Is Wood Ash Good for Plants?

Your soil may not need it, your plants may not want it, and it's possible to use it incorrectly. But when properly applied, wood ash can be good for plants. This guide explains how to add wood ash to your garden for optimal plant growth.

Healthy Soil

Wood ash is high in calcium carbonate (lime), great for reducing soil acidity. It's an excellent substitute for commercial lime, which has a high carbon footprint. Soils in heavy-rain areas tend to be more acidic than soils in dry regions, so wood ash could be a good garden additive in rainy regions. But, if you conduct a pH test and determine your soil is too alkaline, skip the wood ash.

Wood ash can help with soil structure and porosity, allowing water to reach roots more easily. Take a fistful of moistened garden soil and give it the squeeze test. If it immediately falls apart, your soil is too sandy. If it makes a solid ball, it's too clayey. Wood ash can help to break up clay soil.

Healthy soil is rich in carbon, and wood ash returns organic carbon to the soil. This means wood ash also plays a role in carbon sequestration—not a large one at the scale of a backyard garden, but every bit helps.

Rich in Nutrients

Beyond calcium, wood ash also contains potassium, phosphorus, magnesium, and trace amounts of other elements, all of which are essential nutrients plants need.

Potassium, calcium, and magnesium are extremely water-soluble, so their effect on the soil is quicker than other elements. Wood ash is missing significant amounts of nitrogen, however. Adding human urine makes it a nearly complete fertilizer.

Pest Control

Wood ash has proven effective in controlling a wide variety of garden pests—from slugs and snails to beetles and borers—as a diluted solution applied to leaves and as a dust. In laboratory settings, for example, wood ash proved 100% effective at controlling Colorado potato beetles in adult and larvae form, and equally effective against granary weevils.

Don't expect such stellar results in real-world garden conditions, however. Wood ash becomes much less effective once it's moist and easily blown away when dry.

How to Apply Wood Ash

What's the best way to use wood ash? Mix it into your compost pile, especially if the compost is high in vegetable matter, as the ash will lower its acidity. Be sure to mix it in well and use it only occasionally. Ash is fine, so when moist, it can form a barrier layer, reducing the level of aeration necessary for decomposition.

If you want to apply ash directly into your garden, collect it in a fire-proof container during winter, then incorporate it into your soil during late winter or early spring. Before application, ensure no hot embers are present. Sift the wood ash to remove any large pieces. Wear gloves, long sleeves, eye protection, and a dust mask, as the alkalinity of the ash can irritate skin, eyes, and lungs.

Apply wood ash around existing plants, such as trees or emerging perennials, by mixing it into any mulch you might place around your plants. Avoid applying the ash directly onto plants, as the lye and salts in it can burn foliage.

If you're starting a new, unplanted garden, apply one to two pounds of wood ash per 100 square feet, or roughly two handfuls per square yard. Don't apply wood ash on a windy day or before a rainstorm, as the ash will easily blow or wash away. Apply the ash to moist soil so it remains in place and begins leaching nutrients. A barrier layer of moist ash on top of the soil will slow down air and water penetration into the soil. Work the ash lightly into the soil with a garden rake or fork, tilling the soil as little as possible.

Choose Your Wood Wisely

Avoid ashes from any materials containing paint, glue, plastic, or colored paper. Never apply wood ash produced from pressure-treated wood. The U.S. Environmental Protection Agency warns against burning pressure-treated wood because of the potentially toxic chemicals used to treat it.

Hardwoods like oak contain more nutrients than softwoods like pine or fir.

Compared to heartwood, bark contains higher sodium levels, which can stunt plant growth. However, this is only a concern if the wood ash comes exclusively from bark.