gray alder
Alnus incana
A genus split is not a placement verdict.
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Gray alder has mixed fire-resistance findings, so a simple yes would overstate what the sources establish.
The USDA Forest Service and state Firewise programs call it “Moderately Firewise,” one step below their best rating, “Firewise.” Idaho Firewise gives it a fire-resistance score of 6 of 10. These programs use different scales, so their numbers should not be read as matching grades.
Laboratory testing adds detail without resolving the choice. A 2007 study published in the International Journal of Wildland Fire tested conditioned foliage and twigs under a heat exposure of 25 kilowatts per square metre in a cone calorimeter. Gray alder was outside the authors’ named high and low groups for effective heat of combustion. That does not establish a middle rank: the authors warn that rankings from their different combustion measurements are not interchangeable.
The FLAMITS database, published through Dryad and Global Ecology and Biogeography, also carries a calorific value of 3,797.65 kilocalories per kilogram from that same study, for live leaves that were dried before testing. This describes heat released per unit mass during combustion, rather than how readily a living plant catches fire. It is another measurement from the same research, not another planting endorsement. For a homeowner, the guide ratings offer qualified guidance, while the laboratory results describe particular materials under particular test conditions. Neither the heat value nor the absence from the study’s extreme groups establishes gray alder as a clearly fire-resistant planting.
- Form
- tree
- Mature height
- ~15 ft
- Light
- Full sun
- Water need
- high
- Rated in
- Global; eastern US; north-eastern and mid-Atlantic United States; southern Idaho / Intermountain
