At this time of year, many of us are accumulating significant quantities of wood ashes from our wood stoves and fireplaces and wishing that they could be put to some use. Here in New England, most of our native and non-native landscaping plants thrive in our naturally acidic soil. However, there are many plants that we grow for either ornament or sustenance, that prefer an alkaline soil and the by product of our wood fueled heat can contribute to their health. Here’s the skinny on wood ashes in the garden:

Wood ash does have fertilizer value, the amount varying somewhat with the species of wood being used. Generally, wood ash contains less than 10 percent potash, 1 percent phosphate and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc. Trace amounts of heavy metals such as lead, cadmium, nickel and chromium also may be present. Wood ash does not contain nitrogen.

The largest component of wood ash (about 25 percent) is calcium carbonate, a common liming material that increases soil alkalinity. Wood ash has a very fine particle size, so it reacts rapidly and completely in the soil. Although small amounts of nutrients are applied with wood ash, the main effect is that of a liming agent.

Increasing the alkalinity of the soil does affect plant nutrition. Nutrients are most readily available to plants when the soil is slightly acidic. As soil alkalinity increases and the pH rises above 7.0, nutrients such as phosphorus, iron, boron, manganese, copper, zinc and potassium become chemically tied to the soil and less available for plant use.

Ash from a cord of oak meets the potassium needs of a garden 60 by 70 feet. A cord of Douglas-fir ash supplies enough potassium for a garden 30 by 30 feet. Both types of ash contain enough calcium and magnesium to reduce soil acidity (increase soil pH) slightly.

One-half to one pound of wood ash per year is recommended for each shrub and rose bush. Spread ash evenly on the soil around perennial plants. Rake the ash into the soil lightly, being careful not to damage the roots. Never leave ash in lumps or piles, because if it is concentrated in one place, excessive salt from the ash will leach into the soil, creating a harmful environment for plants.

A safe rate of wood ash application for a garden or lawn area would be twenty pounds per thousand square feet or a five-gallon pail full of wood ash. Twenty pounds of wood ash is equivalent to six pounds of ground limestone per thousand square feet. If the soil is in the proper pH range, this rate of application is considered appropriate for yearly treatments. After wood ash application, we should need no additional lime, with nitrogen and possibly phosphorous being the other plant nutrient requirements. Here at Stonewell Farm, we apply nitrogen rich chicken manure in addition to horse manure and wood ash.The wood application will also supply potassium. We should mix the application of wood ash to the garden soil well.

In compost piles, wood ash can be used to help maintain a neutral condition, the best environment to help microorganisms break down organic materials. Sprinkle ash on each layer of compost as the pile is built up. Ash also adds nutrients to compost.

If used judiciously, wood ash can be used to repel insects, slugs and snails, because it draws water from invertebrates’ bodies. Sprinkle ash around the base of your plants to discourage surface feeding pests. But once ash gets wet, it loses its deterring properties. Continual use of ash in this way may increase the soil pH too much, or accumulate high salt levels harmful to plants.

• Protect yourself when applying wood ash. Use the same precautions you would use when handling household bleach, another strongly alkaline material. Wear eye protection and gloves. Depending on the fineness of the ash, you may want to wear a dust mask.

• Do not use ash from burning trash, cardboard, coal or pressure-treated, painted or stained wood. These substances contain trace elements, harmful to many plants when applied in excessive amounts. For example, the glue in cardboard boxes and paper bags contains boron, an element toxic to many plant species at levels slightly higher than that required for normal growth, (however, very useful for beets).
• Do not use ash on alkaline soils or on acid-loving plants.
• Do not apply wood ash to a potato patch as wood ashes may favor the development of potato scab.
• Do not apply ash to newly germinated seeds, as ash contains too many salts for seedlings.
• Do not add ash with nitrogen fertilizers such as ammonium sulfate (21-0-0-24S), urea (46-0-0) or ammonium nitrate (34-0-0). These fertilizers produce ammonia gas when placed in contact with high pH materials such as wood ash.

Here are some of the common garden plants that prefer alkaline soil:

Evergreen shrubs
Green Velvet boxwood (Buxus ‘Green Velvet’; Zones 6–8)
Daphniphyllum himalaense ssp. macropodum (Zones 7–8)
Photinia species (Zones 7–9)
Aucuba species (Zones 7–10)
Euonymus fortunei ‘Emerald Gaity’, ‘Silver Queen’, ‘Emerald n Gold’ (Zones 5–8)
Deciduous shrubs

• Daphne species (Zones 5–8)
• Deutzia spp. (generally Zones 6–8)
• Forsythia spp. (generally Zones 6–9)
• Mock oranges (Philadelphus spp.; generally zones 5–9)
• Lilacs (Syringa spp.; generally Zones 5–9)
• Weigela spp. (generally Zones 5–9)
• Spiraea spp. (Zones 3–8)

Perennials

• Hellebores (Zones 4–8)
• Pinks (Dianthus spp.; generally Zones 4–10)
• Brunnera macrophylla ‘Jack Frost’ and ‘Looking Glass’ (Zones 4–8)
• Clematis spp. (generally Zones 3–8)
• Potentilla spp. (generally Zones 5–8)
• Scabiosa spp. (generally Zones 5–9)

These too, prefer an alkaline soil:

Anemone, Aster, Baby’s Breath, Barberry, Blue Flax, Calendula, chrysanthemum, clematis, Cotoneaster, Crown Vetch, Daffodils, Hens and Chicks, Iris, Lavender, Lilies, Mandevilla, Monkshood, Nicotiana, Oregano, Salvia, Speedwell, Stone cress, Thyme, Verbena, Vinca, Yarrow and Zinnia.