One of the factors behind a successful and satisfying garden composition is the intelligent use of trees and shrubs that have a definite and clear form. A pencil thin Cypress for instance, creates a clear vertical accent, and draws the eye upwards. A contrasting effect, horizontal in nature, is created by trees that possess an umbrella, flat-topped shape. In general, plants with a strong, definite line enhance both the sense of order in the garden, and its natural feel. Umbrella-shaped plants are therefore ideally suited to small home gardens. Two fine examples that stand out for gardeners in Mediterranean climates are Delonix regia and the Silk or Mimosa tree. (Albizia julibrissin)
Delonix is only suitable for warm winter locations. It has zero tolerance of frost, and is best avoided if the winter minimums reach below 10c. However, in the right place it can be one of the most unforgettable garden treats imaginable. In the tropics, where it originates, it grows to about 12 meters in height and as much in width, although in climates with cooler winters, it has a more squat, flat shape, being very wide, but relatively low. Delonix's leaf texture is delicate and feathery, but most people are struck by the spectacular red blooms that adorn the tree in the summer, followed by the dramatic looking fruit that hang from in long pods. Delonix is not suitable for planting as a shade tree near a house, as its roots are very aggressive and invasive.
The Silk tree cannot compare to a truly great Delonix, but as it can be grown in locations that receive light frosts, it is a far more viable option in many Mediterranean regions. Sometimes, the leading shoot has to be clipped, causing the tree to adopt the desired umbrella form. Mimosa's leaf texture, like that of Delonix, is light and feathery. It also adds color to the garden with its pin-cushion, pinkish red flowers. It associates brilliantly with Calliandra, a large shrub from the same botanical family, (Fabaceae) the latter's blooms being almost identical in color and shape. Mimosa is a slow starter tending to take a few years before growing fairly rapidly to 4-5 meters.
A number of shrubs and bushes can also be used for the purpose of creating a low, flat appearance. One of the best and easiest to grow in this manner is the Chaste tree, Vitex agnus-castus from West and Central Asia, or its Australian counterpart, Vitex trifolia. These plants can be limbed-up to reveal interesting branch architecture, while the tops can be pruned now and again to maintain the desired height. Furthermore, their addition to the planting scheme, strengthen the umbrella motif established by the trees.
My name is Jonathan Ya'akobi
I've been gardening in a professional capacity since 1984. I am the former head gardener of the Jerusalem Botanical Garden, but now concentrate on building gardens for private home owners. I also teach horticulture to students on training courses. I'd love to help you get the very best from your garden, so you're welcome to visit me on http://www.dryclimategardening.com