wood ash in the garden

The benefits of wood ashes in the garden have been well known since antiquity. The ancient Romans espoused using wood ash, and burning brush and crop residue in the fields to add wood ash to fields is a timeless agricultural practice. Now we add wood ash to our gardens as a soil amendment and to correct soil pH. 

Wood ash contains a variety of minerals including potassium, phosphorus, and trace minerals. It is also very high in calcium carbonate making it very alkaline so it can easily and naturally balance acidic soil by applying at a rate of 10 kg per 100 square meters every three years. It can also be added to your compost heap or made into tea. 

Wood ash must be used in moderation, however, as it can quickly make the soil too alkaline and the high salt content can easily burn plants.  

Keep reading to learn how to safely use wood ash to really benefit your garden.

Keep reading to find out what Wood Ash/Potash is, what’s in it, and how it can help or hurt your garden. Next, I’ll show you easy and safe ways to use it in your garden!

What Wood Ash Should I Use?

Wood Ash: The Secret Weapon for Your Garden… Or a Potential Disaster Waiting to Happen?! 1

In general, ash from any wood fire will do. In most cases this means from fireplaces, wood (or pellet) stoves, boilers, burning barrels, fire pits, burned brush pits, or incinerators.

We use the ashes from our burning barrel where we burn scrap wood, garden scraps that cannot be composted (such as infectious weeds), and any non-recyclable paper products. Ash from young branches has been found to be higher in nutrients, especially potassium, than ash from older woods. 

Also, hardwood ashes are more nutrient-rich than softwood ash. Never use ash from pressure treated wood. Pressure treated wood contains carcinogens and other dangerous chemicals. Pressure treated wood should never be burned (as the toxins are released into the air) and the ash should NEVER be used in the garden or the ground will become poisoned. 

Why Use Wood Ash – Its Benefits For Your Garden

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In days gone by, ashes were the sole source of potassium in agriculture and were regularly applied to farmer’s fields. Nowadays, however, it is much less common but it still has its place as a natural soil amendment

Here are the main benefits of using wood ash in your garden or on your farm: 

1. It Helps to Balance Soil pH Levels

It Helps to Balance Soil pH Levels

The most common reason to use wood ash is to adjust soil pH. Wood ash is very alkaline with a pH of around 10 to 12 so it can quickly neutralize acidic soil. 

It should be used in moderation, however, as it can easily make the soil too acidic. It is best to start with a DIY soil pH kit to test the acidity of your soil to see how much wood ash to apply.  Raising the pH of your soil can also have other advantages for your garden. For example, the fungal infection club root can be minimized if wood ash raises the pH to around 7.5. 

2. Wood Ash As A Fertilizer

Wood Ash: The Secret Weapon for Your Garden… Or a Potential Disaster Waiting to Happen?! 3

If you were to buy a bag of wood ash fertilizer the N-P-K (nitrogen-phosphorus-potassium) levels would be approximately 0-1-3. 

Because it contains relatively no nitrogen, wood ash is not often thought of as a fertilizer. However, not only is it natural and water soluble, wood ash contains valuable minerals necessary for growing plants including large amounts of carbon and calcium (as calcium carbonate), potassium, phosphorus, magnesium, manganese, sulfur, zinc, copper, iron, and chromium. 

3. Works as Insect Repellent

Works as Insect Repellent

Wood ash can be successful as a slug and snail repellent. Sprinkle ashes in a circle around plants you wish to protect (taking care not to apply directly to foliage or roots).

The concept is that the invading mollusks do not like crossing dry ashes so it will need to be reapplied following rain or watering. 

Potential Downsides Of Wood Ash

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Wood ash, despite its benefits, should be used with caution and moderation. 

  • Too Alkaline: Because of its extreme alkalinity, too much wood ash can quickly make a garden inhospitable for many flowers, vegetables, bushes, and trees. 
  • If you are using wood ash to significantly modify acidic soil, talk with a local garden centre about application rates for your area. 
  • Water Soluble: About 80% to 90% of the total matter of wood ash is water soluble. While water soluble can be a beneficial trait, make sure not to leave wood ash out in the rain before use or all the valuable nutrients will be washed away and you will be left with a foul smelling alkaline paste.

Keep wood ash in a sealed container or out of the elements when not in use to keep nutrients from washing away. 

  • Salinity Burning: Wood ash contains a high concentration of salts, which, if they build up in the soil will burn your plants. (And we should recognize the irony that the residue of fire can burn our crops). A salinity build up in the soil can burn roots while ashes resting on the plant itself can burn foliage. 
  • Potato Scab: Alkaline conditions can cause scab on potatoes, so do not apply to areas where you plan to grow potatoes this year or next. 
  • Germination Problems: In large quantities, wood ash can inhibit germination and slow the growth of seedlings and plants.
  • Heavy Metals: Wood ash has been found to contain certain heavy metals that can be dangerous is large quantities such as lead, cadmium, nickel, chromium, and even arsenic. 
  • Dangerous To Handle: Wet wood ash becomes a very alkaline paste that can burn your skin, especially after prolonged exposure. Be careful when handling wood ash and wear protective clothing such as gloves, long sleeves, and goggles (this is especially important on a windy day).

