Q: We have a volunteer black oak tree growing on a slope above a 4-foot retaining wall above our pool. Our neighborhood has many black oaks but not on our property. (We have blue oaks and valley oaks, but mostly live oaks.) Long term, this is not a viable location for this tree, and I would like to transplant it. It is about 3 feet high and has been watered by the sprinklers for the ground cover planted at its location. Is transplantation possible, and what type of location should we place it in?
Cathy Maloney, Shingle Springs
A: Before moving your tree, let’s look at black oak basics.
In general, black oak grows best on medium- to coarse-textured, deep, well-drained soils, forest sites and moister hardwood rangelands, according to UC master gardener Annie Kempees. These oaks are deciduous and can tolerate some shade as a seedling, but none at maturity.
Bordering counties of the Central Valley where black oaks grow are Placer, El Dorado and Amador, elevation 200 to 6,000 feet. Black oaks often grow with tan oak, madrone and mixed conifer forest species, coast live oak, interior live oak and blue oak. Black oaks generally have a tough time in the Central Valley or near the Pacific Ocean.
The form of California black oak varies greatly. On the fringe of its range and on marginal sites, black oak trees assume a scrubby form. In closed stands on good sites, the oaks tend to be tall with straight trunks and thin crowns. When open-grown, black oaks generally fork repeatedly, becoming multi-stemmed and broad-crowned.
Position on long continuous slopes also influences growth and form. Trees at the toe of slopes or on gently sloping benches, where deeper soils are likely, generally grow best and have good form. Those at midslope are shorter and more scrubby. On upper slopes, trees grow slowly and are even shorter.
Aspect also influences growth, with east being best for maximum growth. Identical 100-year-old trees averaged about 85 feet in height on east-facing exposures, 72 feet on northern aspects, 68 feet on western exposures and 56 feet on southern aspects.
Black oak seedlings often reach heights of 4 to 6 inches and extend their taproots downward as deep as 30 inches in the first growing season. Development of a deep-thrusting vertical root is necessary for seedlings to cope with the hot, dry summers characteristic of California black oak’s range.
For the first few years, therefore, both lateral root development and shoot growth are slow. Shoot growth probably does not begin to accelerate until root capacity is extensive enough to obtain adequate moisture. This may take six or seven years – or longer.
Shoot growth of some seedlings, particularly those stressed by competing vegetation, never accelerates and these seedlings eventually die.
Survival rate for a transplanted California native plant is only 50 percent. The challenge with transplanting oaks is ensuring that you can remove enough of the root system to support the “shoot” system above ground.
It would be best to attempt transplanting in the dormant season (December-February), then provide sufficient irrigation through the first couple of summers.
Young black oak seedlings are killed mostly by drought and pocket gophers. Grasshoppers and other insects damage young seedlings, and freezing by late spring frosts injures them, too. These injuries usually are mitigated by sprouting from the root crown.
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