Is Wood Ash Safe for the Garden?

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Homeowners often ask if wood ash is safe to use in the vegetable garden, ornamental flower beds, lawns, and around trees or shrubs. There are many opinions; some warn wood ash damages tender roots, yet others know they saw their grandparents mix a handful to each vegetable transplant hole. Which is right?

Wood ash can be a valuable source of calcium (20%) and potassium (5%) in the garden. In fact, four cups of wood ash can be substituted for one pound of agricultural lime. Magnesium, phosphorus, and sulfur are also typically found in wood ash at concentrations of about 2%. Iron, aluminum, manganese, zinc, boron, and other trace elements are also used by plants. In terms of commercial fertilizer, average wood ash would be about 0-1-3 (N-P-K), which is basically a low fertilizer analysis.

Useable wood ash is from a known source and grown in natural areas. Fireplace wood ash is a good example. Wood ash contains few elements that pose environmental problems. Heavy metal concentrations are typically low. Avoid ash from industrial sites that may be contaminated with toxin or heavy metals. Also do not use ash that contains garbage, plastics, waste oil, or treated wood.

Soil sample results are used to provide lime and fertilizer recommendations for various land-uses (vegetable gardens, ornamental, turf, and others) and can provide guidance on the wood ash application rate based on your crop needs  Applications are generally limited to 15-20 lbs per 1000 ft2 per year. This equates to a 5-gallon bucket of wood ash over a 20’ x 50’ area. Broadcast wood ash over the area to be treated and rototill, spade, or rake in. It is best to apply in autumn to give ash enough time to react with the soil before rapid spring growth, but mid-summer is also acceptable. Early season applications immediately before planting or during early emergence could cause short-term concentrated alkaline conditions that could interfere with plant growth.

Due to the undesirable handling characteristics of ash (small particle size and dusty nature) wear long pants, long sleeve shirt, gloves, dust mask, as needed to limit exposures that may lead to skin, eye, or respiratory irritation. So, who was right? Both opinions had merit. Safe use of ash is based on proper timing and application. Wood ash can have the same benefits as agricultural lime and benefit crop productivity but could cause crop damage if misused. Landowners should consider ash source, and make sound decisions about application timing and rate to enhance plant growth and make use of this valuable resource.

For more information on gardening tips, contact Horticulture Agent, Aimee Colf, with Anson Cooperative Extension at 704-694-2415 or aimee_colf@ncsu.edu