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.
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.
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.
How Weeds Survive
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.
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.
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.
For hot composting to fully kill all weed seeds and roots, follow these tips:
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.
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.
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.
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
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.
*Note that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!
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.
Heat-resistant weed seeds requiring treatment at 45° F (63 ° C) include:
When the Pile Is Not Heating Up Enough