Are wood ashes good for the garden?

Wood ash was a genuinely valuable garden input when everyone had an open fire, and it still is if you have a log burner. What has changed is the number of things that get burnt which should not go on a garden, and the fact that most people now have far less soil per bag of ash than a Victorian gardener did — so the alkalinity matters more than it used to.

Safety

What you'll need

• A metal container with a lid for storage
• A dust mask and goggles
• Gloves

Step by step

1. Establish what was burnt.Only clean, untreated, unpainted wood. No treated or tanalised timber, no painted or varnished wood, no MDF, chipboard or plywood, no pallets unless you know they are untreated, no coal or smokeless fuel, no household waste, no printed or glossy paper. Ash from any of those carries heavy metals or other residues and belongs in the bin.
2. Let it cool completely and store it dry.Embers stay hot in ash for days. Use a metal bin with a lid, away from the house and from anything combustible. Then keep it dry — potassium in ash is highly soluble and a sack of ash left in the rain loses most of its value in a fortnight.
3. Understand what it supplies.Roughly 5 to 10 per cent potassium, a lot of calcium, some magnesium and phosphorus, and a range of trace elements. No nitrogen at all — that went up the chimney. It is a potash source and a liming agent in one.
4. Treat it as fast-acting lime.Ash raises pH more quickly than ground limestone because the calcium is in a more soluble form. That is useful when you want it and a problem when you do not. If you spread ash regularly you are liming, whether you meant to or not.
5. Use it where potassium and alkalinity are both welcome.Around gooseberries, currants and other soft fruit, which are potassium-hungry. On the brassica part of a vegetable rotation, which prefers a higher pH. Around fruit trees. On a lawn only if the lawn is genuinely acid.
6. Keep it away from the acid-lovers.Blueberries, rhododendrons, azaleas, camellias, heathers and pieris. Also keep it off potatoes — alkaline conditions favour scab.
7. Apply it thinly and rarely.A light dusting raked in, not a heap. A generous double handful per square metre once a year is about right on acid soil; less or none on neutral, and none at all on chalk. Ash concentrated in one spot scorches roots and creates a sterile patch.
8. Add small amounts to a compost heap.A sprinkling between layers adds potassium and buffers acidity in a heap that has gone sour. Do not put in more than a light dusting per layer — a lot of ash in a heap raises pH enough to drive nitrogen off as ammonia.
9. Apply in spring rather than autumn where you can.The potassium is soluble and winter rain leaches it out of the soil before plants can use it. Spring application on growing ground gets the benefit.

Tips

• Ash from hardwood contains substantially more potassium than ash from softwood, and considerably more than ash from a fire that mostly burnt kindling and paper.
• A ring of dry ash around a hosta is a mildly effective slug barrier for as long as it stays dry, which in a British summer is not long. Treat it as a bonus rather than a strategy.
• Do not use ash on seedbeds or around seedlings. The salt and alkalinity are too much for germinating seed.
• If you have more ash than garden, the excess goes in the general waste. Do not stockpile it on the plot — a big pile of ash creates a genuinely sterile patch that takes years to recover.
• Test the pH every two or three years if you use ash regularly. It is the most common cause of a garden drifting alkaline without anyone intending it.

Common mistakes

• Emptying the log burner onto the same border every week all winter — That is a heavy liming programme applied to one spot, plus a concentration of salts. The soil there goes alkaline and sterile and takes years to come back.
• Using ash from treated or painted timber — Copper, chromium, arsenic in older treated timber, and lead in old paint, all concentrate in the ash. This is one of the few genuine contamination risks a gardener creates for themselves.
• Storing ash in a plastic bin or a paper sack — Ash holds live embers for days. This is a common cause of shed and bin fires. A metal container with a lid, away from the building.

Questions people ask