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compost temperature to kill weed seeds

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.

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.

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.

Heating Soil

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.

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.

In general, the bigger the compost pile, the more heat it produces … and heat kills seeds, even weed seeds.

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 that such temperatures will also kill any weed roots and rhizomes placed in the compost. Two birds with one stone!

A Big, Hot Pile

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.

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.