds. But others say it’s safe to toss your morning glory, crabgrass, ivy roots, and blackberry bushes in with the rest of your compost.

Weeds are organic matter, so they’ll decompose with the rest of your garden and kitchen wastes. But there’s a catch: you need to take extra care when composting weeds to avoid spreading them all over your garden.

Here are the must-know recommendations for making weeds good for your garden!

Can Weeds Be Composted?

There are several effective methods for composting weeds and their seeds. Hot composting can eliminate weeds effectively. However, this is difficult to achieve with small home composting systems because sustained high temperatures are uncommon.

Nevertheless, there a still several things you can do:

  • You can hot compost the weeds if you have a sizeable well-monitored system.
  • Kill the weeds and their seeds by sun-drying.
  • Soak them in water before adding them to your compost.
  • Compost the weeds separately in black plastic bags.

Keep reading for all the answers to your questions about how to compost invasive plants.

Tip: try to uproot weeds before they have time to go to seed.

Can I Put Dried Weeds In Compost?

You can add thoroughly dried weeds to your compost.

During the sunny seasons, spread your weeds out on a metal surface (not on your grass or soil!) and let the sun dry them out. After about 2 to 3 weeks, the weeds will be crisp and safe to add to your compost.

Some gardeners use a platform made out of chicken wire as a drier before adding them to compost.

Can You Compost Weeds Killed With Herbicide?

Composting weeds killed by herbicide is problematic. Not all herbicides destroy the seeds. And the chemicals in herbicides can be harmful to the composting microbes.

Herbicides work in different ways to control weeds. For example, some herbicides might kill weed plants but leave their seeds viable to wreak havoc in your compost. And the herbicides that are powerful enough to kill weeds (seeds and all) might make your compost toxic to plants.

Keep herbicide-killed weeds out of your compost to protect your garden against possible rouge weed seeds and poisons.

If you really want to compost chemically treated weeds, compost them on their own and use them on plants less susceptible to herbicides. Don’t use the resulting compost on food crops.

Can Weeds Go In A Compost Pile?

You can add weeds to your pile if you’ve got a hot composting system. Just know that you’ll need to tend to your pile to keep the temperatures high enough to kill the weeds and their seeds.

If you’re letting your compost do its own thing (cold composting), don’t add weeds unless you’ve dried them in the sun first.

Another option is to drown your weeds before adding them to your compost pile.

Put the weeds into a container with a tightly fitting lid, pour water over them till they’re submerged, close the top, and leave them to soak for several weeks. The water will be greenish-brown but full of nutrients and safe for your plants. Next, strain out the remaining bits of the weed plants and add them to your compost.

The resulting liquid can be used as a plant fertilizer or a booster for your compost heap.

Can You Put Weeds In A Compost Tumbler?

You can throw a few weeds into your compost tumbler as long as you maintain high temperatures. You might find it easier to keep temperatures up in a compost tumbler than in a pile because tumblers are better at generating and retaining heat.

You can sun-dry or soak the weeds before adding them to your tumbler to be on the safe side.

Why Should You Not Compost Weeds?

Unless you use the proper technique when composting weeds, their roots and seeds can survive the composting process. You might then introduce the weeds to your garden as you spread compost around.

So, instead of giving your garden a treat to help it thrive, you could expose it to persistent weeds eager to take over and cause trouble.

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However, if you’re careful, you can turn weeds into plant-friendly compost.

How To Compost Weeds

There are ways to stop sneaky weed seeds and roots from making it through the composting process alive.

I’ve let you in on two ways to nuke weeds before adding them to your compost (that is, by drying them in the sun or soaking them). I’ll now share other ways to safely compost weeds.

Does Compost Kill Weeds

Compost that gets hot enough at its core will kill weeds. But weeds need to be mixed into the center of a hot composting system to be effective.

Even the toughest weeds (think: morning glory, buttercups, Bermuda grass, oxalis, quackgrass, and crabgrass) or weeds that have seeded stand no chance of surviving scorching compost.

The problem is: that it’s challenging getting backyard compost heaps to produce enough heat to destroy the weeds.

How Do You Kill Weeds In Compost

The best way to kill weeds in compost is by heating things up!

The hardier the weed, the higher the temperatures needed to kill it. Temperatures around 115°F (46°C) can destroy meeker weeds. Still, more robust species might only be knocked out by temperatures as high as 145 to 160°F (62 – 71°C).

Wondering how hot your compost is? You could always stick your gloved hand into the pile’s center and feel around. However, you’d get a more accurate measurement using a compost thermometer – check out this model on Amazon. Monitor the temperature all over the compost, as some areas can be cooler than others.

How To Hot Compost Weeds

There’s a method to create compost that stands a chance of getting hot enough to exterminate weeds.

Start with these composting basics:

  • Build a pile that’s at least 3x3x3 feet in size.
  • Keep a good balance of nitrogen-rich greens (like kitchen scraps) and carbon-rich browns (for example, dry leaves and grass).
  • Make sure the compost stays about as damp as a wrung-out sponge.
  • Turn the compost regularly.
  • Don’t add new organic wastes to the pile.

To speed up the heating process, try these tips:

  • Position your compost in direct sunlight.
  • Add a compost acceleratorto the mix. (Amazon)

If you see steam coming from the compost when you turn it, you’re doing it right!

