Composting Weed Seeds

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Organic weed management techniques described in detail. Weeds can be safely added to a compost pile if you make sure temperatures are high enough to kill the seeds and roots.

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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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.

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.

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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.
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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.

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