Control of Invasive Plants with Prescribed Fire
Proper timing is essential for effective use of prescribed fires. Therefore, the life histories of the various types of invasive plants, as well as those of the surrounding native plants in the community, must be considered when planning the burn. Long-term suppression for all types of invasive plants also requires reduction or depletion of the seed bank, so it is important to understand the timing of seed maturation, mechanisms of new seed recruitment, and the longevity of existing seeds in the soil. Understanding these aspects of invasive plant biology is important in developing prescribed burning plans as well as more integrated approaches to management.
To control annuals effectively, it is critical to either destroy the seeds with fire before they germinate or to kill the plant before the seeds mature. Prescribed burns have been used effectively on invasive grasses such as downy brome, barb goatgrass and the annual forb weed yellow starthistle.
Prescribed burning is generally not successful for control of biennials because the life cycle of biennial plants creates uneven-aged stands. Only those plants that have flowered in the second year of growth are susceptible to fire mortality; one-year-old plants in the basal rosette stage (ground-level grouping of leaves) are protected from grassland fire damage and can survive to produce seeds the second year. To obtain better results, initiate the burn later in the spring before the bolted plants have set and dispersed seed.
Since the strategy with biennials is to decrease the existing seed bank and prevent new seed set and off-site recruitment, multiple year burns can be used to eventually deplete the seed bank. More intense burns give better control of bolting plants, but control is still difficult when the stands are uneven-aged.
In the eastern United States, prescribed burning has been used to control garlic mustard effectively. However, in areas where the thatch and litter layer were damp, plants survived. Sequential burns under drier conditions were more effective in reducing garlic mustard cover and spread rate. Although burning has been shown to suppress sweet-clovers, garlic mustard and other biennial species, combining burns with timely herbicide treatment would probably be even more effective.
In most cases, successful control of perennial forbs involves integration of multiple control options, such as prescribed burning and herbicide application. Typically, controlled fires or wildfires promote invasive perennial forbs. For example, spotted knapweed seeds are dispersed soon after they mature and neither spring nor fall burns have been successful in the control of this species. While timing of burning may be critical to management of herbaceous perennial forbs, in some cases, regardless of the timing, control failed and invasive species generally increased in abundance.
Most woody species are difficult to control with prescribed burning. For example, Japanese honeysuckle, tree-of-heaven and Russian-olive actually benefit from fire because they readily re-sprout from the base following fire or mechanical damage. However, other shrub and tree species can be controlled using prescribed burns.
In the eastern and mid-Atlantic states, prescribed burning for invasive plants is conducted during the dormant season and is generally unsuccessful due to re-sprouting from rootstocks. It is theorized that growing season burns will be more effective since non-structural carbohydrate reserves are lowest at this time, and a prescribed burn could potentially deplete the energy reserves of the plant’s root system, making it more susceptible to subsequent burns or other control measures.
Another strategy is to take advantage of the stimulation of seed germination following a fire: the above ground growth of a plant can be cut in summer or fall and allowed to dry on-site. Once dried, the site can be burned; this initial fire is usually intense because of the dried material and will stimulate greater-than-normal seed germination in the next season. The cut and burn strategy is repeated, and the second burn provides control of the new large batch of young seedlings. However, it will likely require several more years of follow-up control efforts to assure near-elimination of invasive plants at some sites.