Of course, those who are not horticultural professionals should not be expected to be proficient with grafting but might want to recognize and correct minor problems associated with it before such problems become serious. Basically speaking, grafting is what brings the "understock" and "scion" together as one plant composed of two genetically different parts. Understock is what provides the unseen roots that the scion is grafted onto. The scion is the upper part of the grafted plant that is seen in the garden.

This is done to assemble a plant that exhibits the best qualities of both involved. For example, understock might provide a strong or more complaisant root system for a tree that would otherwise exhibit an inferior root system. Dwarf citrus are grafted onto a dwarfing rootstock that limits the mature size of the grafted trees, which might otherwise become disproportionately large for the garden. Weeping cherries and tree roses are actually grafted twice to assemble an appropriate understock, a mediary stock to provide the strong and straight main trunk and, finally, the scion of the desired weeping cherry or hybrid rose.

Suckers are stems that emerge from below the graft and are genetically identical to the understock. Almost all suckers therefore emerge from below the soil or very near to it. However, suckers may also emerge from the mediary stock of weeping cherries, usually immediately below the graft. When the understock realizes that it can divert resources to its own growth, sustaining the scion is not such a priority. Among weeping cherries, the mediary stock naturally grows more aggressively than the scion and has the advantage of extending vertically, consequently shading the pendulous growth of the scion. Suckers among purple-leaf plums are likewise advantaged because they have green foliage with more chlorophyll. Removal of suckers as they develop is therefore important to preserve the desired effect.

Variegated plants are mostly genetically unstable and consequently occasionally produce mutant growth known as a "sport" that is not variegated. Although physiologically very different from a sucker, a sport with more chlorophyll may easily overwhelm the desired growth and should likewise be removed as it appears. Sports originate from genetically similar growth and most are merely reversions to growth of superior genetic stability. In fact, most variegated cultivars originated as variegated sports among normally unvariegated plants. The most common species that may produce green sports from variegated cultivars include English holly, Algerian ivy, Pittosporum tobira, Euonymus, daphne and pieris (lily of the valley shrub).


Tree of the Week: Tristania laurina

I was reminded of the usefulness of Tristania laurina (which is known primarily by the Latin name) by a photograph of a street tree sent to me by Frank Ramirez of Sunnyvale. It has everything a street tree should have. Like related eucalyptus, it is very resilient and its roots are very complaisant. Unlike eucalyptus, it does not exhibit inferior structural integrity.

Tristania may be too small and too slow for those of us who like large trees, but it eventually becomes proportionate to residential streets and is easily pruned for clearance. As a street tree, it should be grown with a single trunk. It may otherwise be grown like olive trees, with multiple trunks, to exploit the interesting mottled bark. "Laurina," meaning "laurel-like," describes the narrow, three-inch-long leaves. The yellow flowers are small, but occur in profuse trusses in late spring or early summer.

Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.