Composting Weed Seeds

Composting occurs when organic materials—such as yard trimmings, food wastes, and animal manure— decay to form compost, that can be used to improve soil. Organic weed management techniques described in detail. 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 to Kill Weed Seeds

Composting occurs when organic materials—such as yard trimmings, food wastes, and animal manures— decay to form compost, an earthy material that can be used to improve garden soil. Compost benefits gardens by:

  • Supplying many nutrients that plants need
  • Improving the soil’s physical characteristics, such as texture
  • Enabling the soil to better hold water and nutrients
  • Helping aerate the soil

The composting process also naturally kills weed seeds. Properly managed, a compost pile should easily reach 140°F, which breaks down all organic matter, including weed seeds.

The key word is properly . Organic matter that is improperly composted can introduce problems into a garden. Raw animal manure often contains disease-causing organisms such as E. coli and Salmonella, which can make people sick if they eat vegetables contaminated with them.

Manure can also contain live weed seeds. These seeds can spread easily from one farm, field, or garden to another, multiplying the problem from one weed to thousands of new weeds.

Figure 1. Build your compost pile with alternating layers of green matter (grass clippings), and brown matter (dead leaves), to maintain a proper carbon-to-nitrogen ratio.

How does composting reduce weed seeds?

Proper composting occurs under the following conditions:

  • The ratio of carbon to nitrogen (C:N) ranges from 25:1 and 40:1. This ratio balances both energy (carbon) and nutrients (nitrogen).
  • The compost is about 40 to 60 percent moisture by weight.
  • The oxygen content is 5 percent or more.
  • The pH level ranges from 6 to 8.

In these conditions, microorganisms begin breaking down the organic residues and releasing heat. A clear sign that the compost is decaying properly pile is the release of steam when the surface of the pile is disturbed (Fig. 2). As the temperature rises above 113°F, heat-loving microorganisms replace the earlier microorganisms. At that stage, the pile will enter the active phase, with temperatures reaching 131 to 170°F in 1 to 3 days.

Figure 2. The release of steam from a compost pile when its surface is disturbed indicates that the compost is decaying properly.

These high temperatures are the key to killing weed seeds in a compost pile. In general, more seeds will die the longer that the temperature in the pile remains within this range (Table 1).

How to compost properly

Most gardeners have a static compost pile. They believe that composting consists of filling the pile, waiting a few weeks, and then magic happens—the compost is ready. In reality, most compost piles are merely trash heaps of garden and kitchen waste.

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To compost properly, keep the C:N ratio at 25:1 to 40:1 and the moisture, oxygen, and pH in the pile at optimum levels.

C:N ratio: To maintain the correct C:N ratio, build the pile with alternating layers of brown matter such as dead tree leaves, and green matter such as grass clippings. Adding equal amounts of green matter (grass clippings, kitchen waste) and dry matter (dry leaves) will often achieve this desired ratio.

Moisture: Water the compost pile regularly to keep the microorganisms alive and to soak the weed seeds fully. Don’t add so much water that it flows out from the bottom of the pile.

pH: pH meters are available in garden centers and can be used to estimate the pH level of the compost pile. However, an easy and more practical way to tell whether the compost pile is “cooking” properly is by its smell. If the compost pile smells sour or like a rotten egg, the pH is not correct. A compost pile at the proper pH should smell earthy, like freshly dug garden soil.

If the pile smells bad, check to see if it is too wet. You may be adding too much water or wetting too often. Let the pile dry for a while, and wet it less often. Another option is to turn the pile and mix it thoroughly.

If the first two measures do not help, mix lime into the pile to correct the low pH level and reduce the rotten egg smell.

Turning: Periodically mix the materials within the pile to introduce more oxygen and distribute the moisture evenly (Fig. 3). To add as much air into the pile as possible, break up any clumps, and move the drier material from the outer edges into the center.

Figure 3. Watering and turning the compost ingredients regularly will help keep microorganisms alive, aerate the pile, and distribute the moisture evenly.

Turning the compost will also enable the temperatures at the edges and surface of the pile to rise high enough to kill weed seeds. The pile must be mixed thoroughly during the active phase to ensure that all the material is heated for a long enough period to kill the seeds.

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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.

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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.

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,

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.

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*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:

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.