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Foliage can offer a garden color in addition to green
By Tony Tomeo
Back in 1988, while I was still attending Cal Poly, San Luis Obispo, my roommate and I entered the landscape competition of our horticultural department during the annual open house which was then called Poly Royale. The theme of the landscapes was "shades of green" referring to the use of plant species with interesting foliar color. When I look at photographs of our landscape now, I wonder how we could have possibly won second place; it was quite bad.
However, the concept was interesting. Because most garden enthusiasts pursue showy or fragrant flowers, other potentially ornamental features, such as foliar color, fruit (berries for example) or plant form are often not considered.
Plants with interestingly colored foliage may be trees, shrubs, vines, perennials or annuals. Their colors may range from gray, blue, red, bronze, gold and yellow. Many plants are variegated, including various colors on the same foliage.
One of my favorites trees which exhibits brilliant gray during the cold part of winter is the silver mountain gum, Eucalyptus pulverulenta. Of course, Colorado blue spruce, Picea pungens is excellent for blue. Beech, Fagus sylvatica is available in classic bronze, as well as purple, or with white, pink and green variegation. A classic for gold is golden honeylocust, Gleditsia triacanthos 'Sunburst,' though I always thought that they simply look chlorotic. There are many more examples than I can possibly write about.
It is useful to remember that many unusually colored cultivars of species which are normally green may be weaker and slower growing than their green relatives. The golden honeylocust, for example, usually grows slower and is more susceptible to insect infestation than a normal honeylocust with green foliage. (A cultivar is a variation of a species and is cloned so that all resulting individuals are genetically identical. All golden honeylocust are grown from an original mutation which exhibited gold foliage.)
Such weakness is more common in gold, yellow or variegated plants than gray, blue, red and bronze. This is because yellow or gold color, or variegation, usually results from a lack of chlorophyll rather than additional pigment in the foliage as with the other colors.
These weaker cultivars are also more susceptible to "reversion." This means that a healthy plant may put out growth which lacks the desirable color or variegation and continues growing with "normal" green foliage. This happens because most of these cultivars originated from mutations which are genetically unstable.
It may also occur in plants which owe their unusual color or variegation to a viral inoculation, if vigorous growth inhibits the perpetuation of the virus. Some garden enthusiasts find reversion amusing and maintain plants with both normal green foliage and unusually colored foliage. However, unless strictly maintained, green parts grow faster and shade out slower growing "mutant" parts, and the entire plant may eventually be green
Most plants with colored foliage are most colorful this time of year when new growth emerges. With many, color fades as foliage matures. Some start out with colored foliage which turns completely green by summer. Photinia, Photinia fraseri, is not normally grown for the showy red new growth which emerges in spring, but for fast growth and adaptability to hedging. The bronzy red spring growth is an added benefit for those of us who think of photinia as a large green shrub.
Along with the new spring growth appearing on landscape trees and shrubbery, late blooming and summer blooming bulbs planted earlier should be appearing now. Gladiolus and watsonias may be planted as late as the bulbs can be found in nurseries. The blooming season may be prolonged if groups of bulbs are planted in two to four week intervals. Although fewer flowers will be present at any one time, cumulatively, the same amount will be produced over a longer season. If storing bulbs, be certain not to store them so long that they desiccate or rot. If stored bulbs begin to grow, they should be planted immediately.
Tubers and tuberous roots which would rot if planted in autumn with other bulbs may be planted now. Plants should start to grow immediately and through the summer. Calla and canna grow fast and may bloom by earlier summer. Dahlias put on their best show after middle summer and into early autumn. Tuberous begonias grow fast and bloom early; but require rich soil and considerable care. I recommend them for the more experienced garden enthusiasts.
Tree of the Week: Silver Mountain Gum
My father always told me that the best eucalyptus is a dead eucalyptus. However, there are so many useful species available. The silver mountain gum, Eucalyptus pulverulenta, exhibits excellent silvery gray foliar color, particularly in winter. Although it stays small compared to most eucalyptus, it can reach 30 feet in height and is usually low branched and sprawling with an irregular branch structure. At maturity, the trunks are disproportionately bulky and shaggy with coarse bark.
They may be used in close proximity to a garden to display the large trunk; or at a distance, using the silver canopy as a background to a garden. They tolerate just about any growing conditions, except excessive water. Although they are not as messy as most eucalyptus, they drop foliage, twigs and seed capsules which may be toxic to other garden plants if not removed.
Horticulturist Tony Tomeo can be contacted at 408.358.2574.
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Saratoga, California Since 1955
