I have a big sack of the ashes from our log fires last winter. I propose to scatter this ash throughout our garden before winter starts. Is this a good idea? Are there any plants/shrubs/conifers that I should avoid when scattering the ash?
Advice appreciated


As I have never used ash in the garden, I thought I would search around and find what the experts say. I have to admit that my answer is copied verbatim from another Master Gardener in Frederick county, Maryland.  Since the climate is not so different from ours, the information should be transferable to southern Ontario.  www.emmitsburg.net/gardens/articles/frederick/2004/ashes.htm

Using fireplace ashes in your garden

Dawne Howard
Frederick County Master Gardener Program

Since Roman times, wood ash has been recognized as a useful amendment to the soil. In fact, North America exported wood ash to Britain in the 18th century as a fertilizer, and today, 80 per-cent of the ash produced commercially in the Northeastern United States is applied to the land.

Wood stoves and fireplaces are great for warming gardeners’ chilly hands and feet. So, what can we do with the ashes? Since wood ash is derived from plant material, it contains most of the 13 essential nutrients the soil must have for good plant growth and health.

When wood burns, nitrogen and sulfur are lost as gases, and calcium, potassium, magnesium and trace element compounds remain. The remaining carbonates and oxides are valuable liming agents, raising pH, thus neutralizing acid soils. Soils that are acid and low in potassium benefit from wood ash. However, acid-loving plants such as blueberries, cranberries, rhododendrons and azaleas would not do well at all with an application of wood ash. 

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 it is a liming agent. The average ash is equivalent to a 0-1-3 (N-P-K). The chemical makeup varies with the type of wood burned. Hardwoods produce three times as much ash per cord as do softwoods.

Calcium and potassium are both essential to plant growth. Calcium is needed for root development, strong cell walls and protein formation in the plant. Potassium is an important catalyst in photosynthesis and is essential for the movement of sugars, seed formation, protein synthesis and the use of nitrogen in plants.

Wood ash should never be applied to areas where potatoes will be planted as ash can promote potato scab. For most garden soil, 20 pounds (about a 5-gallon pail) per 1,000 square feet can be applied safely each year. That equals about 6 pounds of ground limestone applied to the same area.

The best time to apply wood ash is in the spring when the soil is dry and before tilling. In compost piles wood ash can be used to maintain a neutral condition, the best environment for microorganisms to break down organic materials. Sprinkle ash on each layer of compost. This is especially good if you have oak leaves or pine needles in your compost heap.

Wood ash can be used to repel insects, slugs and snails because it draws water out of these invertebrates. Sprinkle ash around the base of your plants to discourage surface-feeding insects. Once ash gets wet, it loses its deterring properties. Too much ash can increase pH or accumulate high levels of salts that can be harmful to some plants, so use ashes carefully.

Ash should be stored in a metal container with a secure lid. This helps prevent accidental fires from live coals and prevents water from flowing through the ash and leaching out the nutrients before the materials are applied to the soil.

Caution should be used when handling wood ash:

  • Protect yourself as you would if you were handling household bleach or any other strong alkaline material. Wear eye protection, gloves and a dust mask.
  • Do not use ash from burning trash, cardboard; coal or pressure-treated, painted or stained wood. These materials contain potentially harmful chemicals, The glue in cardboard contains boron, an element that can inhibit plant growth if applied in excess.
  • Do not scatter ashes during windy periods.
  • Do not mix ash with nitrogen fertilizer as ammonium sulfate or ammonium nitrates or urea. These fertilizers lose their nitrogen as ammonia gas when mixed with high pH materials such as wood ash. For a lawn, wait at least a month after wood ash is applied before putting down a nitrogen fertilizer to allow for the soil to reduce the alkalinity of the wood ash.
  • Never leave wood ash in lumps or piles. Concentrated piles of wood ash causes excessive salt build-up in the soil through leaching and can create a harmful environment for plants.

I do hope that this thorough answer is of assistance to you. Please follow all cautions regarding the pH levels in the soil of hydrangeas, rhododendrons and other acid loving plants.