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killing noxious weed seeds with moist heat

Of all these factors, heat generated during the active phase of the composting process appears to be the most important in pathogen destruction.

Composting is the controlled management of the normal biological process of aerobic (in the presence of oxygen) decomposition of organic residues by microorganisms such as bacteria, fungi, and actinomycetes. This process is optimized when the various organic residues are mixed to provide certain conditions:

A key feature of organic farming systems is the utilization of organic residues as soil mulches and amendments in an integrated system to maintain and improve soil quality. Organic residues used for these purposes may be produced on the farm, or they may be imported from off-farm sources. Often, fresh organic residues produced in place are used in these practices, such as when cover crops are plowed down as a green manure to build soil organic matter and improve soil fertility, or rolled as a mulch in organic no-till systems to suppress weeds, reduce soil erosion, and conserve soil moisture. Organic residues may also be processed before being used to attain desirable qualities, such as when animal manures are composted to reduce volume and improve stability. Regardless of the circumstances, organic residues that are handled incorrectly can introduce otherwise avoidable problems to the farming system. For example, raw cattle manure may contain viable weed seeds and may spread an otherwise isolated weed infestation more broadly across the farm or, if the manure is imported from outside the farm, introduce a weed problem that previously didn't exist. Similarly, plant residues may be infected with pathogens that can infest subsequent crops. This article provides a brief description of the composting process, discusses the use of composting to reduce weed seeds and plant pathogens, and identifies issues that can lead to the failure of composting to reduce weed seeds and plant pathogens.

How does composting reduce plant pathogens?

Finished compost can become recontaminated with weed seeds if weeds are allowed to grow and go to seed on or adjacent to the pile or windrow. Similarly, compost can become contaminated with vegetative reproductive structures from some weeds—Canada thistle and rhizomateous grasses, for example—if they are allowed to grow on or adjacent to the pile. Keep vegetation adjacent to stored compost mowed short, and tarp piles or windrows to prevent contamination by wind-blown weed seeds. When moving or spreading finished compost, avoid picking up soil or other contaminants from under or around the pile or windrow.

Temperatures at the edges and surface of compost piles and windrows may not be sufficient to kill weed seeds and pathogens. This is an especially important risk in static piles that are not turned and mixed during the active phase of decomposition, but rely on forced aeration to maintain an aerobic environment. Thorough mixing or turning during the active phase is essential to ensure that all the material achieves elevated temperatures for a long enough period of time to kill weed seeds and pathogens.

Under these conditions, populations of microorganisms will thrive and organic residues will be decomposed, consuming oxygen and releasing intermediate breakdown products, carbon dioxide, and heat. As the temperature of the pile rises, the community of microorganisms will go through a succession, culminating in thermophilic (heat-loving) organisms at temperatures above 113 °F (45 °C). If the mass of the compost pile is large enough to be self-insulating, temperatures within the pile during this active phase of composting may reach 131–170 °F (

Bollen et al. (1998) found that only two of 17 plant pathogens investigated—Olpidium brassicae and one form of Fusarium oxysporum—survived when exposed to small-scale static pile composting of infected plant residues, and then only at greatly reduced levels. Thermal mortality during the active phase of composting was found to be the most important factor affecting pathogen destruction.

Ever noticed many weeds collect at the edges of your yard or garden? Keep your grass and garden edges trimmed to cut down on invasions of weeds into your fertile garden soil. The places to watch are the not only the edges of your lawn but also around posts and fence lines as well as close to planting beds. Another idea is to grow perennials or ground roses that will shade those edges and make it easier for you!

This works best, of course, when you are starting a new garden bed or a new garden space. Watch Janice Stillman, editor of The Old Farmer’s Almanac lay down a few layers of newspaper, wet it, adds mulch on the edges of the newspaper, and dumps compost on top of the newspaper bed! You’ll love having almost no weeds to contend with and as a bonus it helps build the soil. It couldn’t get any easier than this.

If dealing with weeds is too much of a hassle, at least resolve to keep them from setting seed. Once a week, use a grass whip or string trimmer and cut off their heads before they flower.

11. Avoid Watering Weeds

When weeds have really sprung into action, nothing beats a good old-fashioned garden hoe with a long handle. Hoeing is best done in the morning when the soil is dry. The weeds will cut cleanly from the soil and this creates a “dust mulch,” which inhibits the germination of new weeds. You can let the weeds simply dry in the sun during the day and then take to the compost heap.

If your soil is rich and drains well, plant your plants closer together. This will cut down weed growth. Start your warm weather plants as soon as you can to keep the soil from being bare for too long. At the end of the season, plant cover crops such as rye grass, winter wheat, or oats to prevent weeds from finding a home in your garden.

Image: Straw used as mulch to suppress weeds, hold in moisture, and break down into soil. Credit: Jurga Jot/Shutterstock

While weeding, try to hold the trowel vertically (like a child holding a crayon) to eliminate strain on your wrist.