Burning & Mowing

How do you know whether to burn or mow?

The following will help you decide without wading through the unsubstantiated dogma. 

Fire’s sole purpose is to remove thatch — the accumulation of dead plant material. Thatch is the build up of dead vegetation which is a good thing until there’s too much of it. Thatch feeds the soil microbes and holds in moisture. There are indirect effects of fire, such as opening the land to sun and warming the soil, but since we have no pyrophytic plants in Wisconsin, any effects on plant germination and growth are indirect.

Fire can be used to topkill unwanted woodies but topkilling doesn’t kill that woody and most likely will increase its growth as most of our unwanted woodies are resprouters.

If the ecosystem has lots of grass, thatch builds up faster than if the ecosystem has forbs as its major component. Tallgrass-dominated ecosystems set themselves up for a feedback loop that reduces biodiversity because it increases the grasses with frequent fires. In these habitats dominated by the tallgrasses (e.g. Indian grass, Big blue, Switchgrass), the thatch builds up faster, which means burning is required more often. The more frequently these are burned, the more biodiversity is reduced because it encourages grass growth over forb growth. We label this as “fire favors grass.” This is an accepted fact although few landowners are warned about it. 

Forbs produce litter that decomposes easily. Mowing chops these senesced plant parts allowing them to decompose faster. The thatch and little accumulation document explains this.

Whether to burn or mow can be dependent on the land’s topography. Very steep slopes may not offer the possibility of mowing. Low-lying wet mesic areas that have some shade may need mowing each fall to keep the dead vegetation from matting down or they may need fall burns; spring burns in these areas are often difficult as they don’t become dry enough to burn before the herptiles are out of hibernation.

To the right are 3 photos of topkilled plants. They all look like they were killed and are dead, but looks are deceiving. If you make the mistake of assuming these are dead and don’t do the follow up, you’ll find these unwanted, resprouting woodies have bulked up their root systems and will be ready to put on a show you don’t want to see next year!

Topkill example
Topkill example
Topkill example

How often should you burn?

It depends. Nature is not predictable. If it’s a drought year, that would delay the need to burn. The only way to know is look at the land and determine for yourself. If you can see quite a bit of soil, you do not need to burn. There is a research paper that has quantified when the thatch is sufficiently built up to require burning.” [See the above “Thatch and Litter Accumulation button.] Choosing to do your own experiment based on this, is enlightening. Once you’ve done your own measurements, it makes “eye-balling” it easier.

Dogma provides us with a formula of burning every 3 years yet nature doesn’t operate on a calendar basis. We should not manage that way either. When to burn is dependent upon the vegetation in the ecosystem and the climate.

Frequent fire decreases biodiversity as it increases the tallgrasses.

prescribed burns

Refugia is essential when burning.

Why?

Refugia provides for an area where insects are not killed and wildlife can seek safety. This area is where the insects repopulate the burned areas from.

Burns kill all insects in the burned area who are not sufficiently deep in the soil.

No refugia means no insect repopulation. Perhaps your neighbor has like habitat that they haven’t burned and its close enough the insects can repopulation from there, but….why would you defer your responsibility to someone else?

This needs to be planned areas left unburned. Patchy burns are unreliable in providing sufficient refugia. With our insect populations in dire straits, we cannot leave their continued existence to chance. The only guarantee of sufficient refugia is to plan for it. 

Plan your refugia. The wildlife depend on us to protect them. 

What’s the best season for a burn?

Dogma says that burning in the fall will increase your forbs yet this has not been substantiated (as is the case with dogma).  Take what you do know and think about it. Here’s an example of how to do that.

  1. If you burn in the dormant end of the season, how does that differ from burning in the dormant beginning of the growing season?
  2. If you burn while the ecosystem is still green and growing in the fall, you will reduce your late blooming plants (e.g. asters, goldenrods) and many other herbaceous plants that have not had time to finish sending nutrients to their roots. Seeds may be harmed, too; expect that it will destroy some.

 

 

Here are some ways to think about burn timing:
  1. Are you wanting to overseed? If so, burning in the fall makes that easier. But, one can sow seeds in the fall without burning, then burn in the spring. The snow and rain over the dormant season will work those seeds into the ground.
  2. Are you wanting to leave sufficient habitat for wildlife? Burning in the spring avoids this. But should a fall burn be decided (for time and resource purposes) plan for plenty of refugia, not only for the insects but the other wildlife.
  3. When will you have the help or money to pay someone? Much of our management work is determined by our human time lines and resource limitations.
  4. Timing may be completely dependent on the ecosystem. If you’re managing a wetland, it may be that spring is seldom dry enough for it to carry a fire. Or if you have a north-facing slope, it may not be dry enough for a burn until long after the herptiles have come out of hibernation. 

The first rule of ecology is that eveything is connected to everything else, and so when one component changes, everything else changes in response.

Bill Wolf

Why would you want to mow?

Mowing reduces the need for burns. It prolongs the time in between.

Mowing certain areas (e.g. wet mesic) in fall reduces the matting of vegetation with winter snows.

Mowing allows for light to reach the seeds and warms the ground.

Mowing provides organic nutrients for the soil. Chopping up the senesced plants allows them to decay into the soil, feeding the microbes.

Mowing is how to maintain your biodiversity as frequent mowing doesn’t reduce biodiversity and it prolongs the needs for fire.

Mowing is gentle on the wildlife. Most wildlife can move fast enough to avoid mower blades. Immobile insects may be hit by the blades but that is remote.

Mowing can be used as a triage move to keep unwanted herbaceous plants from setting seeds.

Mowing when herbaceous invasives are in bloom reduces the nutrients they send to their root storage. Overseeding into these mowed areas increases competition once these plants have their vigor reduced. 

Mowing is such a great tool. Why isn’t it promoted as avidly as burning? This is a question I cannot answer this but I have some theories.  The benefits are different from fire and its a valuable tool.

When mowing our prairies, we use a Kubota tractor. A riding lawn mower would not have the power we need. 

Kubota tractor

As you work with your land and diversify your techniques, you’ll get a feel for how often you need to burn or mow. Both are resource heavy. Few mistakes can be made with mowing but burning carries more risks; risks of collateral damage and unwanted/unexpected results. 

Burning also requires labor- and time-intensive follow up with newly planted or newly restored ecosystems. This follow up, if done faithfully, diminishes over time.

Follow up is essential after disturbance, whether it is mowing or burning, because neither will kill the unwanted woodies. [See Biology of Woody Resprouting at the top of this post.] After a burn is the best time to cut and treat resprouting woodies that fire does not kill. They are easy to see and it’s easy to walk through the ecosystem.

Wisconsin has no pyrophytic plants, meaning none need fire in order to germinate. But one of the indirect effects of burning is that it opens the ground to light and the warmth of the sun. This means ALL seeds can germinate. Expect the herbaceous non-natives/invasives to be very happy after a burn. They often are the ones that come up first, too, so it’s easy to also zap those.

Tip when mowing: Carry a spritz bottle and use it when you mow over a perennial invasive.

Other Considerations when Burning

If burning is your option, it requires thorough planning. These are some elements to consider. Burn plans will specify each of these but it’s good to know what to expect and what to think about.

  • Who will write your burn plan? If you’re burning because of a government contract, you’ll need an NRCS-certified person to write your plans.
  • Who will mow the fire breaks? How wide should they be?
  • How many people will be required to do the burn? If you’re working with volunteers, do you have the equipment (e.g. backcans, drip torch, flapper, etc)?