Desirable Plants PLANTS FROM OLD-WORLD WINTER-RAIN CLIMATES
SUITABLE FOR ROCK GARDENS IN VICTORIA, BC
 Compiled by H. Roemer for V.I.R.A.G.S., April, 1996

INTRODUCTION

The following selection of rock garden plants is based on the assumption that species originating from an area with similar climatic parameters to our local ones are the most likely to do well in our gardens. While such an assumption is dictated by logic, it is nevertheless obvious that the model for gardens and garden plants in Victoria - and maybe throughout the summer-dry Pacific Northwest - goes back to gardens in Britain and central Europe which are subject to a quite different climate. It is true, temperature averages and general hardiness limitations are similar there. However, it is the distribution of precipitation through the seasons which makes the important difference, other things being equal.

One of the most practical and simple tools for comparing climates worldwide is the climate diagram according to Heinrich Walter (e.g. Walter, 1984). Only four examples of these diagrams are shown here, but numerous diagrams, as well as maps of climate types were compared for the purpose of delimiting the area from which suitable plants are available. In these diagrams dark shading above the temperature curve shows periods of water surplus, while light areas below the temperature curve show periods of water deficit. Black shading above the 100mm/50°C line is precipitation amount at 1/10th of the normal scale.


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The Utrecht diagram shows the general precipitation pattern in western Europe, with a peak at the height of the growing season. No water deficit occurs on average at any time of the year. Summer-rain climate is prevalent anywhere in western, northern and eastern Europe and indeed is also by far the most common climate in all temperate zones of the earth. Peaks of summer precipitation tend to be steeper in the more continental areas of Europe and Asia and water deficit or drought periods, if at all present, are between late fall and early spring. Winter drought periods and huge growing-season water surplusses are common in Himalayan countries and China, here illustrated by the Kun-Ming diagram. Both the European and this pattern is clearly opposite to that shown in the Victoria diagram. The distribution of precipitation here (as well as in the entire area between about Comox-Vancouver-Seattle-Portland) is much more similar to that of the Tripolis diagram and to that of the Mediterranean realm generally. Average temperatures are of course lower here, but a large number of mediterranean plants are completely hardy in our area due to the maritime, mild winters.

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The Mediterranean area is the only large area where the summer-dry pattern prevails. For this reason, and because information on mediterranean plants is so readily available in the literature, the following list is restricted to that area. Other, smaller areas with this climatic type - and which may yield yet more well-adapted plants for our region - are parts of Chile and Argentina, coastal California, the southwestern Cape area of Africa, and parts of southwestern Australia. In most of these, plants would have to be recruited from higher-elevation portions to be sufficiently hardy in our latitudes.

The principle to select garden plants according to matching climates of origin has been practiced by the author through the past ten years and has been very successful, judging by the performance of the chosen species. Accurate descriptions of the natural distribution and habitat in the literature is of course a prerequisite to such selections, as is the familiarity with the world's climates. This approach has several attractive features: 1) It holds the potential for adding many species to the list of those suitable for local gardens. 2) It is capable of explaining our failures, for instance with Himalayan plants which are adapted to such deviating conditions, and allows us to make adjustments where possible. 3) Finally, it leads to growing gardens that are much less dependent on frequent watering throughout the summer.

PLANT LIST

Readers are encouraged to search the listed species in gardens and nurseries, seed exchanges and suppliers' catalogues. Asterisks following entries in the list denote plants known to the author from local gardens. Double asterisks indicate species which have done particularly well in the author's rock garden which is located mostly on south-facing slopes. No, or only short-term experience is available for the unmarked species, but they are expected to perform well and should be tried wherever obtainable. Additions to this list and observations on any, in particular the unmarked species, are welcome.

Go to the list of desirable plants