Wood Ashes NOT Recommend for Gardens

Help and Advice from the Contra Costa Master Gardeners' Help Desk

Client's Question:
I've accumulated a considerable amount of wood ashes from the fireplace (or wood stove) over the winter, I've heard or read somewhere that wood ashes are good for the garden. Is this correct, and if so, how can I use wood ashes in the garden?

CCMG's Help Desk Response:
If you are thinking about using those wood ashes you have accumulated over the winter in your garden or compost pile, think again and proceed with caution. Generally, we don't recommend use of wood ashes in the garden and if used, only very limited quantities (e.g. 5 pounds per 100 sq ft once a year... and nowhere near seedlings or acid loving plants, e.g. azaleas, rhodedendrons). 

Though many gardening books and websites encourage homeowners to add wood ashes to garden soil or compost, in Contra Costa County, there are several good reasons why doing this may not be advisable. Too many ashes can cause an excess of alkalinity and salinity.  Most soils in Contra Costa County, and there are over 50 named series ranging from loamy sands to mucky clays, have a pH of 7.0 or greater.  This is considered neutral (7.0) to alkaline (7+).  Adding wood ashes which usually contain 25% calcium carbonate and as a result are very alkaline with a pH of 10 to 12, increases soil alkalinity which creates an adverse condition for growing plants. Many plants prefer a slightly acidic environment (<7.0) to absorb nutrients from the soil.  When soil alkalinity increases and the pH rises, necessary minerals such as phosphorus, iron, boron, manganese, copper, zinc and potassium become chemically tied to the soil and are not available for plant use. In time, due to this change in the soil chemistry, plants will exhibit mineral deficiencies by producing abnormal leaves, stems and flowers. A common symptom of plants growing in alkaline soil is interveinal chlorosis, a yellowing of normally green tissue.

A common symptom of plants growing in alkaline soil is interveinal chlorosis on young foliage. This is due to unavailability of iron, zinc, or manganese at alkaline pH.

About 80 to 90 percent of the minerals in wood ash are water soluble.  This means that when wetted these minerals wash out of the ash and into the soil in the form of salts, which are harmful to plants.  This is especially true if ash is left in a lump as the leached salts are concentrated in one area.  These salts typically contain less than 10% potash, 1% phosphate and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc, but they can also contain heavy metals such as lead, cadmium, nickel and chromium, depending on the other material burned.  Seedlings in particular are very sensitive to salt injury and their growth can be stunted and their foliage turn yellow.  In broad leaf plants, necrosis (death and discoloration of tissue) and defoliation can occur.  Excess salts can also cause medium to fine soils to lose their aggregated structure as soils become impervious to air and water.  Also, if fertilizers such as ammonium sulfate, urea, or ammonium nitrate are combined with wood ash, they produce ammonia gas, a severe respiratory irritant detrimental to the health of the gardener.

For safety reasons, before disposing of wood ashes, wait until they and the hot coals buried under them are completely out and cold.  This may take several days.  You may need to pour cold water over the ashes.  Do not transport or store ashes in plastic or paper bags. While working slowly and carefully, use a metal tool to scoop the ashes and a covered metal pail to remove them from a wood-burning stove or fireplace and store them away from flammable materials.  Do not place cold ashes in the green re-cycling bin.  Completely extinguished ashes can be disposed of in the trash bin and placed at curbside on garbage collection day.

Contra Costa Master Gardeners' Help Desk
(updated and edited version of CCMG article in CCTimes March 2010... we intend to update and publish some more of these articles; more articles such as the one above can be found at http://ccmg.ucanr.edu/Ask_Us/Recent_Contra_Costa_Times_Articles/) 

Note: The Contra Costa Master Gardener Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/

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