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.
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.
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:
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 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.
Heating your soil above 200 degrees Fahrenheit can cause certain types of soil with a concentration of organic matter to release a foul odor. In some cases overheating your soil can also begin a chemical reaction in the soil that will release a chemical that is harmful to plants. Large compost piles in excess of 7 feet in height that are heated to 200 degrees Fahrenheit or higher may combust. The optimal temperature for creating compost and killing seeds is between 130 and 150 degrees Fahrenheit. You can monitor the internal temperature of your compost pile using a temperature probe. If your compost pile begins to overheat, cool it off using water, reduce the size of the pile or add a bulky material such as wood chips.
Compost piles that reach and maintain a temperature of at least 140 degrees Fahrenheit for at least two weeks will kill most seeds within the pile. It is important to turn your compost pile so that it composts evenly since the hottest part of the pile is at the center. Seeds left on the outside or bottom of the pile may not reach a high enough temperature to thoroughly kill all of the seeds inside.
Compost, potting soil and other garden amendments can harbor seeds from unwanted grasses and broadleaf weeds. You can prevent unwanted seeds from competing with your lawn and garden plants by using a heat treatment of the right duration and intensity. The exact temperature needed to kill a seed depends on the species of plant it came from, but in most cases heating seeds to 140 degrees Fahrenheit is sufficient to sterilize them.
You can quickly kill seeds and harmful micro-organisms in soil using your kitchen oven. Heating your soil to temperatures between 180 and 200 degrees Fahrenheit will kill any seed in the soil after 30 minutes. The best way to heat treat your soil is to place it in a pan in the center of the oven. Make sure that the soil is about 4 inches deep so that it heats evenly. You can monitor the temperature of your soil using a standard oven thermometer.
Seeds begin to die at temperatures above 108 degrees Fahrenheit, but require longer periods of exposure at lower temperatures. At temperatures below 140 degrees Fahrenheit some species are not affected by heat treatments. The most effective way to ensure that you kill all of the seeds is to heat them to a temperature above 140 degrees Fahrenheit. Seeds mixed into compost or topsoil require higher temperatures and longer exposure times than bare, unprotected seeds.
Daniel Thompson began writing about analytical literature in 2004. He has written informative guides for a hardware store and was published at an academic conference as part of a collaborative project. He attained a Bachelors of Fine Arts in English literature from Eastern Kentucky University.