Published in the News-Review on January 17, 2025

By: Bonnie Courter

Question: We use our wood stove all winter for heat, so we always have an abundance of wood ashes to clean out frequently. I’ve heard ashes could be used in the garden. Is that true?

Answer: In the old days, our ancestors used wood ash to make lye for soaps and to clean glass, silver and pewter. And for generations they also applied ashes to the garden as an agricultural soil amendment to add nutrients to the soil and raise the soil pH in order to neutralize acid soils. Since Pacific Northwest soils that are west of the Cascades tend to be acidic soils, ash can act as a natural substitute for lime to sweeten our soils.

Since wood ash comes from plant material, it contains significant amounts of potassium, calcium, with smaller amounts of phosphorous and magnesium along with micro-nutrients like zinc and copper. Ashes from hardwoods like oak and madrone contain higher percentages of nutrients than those from softwoods like pine, and have a more significant liming affect on soil.

However, since wood ash is more soluble than lime, it can change soil pH more quickly, requiring some precautions: 

• If you plan on using ash in your garden as a soil amendment, it’s best to test your soil for pH levels first. If your pH is alkaline already, you certainly don’t want to add wood ash. (Douglas County Master Gardeners offer very affordable soil testing for the public. Just contact the Extension Office for details).

• A typical vegetable garden of 1,000 square feet with a pH of 6.5 generally only requires no more than 20 pounds of wood ash, enough to fill a five-gallon bucket. Once spread across the moist soil surface, rake in the ashes into the top 2-4 inches. This should be sufficient for one year. Fall is the best time to apply ash, but it can still be done up until spring.

• Wood ash can also be safely added to your compost pile, mixing in the material as you build up the pile. But because of its high pH, it should not make up more than 5% of your compost  feedstocks by volume.

• Handle wood ash as you would handle household bleach which is another strongly alkaline material. Protect yourself while applying wood ash by wearing eye protection and gloves. If the ash is very fine, you may want to wear a mask. Never scatter ashes on a windy day.

• Ash derived from burning trash, charcoal, cardboard, or pressure-treated, painted or stained wood should not be used. These materials contain trace elements that are harmful to plants when applied in excessive amounts.

• Avoid leaving wood ash in lumps or piles. This concentration of ash in one place can leach excessive salt into the soil which creates a harmful environment for plants.

• Don’t apply wood ash to acid-loving plants such as rhododendrons, azaleas and blueberries. But tomatoes, collards, broccoli, roses and lilacs all love wood ash.

• Don’t apply ash to newly germinated seeds as it contains too many salts for young seedlings. That goes for your potato patch as well since wood ash may favor development of potato scab.

• And it goes without saying, do not apply wood ash to nitrogen fertilizers such as ammonium sulfate, urea, or ammonium nitrate. When placed in contact with such high alkaline materials like ash, these fertilizers will produce ammonia gas.

Lawns that require lime and potassium may benefit from the application of wood ash by applying 10-15 pounds of ash per 1,000 square feet of lawn.

Another benefit of wood ash application is it acts as a deterrent to insect pests. By sprinkling a small amount on or around established plants, the ash literally sucks the water out of the insects. Applying a ring of ash around plants will also stop slugs.

By taking these precautions and using common sense, you can start to make good use of all that wood ash from your fireplace or wood stove.

Do you have a gardening or insect question? Contact the Douglas County Master Gardeners at douglasmg@oregonstate.edu or 541-672-4461 or visit 1134 SE Douglas Ave., Roseburg. Douglas County Master Gardeners are trained volunteers who help the OSU Extension Service serve the people of Douglas County.