"When's The Best Time To Burn?"

Updated: Nov 1

A few years ago, I had the pleasure of attending a talk from Drake University professor and consummate prairie enthusiast Tom Rosburg on his research regarding fire ecology on Iowa’s prairies. He had a dizzying arsenal of charts and graphs comparing individual species’ responses to burn frequency (annual, every other year, every three years, etc.) and burn timing (spring, late spring, early fall, late fall). At the end of the presentation he opened up the floor to questions.

Some of Dr. Rosburg's research on the variable effects of fire

“So when is the best time to burn?” asked one attendee. “When it’s necessary to meet your goals,” replied Dr. Rosburg. It wasn’t snark, per se, but I can see how someone who has made an entire career out of studying the intricacies of ecological interactions could find it difficult to offer a more satisfying answer to a broad question. He had just gone through an extensive presentation about how every choice in terms of burn timing will have pros and cons, with the implication that prairie stewards will have to pick their own winners and losers based on what they want to achieve. Ultimately, that’s the crux of habitat management. There is no one correct way to do it. It is inherently a creative process, and like any creative endeavor, requires establishing goals and studying the tools we may use – or not use - to achieve them. It requires continuously updating one’s knowledge to stay current with best practices. Environments change, paradigms shift, and no one site is exactly like another. We have a responsibility to keep up and think critically if we hope to get where we want to go.

Dogma, then, can end up working against conservation goals. The application of prescribed fire presents just one such example. Drive across the countryside in early March, and plumes of smoke dot the horizon with dozens of burns. Then, after a few weeks, the plumes disappear until next year like the blooms of Hepatica in a spring woodland. However, prescribed burns seek to mimic the historical role of fire in Iowa’s ecosystems. Historically, those fires would have occurred at many different times of year. So why does so much of our mimicry remain restricted to one short span of the calendar?

Spring burn in a county roadside

To be sure, maintaining grasslands with fire is generally better than not using it. But too often, I hear from folks maintaining a grassland who burn based on little more than the fact that they know prairies need fire. I’ve even heard from conservation board employees (not here) who have said they burn every three years because “that’s how often we do it.” But plants and animals don’t use calendars. Graziers don’t move cattle into or out of pastures at an arbitrarily designated interval, they adapt to the conditions of the animal and the vegetation. Gardeners look at the weather forecast before busting out the trowel. They do this because they have specific goals and objectives in their craft. Without a goal in mind, and an understanding of how to achieve it, burning every March “just because” might be a waste of time and resources, or even worse, actively detrimental. Most grassland managers want diversity. Diversity can mean many things in land stewardship – bird diversity, floral and pollinator diversity, or structural diversity to name a few.

Henslow's Sparrow, an endangered grassland bird that depends on a well-developed layer of thatch.

There are participants in an ecosystem – the flora and fauna – and processes, like weather, fire, and animal (including human) impacts. Diversifying the processes can greatly help to diversify the participants in the long term. All of these participants are tugged one way or the other in the ecological web by those processes. If we apply the same process, at the same time, over and over, we end up pulling the web in one specific direction.

Burning outside of the short window in early spring, at least occasionally, can help spread the tension out a little more evenly across the web. Many cool-season grasses, for instance, are actually favored by a fall or early spring burn. The open canopy and increased soil temperatures of blackened earth in the early spring following a burn gives a competitive advantage to those grasses who thrive in those conditions.

Perhaps not coincidentally, cool-season grasses like smooth brome and Kentucky bluegrass present one of the most persistent challenges to prairie managers. Controlling them with fire requires understanding their biology. Shifting a burn to later in the spring, when the bluegrass is flowering (and thus having expended much of its root reserves) will get managers a lot closer to their goals of controlling that target. Smooth brome, if it’s the dominant problem, will require a slightly later burn as its flowering season comes a few weeks after bluegrass.

Smooth brome

Kentucky bluegrass

In many contexts, fire is not the appropriate control at all, and grazing, mechanical removal, or targeted herbicide applications would much more efficiently tug the web the right way. Bird's-foot trefoil, a sprawling non-native legume that can quickly take over a prairie planting, loves a fire. Prairie managers seeking to control an infestation could end up making the problem worse with frequent burns in the absence of other treatments.

Bird's-foot trefoil

Even prairies without cool-season plant problems often tend to shift towards the warm-season grasses like big bluestem and indiangrass, and away from wildflower diversity, with repeated spring burns. A fall burn, when these plants are flowering or holding vulnerable seeds on the end of their stems, can stymie their proliferation and tug the web back towards the midsummer wildflowers who give the prairie its splendor and the bees their nectar.

Prairie at Osborne, dominated by warm-season grasses

Doolittle prairie, a "reference" prairie. Note the difference in diversity and structure.

Looking outside the dormant season, we find even more opportunities to shift the balance based on our goals. A summer burn – yes, it’s possible – might do wonders to control warm season grasses. Most curiously, some evidence suggests that reed canary grass – an exceptionally difficult invasive grass – really hates a midwinter burn, sometime between Christmas and Valentine’s Day. Of course, it would have to land in a snow-free stretch, which can make it difficult to collect more data points on the practice, but anecdotal reports from creative land managers offer a compelling case.

Fire in a stand of reed canary grass at Motor Mill

Often, the application of fire seeks mainly to control trees and brush, but here too, we have to look a little closer at the problem children and potential civilian casualties before deciding when or if to burn. An early spring burn, before a deciduous tree has broken dormancy, won’t do much to actually kill it, though with enough intensity it might at least set the growth and seed production back. An early fall burn, when the plant has not yet completed the job of moving carbohydrates from the stem to the roots, might work much better. Not only is the tree "drier" (not moving moisture up from the roots) and more vulnerable, but the roots won't be protected by leaf litter, allowing cold temps to further weaken it.

Cedar trees, notorious native-but-undesirable invaders where floral diversity is the goal, have a pretty low fire tolerance in spring or fall, but seem much hardier just a few weeks into spring when the plant has begun to hold more moisture. Conversely, dogwood, a native woody shrub that likewise invades grassland at the expense of the grasses and flowers, seems to resprout more vigorously after a dormant-season burn but takes a bigger blow the later in the season the fire occurs - though the effect is often short-lived.

Cedars ignite in a dormant-season burn

The list goes on and on. A literature search will reveal that virtually every plant, native or not, in the Iowa ecosystem has a different response to fire, and a different response to when (timing) and how often (frequency) that fire occurs. The variations expand exponentially when we introduce other management tools like grazing or herbicides into the mix, or even variations within the fire itself like intensity (how hot did the fire get, was the fuel extremely dry, etc.), duration (did the fire rip through in 30 seconds or linger in low wind speeds), and even the weather shortly after the burn.

Human infrastructure further complicates the issue. Managers often focus on spring burns because that’s simply when they’ve got the people and equipment to do it. The grain haulers and combines on my commute suggest that, perhaps, fall burns are less common in Iowa because people have other more pressing matters at hand.

But like a lot of ecological restoration, this dizzying complexity gains clarity when we take the time to settle on a clear, measurable objective. Figure out what the problem is, and your research becomes much more manageable. If you notice a certain plant taking over your prairie, do not simply assume that it means a fire will set things back to