Pushing the Boundaries of Hardiness

Many of the exotic plants grown in temperate gardens today were until fairly recently considered viable only in the warmer southwestern parts of the British Isles and the more southerly parts of the United States. Climate change has been a significant factor in changing the contents of our gardens, along with encouragement to experiment with plants traditionally thought of as warm-zone or hothouse plants. This follows an established tradition: when camellias were first introduced into England they were grown in large greenhouses, and it was only when heating became more expensive and glasshouses fell into ruin that they were discovered to be actually hardy!

A number of exotic plants introduced from the colder extremities of the tropical regions of the world adapt especially well to gardens in the temperate northwestern United States, which runs in a band along the seaboard stretching from Oregon through Washington to southern British Columbia in Canada, as well as many parts of the British Isles and northern Europe. These are hardy exotics, meaning they have a subtropical appearance but take cool summers and relatively cold winters well. It surprises many gardeners to find that palms, for instance, are hardy in a maritime climate. In the last decade or so, windmill palm (Trachycarpus fortunei) has been popping up everywhere from private gardens to parks and municipal gardens, even roundabouts, as it is hardy in all but the coldest locations. Bamboos are also enjoying a revival, with many of the noninvasive species and cultivars being suitable for smaller gardens as well as containers, where they give a very Asian feel with their fabulous culms and frothy foliage. And the false castor-oil plant (Fatsia japonica), with its large, palmate, glossy evergreen leaves, has a subtropical appearance but hails from cool areas in eastern Asia and thus is hardy in temperate gardens.

Numerous cacti and some succulents take considerable amounts of frost in their native habitats and work well in the garden provided you are aware of their need for exceedingly well-drained soil. Many cacti, for instance, will take lows of –10°C for weeks or months in the wild where it is very dry or where they have a blanket of snow over them, but they will not tolerate the damp cold or slushy rain of a maritime climate. If these frost-tolerant plants are grown on excruciatingly well-drained soils or have a canopy put over them to keep out winter rain, or alternatively are planted in the lee of a south-facing building, they will not only survive but also thrive, with a modicum of care and attention.

Over the last few years I have been experimenting with plants more strictly described as houseplants, including many bromeliads as well as Chlorophytum, Monstera, Tradescantia, and Platycerium. Some visitors to my garden look at these and say, “You can’t grow those outside—especially on the supposedly chilly east coast of England!” Luckily, though, the so-called houseplants do not know this, happily taking root and growing vigorously through our warmer months (a style much used by the Victorians), apparently unaware of the fact that they should really be sitting on a window ledge in my house.

Tradescantia species and hybrids (wandering Jew), although normally considered nonhardy, have become naturalised in my garden in recent years, dying down to the ground in autumn and returning in late spring. According to many authoritative reference books, they will take a minimum of 5°C! I discovered their hardiness through trial and error. This is one of the best and most exciting ways to garden. When I planted out my first root-hardy Musa basjoo (banana) some quarter of a century ago, scant information was available at that time regarding their cultivation and hardiness, apart from the classic book The Exotic Garden by Myles Challis (published in 1988 and sadly out of print). I had no idea whether it would survive, but with some experimentation with different types of protection, I now have several M. basjoo grown to perfection and forming a large clump reaching 5.5 m tall or more every year. They give a spectacularly architectural effect with their giant paddlelike leaves. In the last few years I, along with many other exotic enthusiasts, have discovered that with our recent comparatively mild winters, they haven’t needed any protection at all! Even if a hard winter cuts them down to the ground, they will vigorously reshoot the following spring.

One of the most important factors in this style of gardening is your garden microclimate. Microclimates are climatic variations that occur in comparatively small areas. Factors that influence microclimates in a garden can include sloping ground, exposure to sun or the lack of it, proximity to large areas of concrete or bodies of water, and prevailing winds. A garden surrounded by a wall or hedges and/or in a city is likely to be much warmer than those in the open countryside. There are even mini-microclimates within gardens; for example, plants grown near a south-facing wall pick up its latent heat and thus tender selections that might not survive in a more exposed area of the garden will thrive there. A shelterbelt of trees or tall buildings will also protect your choice plants. For instance, palms like Trachycarpus fortunei have a tendency to becoming rather tatty in exposed gardens, while those surrounded by other trees become more lush and verdant and develop larger, healthier fronds.

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