Thanks to the recent hot weather (already surpassing 90 degrees several days in a row), we are starting to see the lush green hills near Fresno transition into gold. The much-needed rain we received this spring could unfortunately create a problem in the coming summer: thick forage growth can quickly turn into fuel for fire season.

So, what does this have to do with weed management?

Before wildfire, weeds can be fuel;after wildfire, weeds can spread.

FUEL

The main fuels in the valley and foothills are typically grasses. Even non-weedy grass species can be an issue if grazing livestock (and hardworking landscapers) can't keep up with grass growth before summer. Annual grasses are great wildfire fuel because they provide a relatively uniform, continuous fuel supply over the landscape, and they can carry low-temperature fire. In contrast, perennial grasses that grow in bunches are less effective at spreading fire because there is nonflammable space between bunches, so flames need to be longer and hotter to continue spreading.

As you move toward the foothills and forest, fire fuels include shrubs and trees. Like bunchgrasses, shrubs can catch fire, but shrubs must be fairly close together to carry a fire across the landscape, or there needs to be other material (such as dry grass) to carry flame between shrubs. Woody plants tend to burn slower and hotter than grasses, so fires in woody areas can be more severe and longer lasting.

Certain plants present a higher fire risk when they have characteristics that make them even less likely to be grazed. Medusahead, for example, accumulates silica, making it less palatable and less digestible than other grasses. It also has long, barbed awns on the seeds that mean livestock tend to avoid the plant once it develops a seed head. Once dried, medusahead creates a thatch that takes longer to decompose than our typical forage grasses, thanks to its silica content.

All those characteristics mean that medusahead, especially if it dominates an area, is almost guaranteed to become prime fire fuel: it won't be grazed down to safer levels, and it won't break down as quickly in the sun and rain, as compared to preferred grass species. More material means medusahead patches (or other ungrazed grasses) can burn longer, and possibly hotter, than a grazed pasture. Some studies suggest that grazed pastures burn less severely than ungrazed pastures. Read more here.

Scotch broom and other weedy shrubs can similarly become fire fuel as their twigs and stems dry out in the summer, and they are typically not grazed. Scotch broom can create dense patches of fuel to sustain a fire moving across the landscape.

Research is ongoing to test the benefits of targeted grazing for fire prevention and mitigation, not only in grasslands but in forests, woodlands, and human communities. Goats and sheep may be able to graze some shrub species that cattle will avoid.
Read more here.

Photos: a site at the San Joaquin Experimental Range in Madera County, showing how annual grasses provide a continuous fuel supply for wildfire.

SPREAD

Then we come to the post-fire challenge with weeds. Fire simultaneously clears the landscape and releases nutrients from the vegetation that burned. This creates a “blank slate” of limited competition coinciding with abundant resources (sun, nutrients, space), which encourages weed establishment. Even if the fire itself doesn't cause a change in plant community, human activities during and after the fire can result in new infestations. Bulldozer lines, first responders, and fire crews can all be vectors of weed introduction.

 Many weedy species take advantage of disturbed sites, and some are particularly stimulated by fire. For instance, fire can stimulate Scotch broom seed germination. Medusahead also exploits fire and will happily germinate in a burned area. Medusahead is one of several species that can instigate a positive (for itself) feedback loop, by encouraging sites to burn, and then recolonizing the burned site to the exclusion of other species.

 Some undesirable species that may increase after fire include:
(click the species to view more information about it)

• Yellow starthistle
  • Scotch broom
  • Tree-of-heaven
  • Medusahead
  • Barb goatgrass
  • Spurges
  • Puncturevine (a common hitchhiker into burned areas, via equipment or people)

There are a variety of ways that all of these species can be managed (mechanical, cultural, and chemical) that have varying degrees of efficacy on each species. The links above can help you decide which methods work best for your management goals.

Key considerations to post-fire weed management:

Know your weeds -

Know what's growing, or what could be;

Check out burn edges or work sites for new invaders from neighbors or equipment

It's all about timing -

Monitor regularly for new invaders: rapid detection, rapid response

Know when you need to treat each weed

Talk to your local UCCE office about any unknown plants - we'll help identify them!

Consider all management tools -

Not every species will respond to every tool

Many weeds are best controlled with a combination of methods, or repeated treatment

DON'T IGNORE WEEDS THE YEAR AFTER FIRE*

If you can't afford to treat every year, spend your resources the year after it burned, to prevent an exponentially worse problem in future years.

 *And, if you can, control weeds around your home or other critical structures, especially if you are in a fire-prone area. No matter where you live, weeds growing next to buildings can become a fire hazard. Proactive management is worth your while.

Steve Quarles, Emeritus UCCE Farm Advisor who set buildings on fire for research purposes, recommends clearing the 5ft closest to your home of any flammable materials. This reduces the risk of catching fire due to airborne embers or radiant heat from nearby burning materials.

Stay safe this fire season!