Garden Docs: How to help plants damaged by fire heat and ash from Sonoma County wildfires

Several readers have asked the same questions in the wake of the devastating wildfires that did so much damage in the Wine Country. One is “What can be done to remove the wildfire ash that has coated plants?” The other question on many people’s minds is, “What can be done for suffering plants and trees in neighborhoods that were hit by wildfire or are very near the fire zones? The intense heat has caused a lot of stress in plants.”

Whether or not fire damaged landscape plants will survive depends on several factors: the species, their health and condition before the fire, how long they were exposed to the high heat, and how badly they were scorched. If you take a small pocketknife and cut into the trunk of a tree, getting into the cambium layer inside, a green or white, moist cambium layer beneath the bark of the tree is a good indicator that the tree will survive.

If you see that most of the buds on any plant are still green, moist and flexible, that is a clue that the plant also has a good chance of surviving. With some trees and plants it’s just a little too difficult for you to tell yet. Be patient, and wait until next spring to see if they sprout.

Sometimes after a wildfire, the soil begins to repel water and then becomes “hydrophobic.” If water can’t or won’t soak into the ground, loosen the soil a bit with a spading fork or shovel to break up the surface. A thin layer of organic matter or straw on top of the soil will help the soil absorb moisture.

Irrigate all stressed plants as soon as you can. Water the ground under the trees, covering the circumference of their canopy of branches, and if possible, a few feet out. Keep watering until the soil is moist down to a foot deep.

Wash off any ash with a gentle spray of water, or use a soft brush or broom to clear it. Not only will the plants look better, but you will be able to get a better look at how they fared through the heat.

Fire-stressed trees are vulnerable to beetle attack and other insect and disease problems. For an indication of beetles, look for pink- to red-colored pitch on the branches and trunk. Beetle-infested trees should be cut down and removed.

Soil erosion can become a major concern after a fire when there’s nothing left but barren soil. Before a fire, fallen leaves, branches and plants with shallow roots help control erosion. But when the plant debris consumed by the fire is gone, and the soil won’t absorb water, it becomes vulnerable.

There are several things that you could do to help control erosion, such as spreading out a layer of straw mulch and felling damaged trees across a slope. Sowing pasture grasses, wildflowers and ground covers also can help control erosion.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.