Many species are so happy in California that they become naturalized, proliferating without assistance. Prior to the introduction of various pathogens during the past few years, eucalypti did not have much to worry about. It is unfortunate that so few of the many species of Eucalyptus are appropriate for compact urban landscapes.

Eucalypti certainly have their share of problems. Some become so large that they must be pruned for clearance of space shuttle traffic like some street trees are pruned for clearance for truck traffic. Many have inferior structural integrity and occasionally drop large limbs that can easily squash a car. All are very combustible. If you drive imports and decide to plant eucalypti in a sufficiently large and unused area where forest fires are not a concern, you should be aware of the primary weakness of the genus. As resilient as they are, eucalypti are very sensitive to being root-bound if confined too long.

"Bigger is worse" in regard to nursery stock of the various species of Eucalyptus, as well as most vigorous large tree species. Eucalypti, pines and most species of oak prefer to germinate from seed and grow into trees in the same location where they are to spend the rest of their lives. Since this is not practical, they are typically planted as canned or boxed nursery stock. Of course, those planting the trees would prefer the largest sizes available, which is contrary to the preferences of the trees.

Trees that have sensitive roots should be planted as the smallest size practical if they cannot be planted as seed. No. 1 (1 gallon) specimens are the "next best thing" to seed, but if such small trees were even available, they are simply too small and would likely desiccate before anyone notices. No. 5 (5 gallon) is a fair compromise but unsatisfactory for most garden enthusiasts. No. 15 (15 gallon) is the most common size for trees and should be the largest considered for the most sensitive trees, such as eucalypti and pines. Although also sensitive to root confinement, most oaks grow slowly enough to accommodate recovery of their root systems, so may be planted as larger boxed specimens.

Of course, trees that have fibrous or more confined root systems, as well as trees that vigorously generate adventitious roots to replace those damaged by confinement, may be planted as larger boxed specimens, such as 24-inch box, 36-inch box, 48-inch box, and so on. Crape myrtles are popular in all sizes and don't seem to mind confinement within boxes. Willows and poplars are so vigorous that they are difficult to contain within boxes, but rapidly replace damaged roots. (However, willows and poplars grow so rapidly that there is no need to plant large specimens.) Regardless of the root-system behavior of the various species, any circling roots should be severed during installation.

Even trees that are not so sensitive to being root-bound should not be disproportionately large for their associated root systems. For example, the root system of No. 15 crape myrtle may not adequately sustain the moisture demands of a disproportionately large foliar canopy.

Foliage of the Week:
Plantain lily

The small pale blue, lavender or white tubular flowers that bloom on delicate stems for a few weeks during summer are merely an added bonus to the bold basal foliage of the plantain lily, Hostas. Foliar color among the various cultivars ranges from metallic blue to deep green to bright yellow and may be striped or bordered with white or yellow variegation. Leaves may be as short as 6 inches among dwarf cultivars or as long as 5 feet among larger cultivars. Foliage abscises during winter dormancy, but is more than replaced in spring. Minor shade is preferred and more substantial shade is tolerable. Irrigation must be applied regularly (but not excessively) during warm weather. Plantain lily is unfortunately very popular among snails.

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