Google any combination of the words mulch, flammability, risk, and so forth and you’ll find oodles of articles with frightful headlines talking about people being burned by smoldering mulch, mulch being flammable, alternatives to mulch, etc. If all you do is read the google headlines, you’d think mulch was out to get you. Please don’t let these articles freak your clients out – they almost always discuss wood mulch as if there is only one kind, and it is the flaming, flying, aggressively-trying-to-kill-you kind.
Look closely at the images in a lot of those posts. Did you see the same thing I did? You probably found different articles, so let’s find out: I saw a lot of shredded mulches (which are easily ignited with a high surface area to mass ratio) and I saw a lot of foundation plantings next to flammable siding that comes down low to the ground. I read nothing about what caused the fires in the example images where a fire happened. Hmm. There’s more going on than was addressed in the articles I read.
The benefits of using organically based appropriate mulches in appropriate ways and in appropriate locations outweighs any risk it may pose. That is why your discernment as a landscape professional is so critical. You can advise your client in selecting the best options for specific conditions (something I’ll be working on making content for as we go). Rather than focus on that right now, let’s talk about those perceived risks so you can discuss with your clients. To be clear, the following are arguments I’ve read against wood mulches used as a protective top dressing in planting areas:
Wood mulch is flammable: It is unwise to pave or use gravel, rocks, etc everywhere merely to save ourselves from flammability – to do so only makes the underlying causes of climate change worse. The use of mulch alternatives (such as gravel, etc) where a wood mulch is the better choice can make those plantings more flammable regardless of plant selection. The problem I see online is not that a top-dressing is made of wood – the problem in most of the articles attacking wood mulch is the use of flammable materials within the 0-5’ zone of a flammable structure, shredded wood mulches, and mulch layers that are too thick.
Wood mulch smolders after the fire is out and can stay hot for a long time: The safest place during a wildfire is anywhere that has already burned. In a previously burned area, the fuel has been consumed and a fire has no means of hanging-out there anymore. Smoldering mulch should be watched in case there is a risk of a flare-up, if it is in close proximity to unburned materials, and because someone could accidentally get hurt. Mulch that is still smoldering is not a walkable surface without protective gear, and so should not be used for egress paths. I was shocked to read an article about firefighters receiving burns while cleaning up smoldering mulch. Were they not taking appropriate precautions? Did they have on their protective gear? It didn’t say.
Wood mulch could blow around: The speed of wind you would need to lift smoldering wood mulch off the ground and turn it into firebrands is high enough that you should not be there anyway. To avoid your mulch selection from contributing to an ember shower, use heavy chunky, partially composted mulch made out of recycled, less flammable species. The heavier the mulch is, the more difficult it is for wind to pick it up, the chunkier it is, the less surface area there is to welcome and hold embers, and if it is partially composted, it will resist ignition better. See the studies below for more on different kinds of mulches and species of mulch wood.
We have a responsibility as landscape professionals to understand the fears of our clients, be able to address them, and to be able to specify the right mulch for the conditions if mulch is an appropriate thing to do.
Below are two fabulous studies on mulches and flammability. They use different methods, so both are good to read. I’d like to address a note or two on each:
The study recommends “not using any organic mulch within five feet of a house located in wildfire-prone areas. Instead, place non-combustibles such as rock, pavers, brick chips or well-irrigated low-combustible plants such as lawn or flowers closest to the house. The more combustible mulches should be used more than 30 feet from the home.”
As landscape professionals, you know that “flowers” is not an adequate description of anything outside what a florist does, so we need a better descriptor. I once quoted this article (unwittingly to one of the authors) both complimenting the study’s contents and complaining about the use of “flowers”. I was challenged in return to provide better language. Someday I’ll whittle my response down to less than a novella-sized reply.
I also want to ask why we are using “more combustible mulches” at all!? Suppose you demo a juniper (highly combustible) or pine tree, cedar tree, anything heavily laden with oils and resins – rather than haul the greenwaste to the landfill where they promote methane generation, it is better to chip them up and spread it to decompose naturally – just not so close to structures and along paths of egress. Better yet would be to send it away to a composter so that it can come back in a safer form. I believe that all too often, we chip and spread without considering what we are chipping and spreading. Where there are options for composting, we should take advantage.
This study concludes: “Inorganic mulches such as decomposed granite, gravel or rocks offer superior fire-proofing as landscape mulches and should be used when mulch is needed that directly abuts flammable structural materials such as siding or decking.”
Given that many existing structures have flammable materials close to the ground (wood siding, deck posts, etc), it would be wise to keep organic mulches from coming close enough to ignite those materials. Better still would be to add non-flammable skirting to older homes, build decks, fences, and the like with concrete footings extended above finish grade so that wood posts are not in contact with the ground (or use metal posts instead), and so forth.
Instead, the authors focused on the mulch as the problem – fine, since it was a study on mulch – but rocks and gravels as mulch can become problematic for many reasons, I’ll have to cover that another time.
If you have an additional point that I missed, or was able to explain this in a really clever way to your clients, please let me know!
