Ask a Master Gardener: Wood ash a poor desert soil additive

Q. Can I use wood ash in compost or as a soil additive?

A. The composition of wood ash will vary by geographical location and the type of wood. Basically, pure wood ash consists of calcium, potassium (sometimes called potash), and magnesium. It has other trace elements and can contain heavy metals, but it has no nitrogen.

Wood ash is used in agriculture and home gardening as a liming agent (it reacts with the soil much like limestone) to raise the pH of the soil, making acidic soils more neutral or more to the alkaline side. Most of Arizona's desert soil pH is alkaline (7.5+). Many vegetables and ornamental shrubs prefer an acid soil, 6.5 and below. Arizona gardeners are always struggling to lower the pH of their soil, not raise it.

Even in parts of the country where soils are acid and wood ash is used, the application is quite sparing.

We do not recommend using wood ash as a supplement in the Mohave County area.

A publication entitled "Wood Ashes Should Not Be Spread on Desert Soils" is available at the Mohave County Cooperative Extension office.

Q. When is the best time to prune outdoor succulents?

A. Generally, the "do no pruning" rule is usually the best method for these types of plants.

• Pruning details for agave: Pruning should only be considered after a severe frost. Otherwise avoid pruning. Do not prune into a pineapple shape. Pruning wounds often lead to pest infestations.

• Pruning details for desert spoon and yucca: Avoid all pruning or shearing. Dry, old leaves can be "peeled" on some species of yucca. You can remove old flower stalks after blooming.

• Pruning details for ocotillo: Do not prune. Plant in an area that will accommodate mature size.

• Pruning details for red or yellow yucca: Avoid all pruning or shearing. Dried flower stalks can be removed.

• The following advice pertains to all succulent varieties noted above:

Only prune to remove dead or diseased wood. This is particularly true for plants in the ground less than three years. Over time, most shrubs and especially trees will develop into attractive and healthy plants when left alone.

Q. Some friends and I are interested in becoming Master Gardeners. Where can we get more information about upcoming horticulture classes?

A. The Master Gardener Program in Mohave County is administered by the University of Arizona's Mohave County Cooperative Extension Office, located at 101 East Beale St., Suite A, Kingman, AZ 86401-5808. The next classes will be held from 2:30 to 6 p.m. Wednesdays starting Jan. 11 and continuing 16 weeks.

The cost is $175 (which includes your book). This is a pre-requisite to becoming a master gardener.

Topics covered include: botany and horticulture; geology of Mohave County, water concerns, facts, soils; fertilizing, composting and plant nutrition, soil amendments and soil analysis; annuals, ornamentals and house plants; how to properly plant, stake and prune trees and shrubs; irrigation, water, plant needs; diagnosing your plant problems and finding solutions; designing your landscape with you in mind; selecting the right fruit and nut varieties; citrus trees for Bullhead City & Lake Havasu City areas; how and where to grow vegetables and herbs; introduction to controlled environment growing/greenhouse; the secrets of cactus and succulents - selection, propagation and care; urban pest management, IPM, pesticide safety; propagating plants from seed, cuttings and grafts; and local plant horticulture facts.