The effect of fire on weed seeds

By Barry tickes, UA Area Agriculture Agent

Vegetation management with fire is a very old technique used in agriculture. It dates back to prehistoric times but is still used today. Fire is a part of nature and is necessary to maintain many grassland and forest ecosystems.

Fire is used in this region primarily as a post-harvest technique to remove dead or dormant plant material. It is most commonly used in the summer after grain is harvested to facilitate ground preparation for the next crop or in the spring prior to the regrowth of bermudagrass to remove stubble and stimulate regrowth.

One of the benefits of burning is it can be effective in killing weed seeds. The effect of fire on weed seeds has been the subject of many studies and is dependent on many variables. These include the weed type and species, the depth of the seed in the soil, the heat of the fire, and the duration of the fire.

The type and species of weeds found most affected by burning are summer annuals. Weeds that produce seeds with hard seed coats are the least affected. These include sweet clovers, mallow, dodder, sesbania, and others.

Fire has little effect on creeping perennials especially during the dormancy period. These include bermudagrass, field bindweed, perennial johnsongrass and nutsedge; all of which have below ground reproductive structures.

In weeds that have become established, seedling grasses are more sensitive to fire than seedling broadleaves.

This is primarily because the growing points of many grasses are below the soil while the growing points of broadleaves are above the soil and unprotected. Seeds that are burned before they become viable are also more sensitive to fire.

The depth of weed seed in the soil also will affect sensitivity to fire. In one study, 88 percent of the seed located in the top 1 millimeter (mm) of soil was killed while only 18 percent of the seed buried 5 mm (0.19”) into the soil was killed. Tillage following burning can also bring up viable seed.

The effect of temperature on seed viability has also been the subject of many studies. In most, the germination of summer annuals increased with a rising temperature to about 140 degrees F. and dropped off rapidly as temperatures approached 212 degrees F. Imbibed seeds were also more sensitive to fire than un-imbibed seeds.

In addition to temperature, the duration of the fire has been found to affect seed mortality. Hot, slow fires kill many more weed seeds to greater depths than light flash fires that raise the temperature only in the top layer of soil.

At one experimental site, wheat stubble with longer straw burned longer and hotter than short stubble.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu