With record cold gripping much of North America this winter, many who find themselves with an abundance of fireplace ashes are wondering whether they can use them in the garden.
Wood ashes from burned untreated wood can be beneficial for your plants — but with a few caveats.
Ashes contain nutrients like potassium, which supports the overall health of plants, phosphorus, which promotes strong root systems, and calcium, which facilitates plants’ absorption of other soil nutrients.
They also raise soil pH, making it more alkaline. That could be beneficial for folks with naturally acidic soil wanting to grow plants like tomatoes, peppers and eggplants, but it could spell disaster for gardeners who want to grow acid-loving plants like rhododendrons, azaleas or blueberries.
And if your soil is already alkaline — with a pH of 7 or higher — raising it further with ashes would risk interfering with many plants’ ability to thrive.
Despite the alkalinizing properties of wood ashes, they should not be relied on as the sole pH-raising amendment for very acidic soils. Although they can be helpful for borderline soils, it would take a tremendous amount of ash to change soil pH from too low to neutral.
The only way to know your soil’s pH level is to test it either with an at-home kit, which you can buy at garden centers or online, or by bringing a soil sample to your county’s cooperative extension office or master gardener clinic for testing and amendment recommendations.
Armed with this information, you can make an educated decision about whether ashes will benefit your soil and plants.
Here are a few more tips to get you on your way.
Know your wood
Whatever the wood contains will be absorbed by your soil and, in turn, your plants. Avoid using ashes from treated wood or from trees grown in polluted areas, which may contain chemicals and contaminants, such as heavy metals.
Coal ash and charcoal briquette ash should never be incorporated into garden soil because they contain toxins.
Make necessary preparations
Before spreading, ensure ashes have cooled completely. This can take a week or more.
Sift the ashes through a compost sifter to remove pieces of burned wood and debris. If you don’t have a sifter, you can use an old window screen or make your own by affixing 1/2- or 1/4-inch (1.3- or 0.6-centimeter) hardware cloth to a large art frame (or make a wood frame yourself).
Don’t overdo it
If you obtained dosing recommendations with your soil test, follow them precisely. Otherwise, apply no more than one 5-gallon (3.8-liter) bucket of ash, roughly 20 pounds (9 kilograms), to a 1,000-square-foot (93-square-meter) area of garden per year. This limit should allow plants to reap the nutritive benefits of ash without adversely raising the soil’s pH. Retest annually before reapplying.
You can also sprinkle similar proportions of ash over the lawn or add it to compost.
Leave time between application and planting
In winter, apply ashes to bare soil (without snow cover) on a windless day. Moisten well with a gentle shower stream to facilitate absorption and prevent ashes from blowing around.
Springtime applications to prepared beds should be made no less than two weeks before planting time. Work ashes 4-6 inches (10-15 centimeters) deep with a stiff-tined metal garden rake.
Dispose of the excess cautiously
Avoid disposing of an abundance of ashes by dumping them in a corner of the yard. Large amounts can severely damage your soil, pollute groundwater, and lower the pH of nearby water sources, harming fish and other wildlife.
Instead, allow ashes to cool in the fireplace or stove for several days before collecting them into a metal bucket. Buried embers may still be burning, so take care and wear gloves.
Cover the bucket and place it outdoors, away from the house, porch, deck and other structures, for about a week. When you are certain they are completely cooled, bag the ashes and place them in the trash.
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Jessica Damiano writes weekly gardening columns for the AP and publishes the award-winning Weekly Dirt Newsletter. You can sign up here for weekly gardening tips and advice.
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