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Prescribed Burn FAQs

Burn Frequency and Time of Year

Although there are no hard and fast rules about how often to burn, there are a few generally accepted practices. Newly created prairies can be burned every year initially, then every two to three years after the vegetation is well established. Healthy remnant or intact prairies will benefit from burns every three to five years. Savannas need to be burned less frequently, every 10-15 years. The encroachment of woody or non-prairie vegetation is the determining factor.

For practical and ecological reasons, most prescribed burns are carried out in early spring. In southwestern Ontario, that is mid-March to late April and in more northerly areas, lateApril to mid-May. Since most prairie plants are warm-season perennials, they remain dormant at this time while the cool-season competing weeds are far enough advanced to get hit hard by the fire.

Some tallgrass managers vary the timing of the burn to benefit specific species. For example, summer burns decrease prairie grasses and shrub invasion and favour wildflowers. Consult an expert if you would like help fine-tuning the timing of the burn to manage certain plants.

How much to burn?

Because of the subtle patchiness of the prairie landscape, fires do not burn evenly or completely. This is a good thing as unburned areas become refuges for species which, in turn, help re-populate the burnt areas. This is especially important for insects. For this reason, some managers burn only half of the site each time on a rotating basis.

Alternatives to burning

Where burning is not an option, mowing can be a partial substitute for fire. Mowing should take place late in the fall after the prairie plants have set seed and the birds have finished nesting. This can be done with the same frequency as burning. It is best to mow only half the site on a rotating basis to leave winter cover for wildlife. The clippings (thatch) should be removed so they don't blanket the ground and prevent sunlight from penetrating. As well, the clippings can be burned in a container and the ash returned to the prairie.

How do grassland plants survive a fire?

The growing points (meristems) of many prairie grasses and flowers are below ground and this feature protects them from both drought and fire. The previous year's stems and leaves provide fuel for the fire, while the growing point remains cool underground. During a fast-moving grass fire, the soil surface can reach 680oC, while only 1 cm below, no temperature changes are detected.

Unlike prairies and savannas, meadows are not fire-dependent and do not benefit from burns. Meadows of Queen Anne's lace, giant ragweed, Canada thistle and common milkweed grow where the soil has recently been disturbed or tilled.

How does fire help grasslands?

A fast-moving grass fire helps the prairie in many ways. Firstly, fire kills plants that are not specifically adapted to tolerate burning. These plants have their meristems above ground. Shrub and tree seedlings and other non-prairie weeds are kept out by regular burns.

The burning and elimination of the standing dead plant material is another important factor. Once it is gone, sunlight and wind can warm and dry the soil surface more readily. The black ash absorbs the sun's energy during the day and insulates the soil against heat loss at night. This warmed soil speeds up the development of underground shoots. The new above-ground shoots receive full sunlight, providing them the energy for photosynthesis.

Moisture may be more available to prairie plants after a fire. Dead standing material intercepts and absorbs a great deal of moisture. Most of that moisture will evaporate once the sun comes out. By burning off the material, water from a slow, gentle rainfall is more likely to be delivered to the ground where the shoots need it.

Finally, the burning of plant material releases nitrogen into the atmosphere. Since prairie species are adapted to low nitrogen levels, this gives them an advantage over weedy species that require more nitrogen.

How do wildlife species survive a fire?

Most animals have adaptive behaviours that help them escape from fire. Mammals, for example, can easily out-run small ground fires or retreat to burrows or previously burned areas. Reptiles and amphibians may remain in the soil, retreat beneath logs and damp leaves, enter burrows, or escape to water. Adult birds can fly away but fires may destroy nests, eggs and fledglings. Insects can be the hardest hit from grassland fires, especially those in the larval stage.

However, most tallgrass prescribed burns in southern Ontario are carried out in very early spring, a time when most reptiles and amphibians have not yet emerged from hibernation and few birds have begun to nest. Overall, most animals benefit from the new growth that follows a fire and the open type of habitat it maintains. Some animals such as quail, turkey, coyote and birds of prey will move to recently burned areas looking for food. 

See Prescribed Burn Worker page for information about volunteering at prescribed burns.