Using wood ash to feed a new or established tree is a great way to add potassium, magnesium and calcium to its diet and make the surrounding soil less acidic. For those who burn logs to heat their homes, wood ash represents a free alternative to commercial lime as well as a resourceful way to use up wood stove or fireplace waste.
Start a cozy fire with oak, ash or beech logs. Hardwoods make for a longer-lasting fire than softwood and yield more nutritious fertilizer. Use plenty of bark and dried leaves as kindling; those parts of the tree contain more nutrients than the inner logs.
Once the ashes cool, place them in a metal container until you are ready to use them. If stored outside, the container needs a tight-fitting lid to keep the ashes’ nutrients from leaching away in rain or snow.
When you are ready to feed your tree, calculate how much fireplace ash you will need. The University of California Cooperative Extension Office recommends 5 to 10 pounds per 100 square feet. For a sizable tree with a 25-foot radius, you would use one to two pounds of wood ash. For a new tree, use about a quarter of a pound.
Wait for a dry, windless day to spread wood ash. The material’s nutrients leach away quickly in the rain, and a windy day will scatter the ashes before they can settle into the soil.
Wear eye goggles, gloves and a surgical mask or bandanna. Wood ash is extremely alkaline and can irritate the skin and eyes, and its fine particles are easily ingested without airway protection.
Determine the tree's “drip line." As you might guess from the term, a drip line refers simply to the outlying edge of the tree’s canopy. This above-ground circle corresponds to the underground feeder tips of the tree’s roots.
Remove any mulch around the drip line and lightly till the soil. Gently rake the ashes into the soil and replace the mulch. The older and more deeply rooted the tree, the more freely you can till. For a brand new sapling, simply top-dress the tree by sprinkling ashes right on the surface of the soil.
Wait a month before applying nitrogen fertilizer and a year before reapplying wood ash.