Hardiness information is given for each plant. In general, the ranges of minimum temperatures for plants of varying degrees of hardiness are as follows:
hardy – tolerates –15°C (5°F) and below
frost-hardy – survives down to about –5°C (23°F)
half-hardy – survives down to 0°C (32°F)
semi-tender – damaged by temperatures below 5°C (41°F) unless protected
tender – must be overwintered indoors at 7°C (45°F) or higher, depending on provenance
The hardiness ratings I assign are based on my own experience in the British Isles. If you live in a temperate climate with hotter summers like the ones in much of the United States, you may find that the exotics you grow are hardier than I indicate. This may be due to the fact that the root mass grows larger during warmer summers and thus the plant is able to withstand cooler winter temperatures than it would were its root mass smaller.
I have deliberately avoided discussing the hardiness zones established by the U.S. Department of Agriculture (USDA) in the United States as I want to encourage you to push against boundaries, experiment with different care and maintenance regimes, and make the most of microclimates in your garden. Gardeners in a wide range of zones may be able to provide for an exotic plant’s cultural requirements using any number of methods, the most extreme of which is overwintering indoors or under cover. Proving my point, it is interesting to note that a growing number of members of the Pacific Northwest Palm and Exotic Plant Society, based in British Columbia, live in USDA hardiness zones 5 (minimum temperatures –20° to –10°F or about –29°C to –23°C) and 7 (minimum temperatures 0° to 10°F or about –18°C to –12°C), with a few members having success even in zones 3 (minimum temperatures –40° to –30°F or about –40°C to –35°C) and 4 (minimum temperatures –30° to –20°F or about –35°C to –29°C)!
The best way to determine how cold your garden gets in winter is to purchase a “maximum-minimum” thermometer and place it in different parts of your garden, measuring the warmest and coldest spots, especially during the night. The warmest areas are invariably near the wall of a building. A south-facing wall will always be warmer than a north-facing one.
Many of the plants included in the Exotic encyclopedia are generally available at your local garden centre, and a good number can be found at specialist nurseries dealing in exotics (some of which deal in only one or two plant families; see “Where to Find Exotics” at the back of this book). Most specialist nurseries have a Web site where you can purchase plants online and have them delivered to your door, from locations all over the British Isles and the United States. Buying plants from abroad via the Internet is fairly easy, though comparatively expensive because of the often-hefty shipping costs and the necessity of purchasing a phytosanitary certificate so that they can legally enter the country. For those who aren’t computerised, the annual RHS Plant Finder is an essential book listing more than seventy thousand plants and where to buy them, a great source for those difficult-to-find must-haves. Finally, some plants you can only obtain through who you know. Making friends in the exotic world is a sure way to expand your exotic garden while at the same time adding like-minded people to your circle.
You should note that some exotic plants, like tree ferns, can be rather expensive acquisitions, as they have been transported from the Southern Hemisphere. They are also slow growing, taking many decades to get up to a reasonable height. Nevertheless, tree ferns are still worth every penny as they dramatically change the feel of the garden with their rugged trunks and delicate palmlike fronds. Some of the other plants mentioned in this book, such as eucalyptus, should be bought as small 45-cm whips, as they will soon overtake larger specimen plants and grow more rapidly, at a fraction of the cost.
If you want to know what other exoticists are growing in your country, it is a good idea to visit exotic gardening chat groups, boards, and forums on the Internet. In the British Isles there are several such cyberspace meeting places, such as the UK Oasis, where all types of exotic plants are discussed in great depth. In the United States there are likewise many forums dealing with exotic plants to a greater or lesser degree.