How Do You Compost A Large Amount Of Weeds?

One of the best ways to compost lots of weeds is in black plastic bags. You compost the weeds alone with this method, preventing them from infiltrating your compost and garden.

Composting Weeds In Black Plastic Bags

Composting weeds in black plastic bags is a simple process. Here are the steps:

  • Place the weeds in heavy-duty black plastic bags.
  • Add a handful of soil and a bit of water to each bag.
  • Tie the bags tightly and put them somewhere out of the way for at least a year.
  • Open the bags, and ta-da! The weeds will have decomposed into nourishing compost for your garden.

How Long Does It Take To Compost Weeds?

How long it takes for weeds to transform into compost depends on how they’re composted.

Although sun-drying weeds for 2 to 3 weeks or soaking them for 2 to 3 months kills the weeds and their seeds, it won’t break them down completely. Compost that’s 115 to 140°F can kill 90% of some weed seeds within hours to days, but these weeds won’t be fully composted.

The weeds won’t be viable after being dried out, soaked, or scorched, but they’ll still take time to decompose. How much time they’ll take depends on whether they’re being hot or cold composted. The type of organic matter that makes up the compost and whether they’re being composted in a bin, heap, or tumbler also impacts the decomposition rate.

Chopping up or shredding the weeds will make them break down faster.

Weeds that have been composted alone in black plastic bags take up to 2 years to fully compost.

Final Note:

Some weeds are considered highly invasive. Government organizations try to control them because they can harm livestock or crops. You’ll find a list of undesirable weeds determined by the US noxious weeds act here. And for the UK, you’ll find a list of invasive plant species here…

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.

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Amanda Rose Newton holds degrees in Horticulture, Biochemistry, Entomology, and soon a PhD in STEM Education. She is a board-certified entomologist and volunteers for USAIDs Farmer to Farmer program. Currently, she is a professor of Horticulture, an Education Specialist, and pest specialist.

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.

Composting Issues

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.

Hot Composting

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.

Cool Composting

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:

  • Don’t compost pernicious weeds. There are certain perennial weeds that require lots of heat to kill, and if you don’t have the time for hot composting, it is best to keep them out of the compost pile altogether. In a cool compost pile, weeds to avoid include morning glory, buttercups, bermuda grass, oxalis, quackgrass, and crabgrass. Any garden plant that spreads by runners, such as mint or raspberry canes, should also be kept out of a cool compost pile.
  • Don’t compost weeds that have gone to seed. Most annual weeds pose no problems if they are added to a cool compost pile before they are mature and set seed. But throw those same dandelions into the pile after their flower heads have produced thousands of seeds, and you may experience a dandelion epidemic when that compost is later added to the garden. If you are cool composting, weeds that have gone to seed should be thrown in the trash, not added to the compost pile.
  • Prebake the weeds. Pretty much any plants, even the pernicious varieties that spread by runners, become safe for any compost pile if you heat them up to the temperature necessary to kills seeds and roots. There are a number of ways to do this. For example, you can solarize them by baking them inside a black plastic bag in the sun for a few days. Other gardeners bake weeds on a sheet of metal laid in the sun; once the weeds are baked to a dried crisp, they pose no risk in the compost heap. Some gardeners have even been known to keep an old microwave oven in the garage or garden shed, using it to “nuke” the weeds into oblivion by heating them until they steam before adding them to the compost heap.
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Seeds in compost

Many materials used for making compost are contaminated with weed seeds. Late cut hay will certainly contain weed seeds. Straw can be examined for fruiting stalks of weeds. All manure other than poultry manure should be considered contaminated unless you have tested it. Horse manure and manure from other animals that have access to weedy pastures or pastures along roadsides are most likely to be contaminated with weed seeds.

In general, it is easier to use weed free materials to make compost than it is to try to kill weed seeds during the composting process. The problem with adding weedy compost to your garden is rarely that you will be immediately overwhelmed with weeds: usually the weed seed density of garden soil is higher than that of manure or poorly made compost. Rather, the problem is that you may introduce some new pernicious weed species that will cause management problems for years to come.

You can test manure for weed seeds by mixing several quarts of manure taken from various parts of the pile with potting mix in a 1:1 ratio and spreading it in flats. Keep the flats warm during the day and cool but not cold at night. For example, run the test inside in the winter, outside in the summer and in a cold frame during the spring or fall. Water the flats regularly, and observe any weed seedlings that emerge over the following two to three weeks. This test will usually show if weed seeds are present, but it may not accurately predict their density since some seeds may be dormant.

To kill weed seeds during the composting process, the pile should reach 140? F for at least two weeks. Some of the more resistant species may not be killed even by this treatment. For a small compost pile achieving a high sustained temperature may require insulating the pile (e.g., with loose, straw over plastic). If the weather is warm and sunny, the heat generated by biological activity can be supplemented with solar energy by covering the pile with clear plastic. Since the outside of the pile is unlikely to attain the required temperature for a sustained period in any case, thoroughly mixing the pile several times will probably be necessary. For a very small pile (e.g., < 1 cubic yard) attaining a high enough temperature for a long enough time to kill most weed seeds may be impossible.

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