ORNAMENTAL PLANTS OF HORTICULTURE VALUE

How to succeed with the cultivation of new plants

It is desirable be able to predict the successful growth of a plant in a new environment. Climate is the principal factor that determines the general nature of the vegetation in a given part of the Earth. This explains why the method of climatic analogies has become very popular. When botanists transfer plant species from one place to another, they compare the climates of both places. In general, the more similar the climates are, the more successful the transfer will be. Several examples can be given to demonstrate the importance of climatic similarity. Thus, Eucalyptus species do well in California because of the similarity of the climate to that of their native home in Australia. The Mediterranean region is the native home of the olive (Olea europaea) and the fig (Ficus carica), and both species are widely cultivated in California, where the climate is about the same. Trees and shrubs have trunks and branches exposed to the elements all the year around. Evergreen trees and shrubs are usually show more obvious damage from harsh weather than deciduous trees because their leaves are exposed to climatic extremes. Although climatic comparison helps to predict future adaptation, there are many exceptions to this rule, and problems of plant introduction can rarely be settled in such a simple way.

The successful introduction of herbaceous plants depends on factors other than climatic similarity. It depends primarily on the properties of the plants themselves. Thousands of plant species taken from all over the world have been tested at the Komarov Botanical Institute in St. Petersburg. The city is located at 59°55' N, and 30°18' E, and has an extreme annual minimum temperature of -36° C (-33° F), and an extreme annual maximum temperature of 34° C (93° F), and an average annual rainfall of 673 mm (26.4²). As expected, many, more southern plants perished during the first year after transplanting. Survivors fell into three adaptive groups. One group grew successfully, developed flowers with mature seeds every year. Plants of the second group flowered but set no fruits, whereas those of the third group did not flower every year and did not regularly produce fruits. All three groups included plant species of different origins, but herbaceous plant species taken from any one region would vary, demonstrating all three levels of adaptation. The successful introduction of herbaceous plants depends on the intrinsic rhythm of their seasonal growth as well as structural characteristics.

Herbaceous plants are enormously diverse, differing in their aboveground structure (erect or prostrate stems), underground organs (taproots, rhizomes), life span (annual, biennial or perennial) and growing season behavior (evergreen, deciduous or ephemeroid). Unlike perennials, annuals do not grow through winter conditions and thus many of tropical origin flourish in northern cities. For example, the Brazilian Salvia splendens grows successfully in St. Petersburg and in Kirovsk, above the Arctic Circle, flowering there. These plants however, can not produce seeds during the short summer.

Perennial herbaceous plants may be divided into groups using their structure and habit. Classification based on underground parts distinguishes groups with taproots, rhizomes (long and short), corms, bulbs, tubers, and stolons. Each of these groups is characterized by a special way to grab the territory. Growth of the shoot system is determined by its apical meristem. When branching is monopodial, the shoot is formed by the activity of a single meristem, and such herbaceous perennials usually produce evergreen rosette leaves and axillary, often leafless, flowering stems known as scapes. When branching is sympodial, the plant produces a series of shoots, each derived from its own apical meristem. Such plants often have stems elongating at flowering time. Both types of branching are closely associated with the seasonal rhythm of the plants. Life forms vary in habit, responsive to the length of the growing season. Some plants have green leaves throughout the entire year, while others have a short dormancy, and yet others have a long and deep dormant period that cannot be broken. This growth rhythm is very important when a plant meets new climatic conditions. Underground organs are also significant to sustain a plant through environmental stress. Gardeners should consider all these structural characters when growing a new plant.

In order to thrive, the plant rhythm of an introduced plant should coincide with that of its new climate. In this respect, the shoot system is the most important indicator for success. Any particular place in the Temperate Zone may have its own crucial season for new plants. A short growing season and a long adverse winter characterize St. Petersburg. Winters in some years have many thaws and the snow cover may melt repeadetly. To best adapt here, plants should have a short growing period but a long deep dormant one, and such plants do well in St. Petersburg regardless to their place of origin. For example, Incarvillea delavayi of western China, which belongs to the tropical family Bignoniaceae, does well in St. Petersburg. Its growing period coincides with St. Petersburg’s short summer and it requires only a short period of warm weather to grow. It starts to grow in late spring (mid May), when the average air temperature is about 12-15o C (54-59° F), and after flowering and fruiting, its above-ground parts die back before the first frost comes. Its renewal buds are situated deep in the soil, and they over-winter successfully. Many other introduced plants similarly behave, and their phenology (flowering, fruiting) corresponds to the weather, likewise as native plants. They come into flower at a definite air temperature sum (effective temperature), varying by months depending on the weather conditions, but always track to the effective temperature. For example, wild peony (Paeonia anomala) flowers every year exactly when the sum effective temperature in St. Petersburg is 450-470° C. Every plant species requires sufficient warmth (sum temperature) for its annual growth, including new shoots, flowering and fruiting, and the formation of renewal buds.

Failure to adapt results when a plant requires much more warmth than it receives in a new place to produce flowers, buds or seeds. Unfortunately, such a plant species then shows little promise of successful introduction. However, some desirable ornamental plants that flower and produce renewal buds every year can still be grown in a new region even though they fail to produce mature seeds. New England aster Aster novae-angliae and Michaelmas daisy A. novi-belgii are grown in the northern regions of the FSU, but simply do not produce seeds. Therefore, many herbaceous species can be grown beyond their hardiness zones, in colder climates.

In some plants almost all the leaves are crowded in a rosette, and the flowering stems may be leafless or few-leafed. This leaf rosette persists the entire growing season, whereas the flowering stem develops a shorter one. Under certain conditions, these plants can grow in an area with a climate not coincident with their own rhythm. Given an appropriate period during the growing season, the plants produce flowering stems and fruits, and overwinter under the snow cover. However, these plants suffer greatly if during the winter the snow melts and freezing temperatures follow. The burgeonic leaves and open buds can then be killed by the frost. Successful growth depends on the type of life form, particularly on shoot structure or "plant architectural model." In one "architectural model", the epicotyl axis is short and either unbranched or little branched, represented by a rosette of leaves. In this case new annual growth lies contiguous to the old growth from previous years, with underground stems dwarfed. Damage to the rosette leaves and open central bud will lead to the death of the plant. In second "architectural model" the formation of a rosette of leaves (stem with short internodes) is only the first developmental stage of a perennial shoot; the shoot system is sympodial and each shoot produces an elongated stem when flowering. Plants such as these usually have many dormant buds that can resume growth when the rosette leaves are killed by frost. Consequently, these plants can grow in areas with a climate markedly colder than that of their native habitat. Life form and seasonal rhythm (growing season) of herbaceous plants are responsible for successfully growing in a new climate.

It is important to maintain a record of Man’s achievement in plant introduction whether accidental or intentional. The experience accumulated with plants at different botanical gardens is of great interest. The problem of cultivating native plants and also special groups are presented in numerous Russians botanical gardens publications and should be taken into consideration. As is known the costs of handling ecologically specialized species are often very high and can only be justified in relation to research. There are often difficulties in transferring what is learned to situation in the field. For any future programmers of new wild introduction cultivated collections provide an important source of research material. The list of the main FSU botanical gardens is given in Appendix; the main publications are presented in the list at the bottom of this part.