Composting Weed Seeds


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Organic weed management techniques described in detail. 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. Want To Keep Your Compost Weed-Free? Compost can be a gardener’s best friend. It can deliver dramatic improvements in soil quality and nutrients to nourish your plants throughout the growing

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.

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.

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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
  1. Put the roots of your perennial weeds into a bucket.
  2. 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.
  3. Cover the bucket to keep out light, prevent evaporation and to stop rain making the bucket overflow.
  4. Leave for approximately 1 month. That should be long enough to drown even perennial weeds.
  5. The strained liquid makes a great liquid fertilizer. Dilute 5 parts water to 1 part weed liquid.
  6. The drowned roots can now be added to your compost pile and composted as normal.

3. Desiccating

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:

  1. 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.
  2. Layer your weeds with the bran. Add approximately 1 inch of weeds and then sprinkle with 1-2 heaping tablespoons of bokashi bran
  3. Repeat until your bin is full. Compact the weeds down really well; you’ll be amazed how much you can pack into one bin.
  4. 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!
  5. After fermentation is complete, you can add the bokashi bucket contents to your compost pile or bury them straight in your garden.
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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.

Want To Keep Your Compost Weed-Free?

Compost can be a gardener’s best friend. It can deliver dramatic improvements in soil quality and nutrients to nourish your plants throughout the growing season. But unless you use the right techniques, you may also find yourself with a bumper crop of weeds.

“Composting is a biological process that decomposes leaves, lawn clippings and other organic materials until they look like rich soil,” said Dr. Gary Wade, extension horticulturist with the University of Georgia. “But many of the rich ingredients in the mix can also be a source of weed seeds.”

When lawns are mowed, for example, seeds can be collected with the clippings. Seeds can also survive in leaf debris or on mature weeds that you pull from your garden.

Pay attention to time and temperature

If you want to keep weed seeds from sprouting and flourishing in your flower beds or garden, you’ll need to ensure your compost gets really warm. With the proper temperature, the volume is reduced, odors dissipate and those pesky weed seeds die.

Within a week, temperatures in a properly constructed compost pile will reach 130 degrees Fahrenheit. That quickly kills many seeds and stabilizes the composted material. But it takes 30 days of exposure to temperatures of 145 degrees or more to kill seeds from tougher weed species. To achieve that temperature, you’ll need the right mix of materials and moisture. (See composting tips below.)

Among the weed seeds that need high temperatures to decompose are common groundsel (Senecio vulgaris), bird’s-eye speedwell (Veronica persica), round-leaved mallow (Malva pusilla), common lambsquarters (Chenopodium album), spiny sowthistle (Sonchus asper), ladysthumb (Polygonum persicaria), wild buckwheat (Polygonum convolvulus), field bindweed (Convolvulus arvensis) and broadleaf dock (Rumex obtusifolius).

How do you know whether you’ve achieved the right temperature? Though compost thermometers are ideal, you can also simply reach into the pile. If it is uncomfortably hot to the touch, you’ve probably achieved the temperatures you need.

Turn your compost pile

For maximum seed destruction, it’s also important to turn your compost. That’s because the exposed surface and localized internal cool spots give weed seeds an opportunity to survive. By turning and mixing, you can minimize the problem.

Turning also can help you determine when your compost is ready to use. If the pile fails to reheat after turning, it most likely has reached a biologically stable state. It should have a fine texture and little or no odor.

Remember, though, that effective weed control doesn’t end with proper compost preparation.

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“The same nutrients that benefit bedding plants and crops will also benefit any existing weeds and give them a real boost,” said Lee Van Wychen, science policy director for the Weed Science Society of America. “So it’s important to make ongoing weed management a priority on composted beds and gardens.”


Ten Composting Tips for Home Gardeners

Here is a ten-step formula for building an effective compost pile – based on tips from the U.S. Department of Agriculture and the University of Georgia’s College of Agricultural and Environmental Sciences Cooperative Extension Service:

  1. Select the right site. You’ll need a level location with good drainage. While a compost bin or wire mesh structure can be helpful, it isn’t a requirement. You can start your pile directly on the ground.
  2. Build from the bottom up. Use a coarse layer of woody material as your base. You’ll have better air circulation and will create a chimney effect that will help to heat your compost pile.
  3. Add organic waste. You’ll need an eight-inch to 10-inch layer of leaves, grass, plant trimmings or similar material. One caution: Don’t use grass clippings if your lawn has been recently treated with herbicides; instead, leave the clippings on the lawn for at least three mowings. Also, check the herbicide label to make certain there is no restriction on using treated lawn clippings in compost.
  4. Add a microbial source. Top the organic layer with a one-inch layer of soil or completed compost. Doing so delivers the microbes needed for decomposition and reduces the leaching of mineral nutrients during the composting process. Note: If layers aren’t your thing, you can simply mix each of your composting ingredients into a single heap.
  5. Add nitrogen. You can use a nitrogen fertilizer such as ammonium sulfate (one-third cup for every 25 square feet of surface area), green grass clippings or even lake plants. Manure is also a good source of nitrogen. But if you decide to use it in your compost, contact your county extension service for advice on safe handling practices.
  6. Top it off. Repeat the layering process as needed, and then top your pile with five to eight inches of straw or hay.
  7. Water each layer. You want your compost pile to be moist, but not soggy. Also, scoop out a section in the center to collect rainwater, which serves as an ongoing moisture source.
  8. Give it air. Punch holes in the sides of your compost pile for aeration.
  9. Turn it over. Periodically flip your compost to move material from the center to the outside and material on the outside to the center. That will keep the temperatures even throughout.
  10. Pull them out. If despite your best efforts weed seeds find a cool spot and sprout, make certain you pull them before they go to seed.

Adding compost to your planting bed can dramatically improve soil quality and deliver nutrients that will nourish your plants throughout the growing season. Photo courtesy of Dr. Gary Wade, University of Georgia.

If you choose, you can use a compost bin or wire mesh to keep your compost contained. Photo courtesy of Dr. Gary Wade, University of Georgia.

Add a layer of finished compost two to three inches thick on your planting bed and incorporate it to a 12-inch depth. Photo courtesy of Dr. Gary Wade, University of Georgia.

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