Wood ash contains nutrient rich potash and raises the soil pH level, otherwise known as "Sweetening the Soil." It makes a wonderful fertilizer for plants that would suffer in more acidic soil such as Lavender, Lilac, and Rosemary, and veggies like Cauliflower, Beets, and Spinach. Simply search "alkaline-loving plants" to see what else makes the list.
In terms of application every 2nd Fall is sufficient. Sprinkle a 1/4 cup around established plants, or rake lightly into the top layer of fresh soil before planting (about 1 lb per 50 square feet). You can also add the ashes directly to your compost bin; just make sure to keep the ratios low—about 5 percent of the overall mix.
(Image: MaryAnne Petrella)