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, 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.
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.
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 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.
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.
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.
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.
When the Pile Is Not Heating Up Enough
*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:
Bury compost to prevent weed seeds from germinating. Source: thelegitimatenews.com
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.
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, 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.