Wood Ashes in the Garden

A toasty warm fire in the fireplace or wood stove is a great comfort on cold winter days but what can be done with the wood ashes that accumulate over the winter season? A common question that we get this time of the year is whether or not it’s a good idea to spread these wood ashes in the garden.

The answer is … it depends.

In many circumstances, wood ashes can be very beneficial to lawns and gardens by providing important nutrients such as potassium, calcium, magnesium, and some trace elements to the soil. Good stuff, right?

Well – not so fast.

Whether or not it is beneficial to YOUR garden depends a lot on the makeup of your soil and even what you are growing in the garden.

You see, wood ashes are quite alkaline and unless they are used carefully, they can significantly raise the pH of your garden soil. This might be okay if your soil tends to be naturally acidic but for areas where the soil tends to be more alkaline like here in the Shenandoah Valley, wood ashes can raise the pH to the point that it becomes detrimental to many garden plants.

Maintaining proper pH levels in your soil is as important to the overall health of your plants as fertilizing, watering, and pest control. Why?

Soil pH affects nutrient availability for one thing. Certain nutrients like iron, copper, and aluminum become less available to plants in alkaline soils and other nutrients such as calcium and phosphorus become less available in moderately acidic soils.

The optimum pH for most garden plants is between 6.0 and 6.5 which is slightly acidic. Certain plants, however, prefer more extreme pH conditions. Shrubs such as rhododendrons, azaleas, camellias, and blueberries prefer a more acidic soil; between 4.5 and 6.0. That’s why we often have a hard time growing beautiful, lush rhodies and azaleas here in the Shenandoah Valley unless we add a soil acidifier; our limestone-based soil has a naturally high pH.

Before you spread any wood ashes on your lawn or in your vegetable or flower gardens, have your soil tested – at least for pH but knowing the nutrient content would be helpful, too.

If your soil is low in potassium, wood ashes can provide this important nutrient but in general, it isn’t advisable to spread wood ashes in your garden if your soil pH is above 7.0 and you definitely should never spread them around acid loving plants such as rhododendrons, azaleas, and blueberries.

For use on the lawn, wood ash works much like lime except that it is more water soluble and thus works more quickly to change the soil pH. If lime is recommended, dry wood ash can be substituted at the same rate (or even a little higher) – just don’t spread them on a windy day!

In the vegetable garden, wood ashes can be used sparingly around tomatoes which grow fine in slightly alkaline soil and the extra calcium in the ashes can help prevent blossom end rot. Root crops like carrots, beets, and turnips can benefit from the extra potassium that the ashes provide.

A few words of caution about spreading ashes in the garden:

• Never spread wood ashes in an area where you plan to plant potatoes as they can raise the pH to the point that potato scab becomes a problem. Potatoes are less likely to develop scab if grown in acidic soil with a pH between 5.0 and 5.4.
• Do not apply wood ashes at the time of seeding or around new seedlings. Excess salts in the ash can harm young seedlings.
• Always spread the ashes out evenly and then work them into the soil. Never leave piles or clumps of ashes in the garden or on the lawn because excess salts from a concentration of ashes can leach into the soil and cause problems for the plants.
• Ashes can also be added (sparingly) to your compost pile. Sprinkle them evenly in a layer and mix them into the pile. Don’t add too many at once.

If a soil test indicates that your soil will benefit (or at least not suffer) from a slight increase in pH and some additional potassium and/or lime, then by all means spread your wood ashes! It’s a great use for them and is certainly a cheap source of nutrients and trace elements. However, never use the ash from burning coal, pressure treated or painted wood, or from burned trash or cardboard in the garden as these contain harmful byproducts.

The bottom line –

Wood ashes can be a great soil amendment if used carefully and moderately. BUT, before you start spreading, be sure to have your soil tested!