Using Fireplace Ashes in

Using Fireplace Ashes in the Garden—or Not

Allen Robinson: Ask a Master Gardener

Tuesday, December 5, 2017

Q: I have heard that fireplace ashes are good for vegetable gardens. Is that true? If so, how much is appropriate to use? Tyler M., Tulsa

A: The use of fireplace ashes is a rather complicated subject. While it is commendable to recycle any and all possible waste materials back into the environment, applying ashes should only be done with considerable forethought and planning.

First, ashes resulting from the burning of standard firewood vary as to chemical content. That content depends on the type of wood that was burned and how hot the fire. On average, ashes contain up to 22 percent of undesirable chemical salts, which may actually convert your soil to a high-salt area. This alone can make the soil unfriendly for plants.

In addition, fireplace ashes are highly alkaline, with an average pH of 11.6, which is in the range of household bleach. This reduces the acidity of the soil perhaps to a range unfavorable for most plants, especially vegetables. While there is some nutrient value in ashes (they contain about 6 percent potassium), most gardens that have been fertilized recently already have more than enough potassium. Further, ashes contain little phosphorus and no nitrogen.

Consider that soils in eastern Oklahoma on the whole are slightly acidic, but as you go west past Tulsa, the soils lose acidity and become alkaline. Given that, most ornamental plants and turf grasses prefer the acidity (or pH) of the soil to be neutral or slightly acidic. Most vegetables prefer slightly acidic soil. And some plants, such as azaleas and blueberries, prefer the soil to be strongly acidic. So, adding ashes will tend to drive the soil pH in the wrong direction.

Always consider having a soil test performed before applying any fertilizer or ashes. Soil samples can be dropped off at the Tulsa County OSU Cooperative Extension Service, 4116 E. 15th St. in Tulsa. They will send them to OSU for analysis for about $10. If your soil test indicates the need to make the soil less acidic (raise the pH) and/or if you need to correct for a potassium deficiency, ashes could be used. However, if you do add ashes to your soil, do not exceed 15 pounds per 1,000 square feet of garden or lawn, and do this only once every 10 years.

Because the use of ashes can be complicated and the chance of damaging the soil is so great, it may be wise to forego the use of ashes in your garden or lawn altogether. For further information on the usage of ashes, refer to the Tulsa Master Gardener’s website, tulsamastergardeners.org, and search for OSU’s Fact Sheet PSS-2238, titled “Fireplace Ashes for Lawn and Garden Use.”

Garden tips

• Don’t forget to keep the compost pile watered. The decay process to produce garden-friendly compost continues in winter if the pile is large enough and kept watered and turned.
• If your roses have not been mulched, do so now. This is a good place to use those fall leaves which have been shredded with a mulching mower. Mulch not only will prevent cold damage to those plants that are susceptible, but also will prevent warming of soil on warm winter days, which may promote premature, cold-sensitive new growth.