Is Firewood Ash Good for Plants?

Firewood ash can be great for plants when used correctly. It’s rich in nutrients such as potassium, phosphorus, calcium and magnesium, among others, and gives your plants a boost that may have otherwise been missing. However, applying too much or using ash from treated or painted wood can have a less-than-desired effect.

In this guide, we explore how to use firewood ash for your plants, when you shouldn’t, the advantages it brings and how to use it safely.

Wood ash is alkaline. Added to soil, it acts like a light lime, raising the pH and making the soil less acidic. That’s why it’s useful on acidic soils, and works against plants that thrive in more acidic conditions.

Firewood ash for plants: Summary

·         Can add valuable nutrients to the soil

·         Supports flower and fruit development

·         Aids root growth

·         Contributes to overall plant health

·         Can help deter pests when dry

·         Only ever use clean ash

·         Avoid using it on acid soil loving plants

·         Apply it to your garden once or twice a year

·         Test the soil level before applying

What plants is wood ash suitable for?

Your firewood ash can benefit a wide range of plants in your garden, although plants that prefer acidic soil are best avoided, as the ash helps neutralise some of the soil's acidity.

Plants that tend to benefit from a sprinkling of firewood ash in the soil or compost include:

·         Grasses and most root vegetables

·         Perennials such as rudbeckia, phlox and foxglove

·         Onions & garlic

·         Herbs including basil, sage and lavender

·         Broccoli, cabbage, cauliflower

What plants is wood ash not suitable for?

Two groups of plants are best kept away from wood ash. The first is acid-loving plants, which rely on low pH levels that ash raises. The second is a handful of plants that tolerate neutral soil but suffer under alkaline conditions for other reasons.

Acid-loving plants

·         Azaleas

·         Hydrangea

·         Rhododendrons

·         Blueberries and most other berries

Avoid for other reasons

·         Potatoes – alkalinity encourages scab growth

·         Rhubarb

·         Apples

At a glance: Which plants?

Plant group	Wood ash?	Why
Acid-loving (blueberries, rhododendrons, azaleas)	Avoid	Need low pH; ash raises it
Most veg, herbs and perennials	Yes, sparingly	Tolerate neutral, alkaline soil; benefit from potassium
Potatoes	Avoid	Alkalinity encourages scab growth
Fruit crops	Avoid	Prefer slightly acidic soil

When is the best time to add firewood ash to your soil?

If you add ash directly to your soil, it’s best to apply it in winter and either rake it or dig it into the soil. This allows the ash to react with the moist soil and have no ill effect on the plants by the time you plant them.

If you add ash to compost, do this sparingly. Too much risks high levels of alkalinity that could damage plants and the soil. Aim for adding some every 15cm of compost. The ash shouldn’t be noticeable in the compost heap after you have mixed it in. If it is, you’ve used too much.

What are the benefits of using wood ash in the garden?

Using your firewood ash in the garden can benefit the plants and the wider garden area. From raising the soil’s pH level to keeping pests away, you can make a positive difference to your outside space.

·         Compost acidity: Adding ash to the compost can help balance its acidity, resulting in an enriched compost filled with valuable minerals that can help the garden thrive.

·         Pest deterrent: A ring of ash around vulnerable plants can help deter pests like slugs and snails, but only when it stays dry, so reapply after rain.

·         Nutrient boost: Wood ash contains various nutrients that plants love. Adding ash to your soil gives potassium-loving plants an extra boost.

·         Soil pH levels: A soil with a pH level below 6.5 might be too acidic; adding the ash helps raise the pH level and gives most plants soil that benefits them, rather than hinders them.

How should you add the ash to your soil?

How you add the ash to your soil depends on what you are using it for. If you are adding it directly to the soil to support plant growth, you’ll need a slightly different approach to the one you’d adopt if you are purely trying to keep pests away:

·         Adding directly to the soil: Sprinkle the ash over the soil and work it into the soil with a rake. Use about 50-70 g per square metre.

·         Adding it to compost: Mix the ash into your compost pile in thin layers.

·         To deter pests: Sprinkle a ring of ash around the plants, topping up after rain.

Before applying ash to your plants or soil, consider inspecting it for debris by filtering it through a sieve. This can help remove some contaminants that could harm your plants. Furthermore, protect yourself too. Dust from ash can be harmful to your health, so consider using a face mask, gloves and long sleeves to limit skin exposure and inhalation.

Potential problems of using ash in your garden

Whilst wood ash can be great for your garden, you should be careful with how you use it. Too much ash can increase the alkaline levels too high, which can cause issues for many of the plants you are growing. Ash from treated wood may also contain contaminants, so only use untreated wood or wood that hasn’t been painted.

If you have ash from coal or charcoal briquettes, it should be kept away from your soil as it can contain harmful substances that could affect the soil, plants and wildlife. However, if you have used a BBQ and fuelled it with lumpwood charcoal, without any other fuels present, you can safely sprinkle it into your soil.

Start with the firewood you can trust

Your garden only benefits from ash that is clean to begin with. Treated, painted, or mixed-fuel ash can do more harm than good. Therefore, knowing what you are burning is essential. At Black Dog Forestry we supply seasoned firewood, sourced from our own sustainably managed woodland and lumpwood charcoal made from Oak, Ash and Beech. Our hardwood burns clean and leaves behind ash you can use in your garden. Available in a range of sizes, our firewood log mix and charcoal are tailored to your needs, whether for a BBQ, fireplace, firepit or chiminea. Contact us today to place your order or ask any questions you may have.