Plants That Benefit From Wood Ash

While wood ash provides a variety of benefits for the garden as a whole, certain plants will do better with a generous application than others. 

Crops That Grow Well In Alkaline Soil

Most vegetables prefer slightly acidic soil (ph between 5.5 and 7.0), so if the soil is more acidic than this they will benefit from a “sweeter” soil. However, some vegetables grow better in a higher pH than others. 

Here are some vegetables that will grow fairly well in soil enriched with wood ash. 

  • Asparagus (can easily handle a pH of 8.0)
  • Alliums (onions, garlic, chives, etc) can be protected from onion worms by wood ash.
  • Cauliflower
  • Cucumbers
  • Celery
  • Cabbage
  • Mustard greens
  • Turnips
  • Brussels sprouts
  • Lavender
  • Roots like carrots, parsnips, turnips, radish and beets benefit from the potash element of wood ash. 
  • Cherries and plums grow well in slightly alkaline soil.

Plants That Do NOT Like Wood Ash

There are undoubtably some plants in your garden that will not like wood ash (they prefer acidic soil). Here are a few vegetables you should use caution when applying wood ash to the garden:

  • Most berries including strawberries, raspberries, blueberries, etc.
  • Rhododendrons
  • Roses
  • Azaleas
  • Potatoes (wood ash can cause scab)
  • Hydrangea
  • Apple, pear and peach trees like acidic soil.

The key to successfully using wood ash is to apply it properly at the right rate. Here are the three best ways to incorporate wood ashes into your garden: in your compost, as a tea, or directly into your soil.

How To Use Wood Ash In Compost

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Wood ash can be added to your compost heap to add extra nutrients to your finished humus. As you layer your carbon and nitrogen matter in your compost, sprinkle in a dash of wood ash with each layer. 

As compost decomposes, it usually settles at a fairly neutral pH (6 to 8), so do not add too much wood ash, or your compost will become overly alkaline and the resulting pH will be too high for most garden plants.

In general, about 500g to 1 kg (1-2lbs) of wood ash can be added to an average size pile throughout the year.

In this case, we assume an average pile is approximately 4ft x 4ft x 4ft). Wood ash should be used in small quantities and well incorporated so it is never visible in the compost. 

Making Wood Ash Tea

You can also make a wood ash tea and apply it to your garden. Put roughly 1.5 kgs (3lbs) of ash in a semi-permeable cloth bag and soak it for 4 to 5 days in a 30-gallon garbage full of water. Apply the finished tea to your garden.  

There is very little research done on the exact benefits or nutrient composition of wood ash tea, or on the proper application rate so use with caution. 

Do not use to directly water plants as the concentrated tea could be extremely bad for your plants. 

How Much Wood Ash Is Too Much?

Wood Ash: The Secret Weapon for Your Garden… Or a Potential Disaster Waiting to Happen?! 6

The most common practice for using wood ash is to add it to the soil. Here are some tips for applying wood ash directly to your garden:

Apply In The Fall: apply wood ash to the garden in the fall. The wood ash will react with the soil so it is less likely to burn plants or inhibit germination. 

Incorporate Into Soil: Lightly work the wood ash into the top layer of the garden. This will aid in reducing burning as well. If you are using ashes to repel slugs or snails, keep the ashes on the soil surface. 

Use The Right Amount: Remember, you don’t want to add too much wood ash but you don’t want to apply too little either or it won’t benefit the soil. 

  • On average, add 100g of wood ash per 1 sq m (.22lbs per 10 sq ft) every three years. 
  • If you prefer a yearly application, spread wood ash at a rate of 50-70g per sq m (0.11lbs to 0.15lbs per 10 sq ft). 
  • If your soil is overly acidic, you can apply wood ash at a higher rate to increase alkalinity, but do not apply more than 200g per sq m (.5 lbs per 10 sq ft) each year.


In our modern world, fire is seen as a devastating force. Nature, however, uses this impressive force for its benefit.

Not only does fire rejuvenate forests and grasslands, but the resulting burned “wasteland” is creating a perfect habitat full of life and fertility. 

While I would never advocate setting anything on fire (I am overly cautious using our burning barrel after taking necessary precautions) it is amazing to see all the benefits the ashes can be put to. Now we can harness this powerful force of nature and see the marvels it can do in our gardens.

Amber Noyes

Written By

Amber Noyes

Amber Noyes was born and raised in a suburban California town, San Mateo. She holds a master’s degree in horticulture from the University of California as well as a BS in Biology from the University of San Francisco. With experience working on an organic farm, water conservation research, farmers’ markets, and plant nursery, she understands what makes plants thrive and how we can better understand the connection between microclimate and plant health. When she’s not on the land, Amber loves informing people of new ideas/things related to gardening, especially organic gardening, houseplants, and growing plants in a small space.

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