Fireplace ashes can harm
gardens
Bill Sevier: Ask a Master Gardener
Saturday, December 17, 2016
Q: Is it OK to spread my fireplace ashes over
my vegetable garden every winter? N.G., Sand Springs.
A:
The best answer is
“probably not.” Ashes from burning various types of wood have a unique
chemistry, which might be useful in some situations but are not recommended for
vegetable and ornamental gardens or for lawns, with few exceptions.
The problem with ashes is three-fold, two of
them significant. One is that the pH is high. The pH is a measurement of level
of acidity on a scale of 0 to 14, with 7 being neutral, 0 very acid and 14
extremely alkaline. Examples of the pH of some items are: Battery acid pH of 0,
lemon juice pH of 2, pure distilled water pH of 7, baking soda pH of 9 and
household bleach has a pH of 12-13.
The pH range of fireplace ashes has an average
value of 11.6, right up there with household bleach. Added to soil, it removes
acid, driving the pH value up and out of the range of acidity that most plants
prefer.
Most vegetables and lawn grass perform best in
slightly acidic soil of an average pH range of 5.8 to 6.5. Many shrubs need even
lower pH values. Azaleas, blueberries and gardenias need an acidic pH of 5 to
5.5. Other acid-loving shrubs include most conifers, camellias and hollies.
When the soil is out of the pH range needed by a
particular plant, it cannot absorb nutrients, such as iron. This is the reason
for yellow leaves on many azaleas growing in soil that has become too alkaline,
producing yellow leaves that have a distinctive pattern called iron chlorosis.
Another downside to using fireplace ashes is the
amount of salt. Excess salt in soil will predictably kill plant roots. An
example of this is seen with the use of salt on roadways in winter with
subsequent death of roadside plants the following spring. Once the salt is in
the soil, it may take a long time for correction to occur.
The last concern about fireplace ashes is the
amount of potassium. There is about 6 percent potassium in ashes, and they will
add this nutrient to soils, which if deficient, could be helpful. However, the
results of soil tests of Tulsa-area gardens by OSU show that in previously
fertilized soils, the majority have adequate or excessive potassium.
OSU has a fact sheet “Fireplaces Ashes for Lawn
and Garden Use,” which offers a good summary of chemical content of various
ashes from burning different hardwoods. The fact sheet recommends that if ash
is used, one should use no more than 10 gallons per 1,000 square feet in sandy
soils and 20 gallons for the same area in other soil types. The key take-home
fact from this document is that if you apply ashes to your soil as outlined
above, you should do so no more often than once every 10 years.
Garden tips
§ All birds need and appreciate clean feeders and
unfrozen water on cold days. Place feeders close to protective shelter, if
possible.
§ Select a freshly cut Christmas tree. Make a new
cut prior to placing in tree stand. Add water daily.
§ Light prunings of evergreens can be used for
holiday decorations. Be careful with sap that can mar surfaces.
§ With the warmer weather, newly seeded fescue
will continue to grow roots and make energy if you keep them free of leaves.
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