Following the December 2021 Marshall Fire, we reached out to Chief Brian Oliver of the Boulder Fire-Rescue’s Wildland Division for his analysis of the event, in particular the role the grassy fields played in the tremendous loss of property in Louisville and Superior. This was his response:

Grass is the primary carrier of the fire, that is not in question.  The other factors were the extreme conditions (almost 0 precipitation in 6 months, 100 MPH+ winds). I know those conditions exist probably more often than we like here; there was also a resource shortage.  With the widespread wind and several other small fires, the available resources to respond to the incident were spread too thin.  In most cases, if a grass fire impinges on a structure, it takes a bit of time to ignite the things that produce enough heat to ignite the structure itself. (mulch, ornamental plants, patio furniture, etc.).  In a typical fire, there are engines that would be in the area as the front hit or shortly thereafter, keeping the structure from igniting or putting out the larger heat sources that would ignite the home.  As [a 9News article] pointed out, the heat and large embers generated by a structure burning are much larger, hold more heat and can travel farther in the wind, creating spot fires and new home ignitions past where firefighters can see.  More homes ignite, and the process repeats.  That is truly an urban conflagration.  I believe the home ignition early in the Marshall incident was combustible fuel around homes and ornamental trees and shrubs.  Yes, the grass delivered the fire to those things, but rarely does grass by itself carry enough heat energy to ignite a home.  

That being said, as we well know, it is a holistic approach.  You could mow all the grass, and from a fire operations standpoint, we support that.  Any mitigation effort builds an  incremental shift in the structure ignitability.  There may be no grass, but an ember could land in the trash can next to the car, creating a fuse to the house. There is no silver bullet. Mowing is a step, removing flammable vegetation from around the home is another. So on and so forth.