Wood Ashes in the Garden

As temperatures rise (or fall), many of us head outdoors to enjoy a nice evening fire (assuming there’s no Spare the Air alerts, of course!). There’s just something primal about sitting around an open fire with family and friends, enjoying the stars, good conversation, and maybe a bottle of wine. But what about the ashes left over from the fire? Can they be used in the garden? The answer is yes, and no.

Fire pit made by a dear friend from a discarded washing machine tub

What is in your wood ash?

Before adding wood ash to the garden, it is important to know what it was before it was burned. Ashes from BBQ grills, plywood, pressure treated, stained or painted wood, cardboard and even paper bags should never be used in the garden. These materials can contain toxic chemicals that you certainly don’t want in your soil, especially if you are growing food plants.

Wood ashes vary in their nutrients, depending on the type of wood that was burned. Soft woods, such as pine, contain only one-fifth of the nutrients held in hardwood. Ashes from good quality hardwoods contain a lot of potassium, or potash. This is the “K” in the standard N-P-K found on fertilizer packaging. On average, wood ash contains 0-1-3 using this scale. Wood ash generally contains the following micronutrients as a percentage of weight:

• calcium 15%
• potassium 2.6%
• aluminum 1.6%
• magnesium 1%
• iron 0.8%
• phosphorus 0.5%
• manganese 0.4%

Potassium supports root growth and cell structure. Stronger roots and cells make plants more resistant to pests, diseases, and environmental stresses. Wood ashes can also improve soil structure, but there’s a price.

Wood ashes and pH

Wood ashes are very alkaline. Using the pH scale, substances are measured on a range from 0 to 14, with 7 being neutral. Lower numbers are more acidic and higher numbers are more alkaline. Wood ash can have a pH of 9-13! Our San Jose soil tends to already be alkaline, so adding wood ash might end up being a bad idea. The only way to really know what you are working with is through a soil test from a reputable lab. Over-the-counter soil test kits are not currently reliable or accurate enough to be useful.

If your soil could benefit from applying wood ash, the best time of year to do this is in the fall and winter. Save cold wood ashes until then and apply over a period of time. Smaller plants and seedlings can be dramatically impacted, for better or worse, with sudden changes in pH.

Other garden uses for wood ash

If your soil does not need its pH altered, there are several other uses for wood ash in the garden: 

• Wood ash can be added to compost piles to help maintain the neutral pH preferred by microorganisms that break down organic material. Simply sprinkle wood ash on each layer of compost as it is added.
• Plants can also be protected against slugs and snails with wood ash. As long as the wood ash stays dry, it will desiccate any mollusks before they get a chance to feed. Of course, irrigation and rain will wash the wood ash into the soil, altering its pH and raising salt levels, so it must be used judiciously.
• Adding a ring of wood ash around basil plants may help keep cutworms away.
• If you raise chickens, wood ashes make an excellent dust bath that helps get rid of lice and other irritants.

If you’re feeling really adventurous, you can use those wood ashes to make soap or shine up Granny’s silver. Just be sure to wear protective clothing when working with wood ashes. Wood ash particulates can irritate your lungs and wet wood ash (lye) can dissolve your fingernails, so be careful!