Using wood ash in the garden can be a good source of free fertilizer, but there are a few things you should know before you start spreading. We'll share the benefits of wood ash, the right and wrong ways to apply it, and how to use it for pest control.
Potential Benefits of Using Wood Ash in the Garden
Wood ash nutrients very depending on the wood and growing conditions of the wood. In general, wood ashes include significant amounts of Calcium, Potassium, Magnesium, Phosphorus, Sulfur, Iron and Sodium, as well as other trace minerals.
Hardwoods such as oak and maple tend to have a higher potassium content, while softwoods like pine have a lower potassium content.
The calcium in wood ash (calcium carbonate) may be 20% or more of the ash. This calcium can help tomatoes avoid blossom end rot.
Ashes also raise the soil pH, making them a good fit for plants that like “sweeter” soil, such as:
- Brassicas (cabbage family plants like cauliflower, broccoli, kale, etc.)
- Apples, figs, and pears
- Bulbs, including onions, garlic and leeks
- Rosemary and lavender
How to Apply Wood Ash
Before you apply wood ash to your vegetable garden soil, it's best to do some soil testing. Most plants prefer a pH level between 6.0-7.0. If your soil is alkaline (with a pH of above 7), wood ash is not a best choice for your garden. Use less, or avoid using it altogether.
For fertilization, apply a thin dusting of wood ash over the garden bed, orchard floor, grass, etc. Make sure the ash is cool with no live embers before applying.
Application guidelines vary from 5-10 pounds per 1,000 square feet up 15 to 20 pounds (approximately a five gallon pail) per 1000 sq. ft. per year. More isn't necessarily better. In fact, too much ash can throw off the soil pH and nutrient balance.
As an example, some years ago my brother applied a light layer of ash in his apple orchard. My nephew saw the ash, and, wanting to help, added a lot more ash. The trees got “burned” from the excess nutrients and their growth that year was stunted.
In our garden, we apply a dusting of ashes in late fall or early winter. This gives the ashes time to break down. We also apply soiled coop bedding at the same time for nitrogen.
Chicken manure needs time to age before it's safe to have in contact with plants. Top dressing the garden beds in fall effectively acts as sheet composting.
Dealing with Charcoal and Other Types of Application
Some people sift out the charcoal chunks, because they can tie up nitrogen as they break down. When we use the sheet composting method, the charcoal soaks up the nitrogen from the coop bedding. In spring, we scrape back the bedding/charcoal that hasn't broken down to plant in the soil below.
If you're worried about charcoal tying up nutrients, separate it and use it to make biochar.
If you want to mix wood ash directly into garden soil, do so in early spring, preferably at least a month before planting. This gives the soil ecosystem time to adjust.
To make a wood ash tea (like compost tea), fill an old sock with wood ashes and let soak in a couple gallons of water for two to three days. Use to water plants, pouring on the soil below the plants.
Things to Avoid
Do not use ashes from charcoal grills, garbage, or treated wood. Avoid ashes from wood that may be contaminated with heavy metals or other pollutants. Ashes from wood burning stoves are easy to gather, but you can use any source of clean ashes.
Don't use too many ashes at once, or apply them in a pile. As noted above, heavy applications can cause more harm than good. If plant leaves turn yellow after ash application, you used too much.
Wood ashes will raise pH, so avoid using wood ashes on acid loving plants, such as blueberries, strawberries, rhododendrons, azaleas, birch trees, red maples, etc. Increasing pH may make potatoes more susceptible to potato scab than plants grown in acidic soils.
Don't use ashes near seedlings or in direct contact with leaves. The ashes are caustic, and can burn tender foliage.
It's best to keep wood ashes out of compost piles. Though many guides suggest composting to “mellow out” the ashes, they tend to reduce the worm count in your pile. They can also throw off the pH balance.
When you're working with ashes, try not to breath them or get them on your skin. They can irritate your lungs the same way they irritate tender plant tissue. Wear protective gear like gloves and a mask, and avoid spreading on windy days.
Using Ashes for Pest Control
Wood ashes can help deter and kill some soft bodied garden pests, such as slugs, snails, and cutworms. The ashes create a barrier that the pests find uncomfortable to crawl over. The high pH of the ashes can also dry out the pests' bodies, killing them.
If you handle ashes with your bare hands, you'll notice that they dry out and irritate your skin. They do the same thing to soft bodied garden pests.
For garden pest control, apply a light side dressing of ashes to the soil surface near the plants. Do not apply the ashes directly to the leaves. If you want to treat worms on leaves, use another option, such as diatomaceous earth.
Wood ashes don't work well for hard shelled pests, and of course winged pest fly right over. Use ashes in combination with other pest control methods, such as companion planting. See Natural Pest Control in the Garden for more ideas.
More Gardening Tips
We have a big annual garden, plus orchards, berries, nuts, and more. Each year we learn new things about gardening, and share them here on the site.
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Laurie Neverman is the creator of Common Sense Home and author of “Never Buy Bread Again“. She and her husband and two sons live on 25 acres in rural northeast Wisconsin, where they are working on their permaculture oasis. She firmly believes that life is better with ducks and nutrient dense food.