BOTANICALS: White alder

Published online on Wednesday, Apr. 02, 2008

These trees are useful in problem areas such as spots that are nutrient-deficient or have poor drainage.

General: White alders (Alnus rhombifolia) are appealing for their distinctive white bark, upright shape and contrasting, deep green foliage that covers slightly weeping branches. These fast-growing trees are used as shade trees, windbreaks and background trees as well as trees to control for erosion along waterways. Alders are found where the soil is deep and constantly moist. They tolerate temperatures as low as minus-10 degrees, but they also thrive in temperate zones.

Size: 25 to 40 feet high, 40 feet wide

Flowers: The unisex flowers appear as catkins that periodically blossom throughout the year, except for winter.

Cultivation: Plant from a container. The best times to plant are fall or early spring. Select 5- to 15-gallon containers, but make sure these plants aren't root-bound. If planting in a group, space them 20 to 30 feet from the nearest structure or paved area. Never plant near leach lines. Encourage lateral root development by building a drip line and mulching the basin's surface with 2 to 3 inches of compost or humus mulch.

For optimum development, plant in full sun where the soil has adequate access to moisture and the pH is neutral (6.5 to 7.2). Because these trees are "water babies," it's almost impossible to overwater them. Watered too infrequently, the trees will stress and possibly perish.

The most efficient time to fertilize is in the early spring and fall. If your white alder is your garden's focal point and you want a standard tree form, prune for shape early. Remove all but a single trunk, and remove any branches below 4 to 5 inches.

-- Source: "California Gardener's Guide" by Bruce and Sharon Asakawa (Cool Springs Press, $24.99)