JARS v63n2 - Tips for Beginners: How Plants Survive Dry Summers

Tips for Beginners: How Plants Survive Dry Summers
Reprinted from the North Island Rhododendron Society Newsletter, March 2007

Will we have another dry summer this year like that in 2006? Who knows at this point! But we can plan for it by considering ways to prevent the loss of plants.

Plants use various strategies to help them survive in hot, dry conditions. For example, some have long tap roots that delve down to find all available moisture. Others, such as Hebe and the Staghorn Sumac, Rhus typhina , have shallow, wide-spreading roots that will seek out all available water.

Some plants' leaves have adapted to limit the amount of moisture lost through transpiration. Thyme and Baby's-breath ( Gypsophila ) leaves are tiny, for example, so there's just a small surface area for moisture loss on hot days. The feathery foliage of Artemisia (e.g., sagebrush) work in much the same way. Lavenders and Eryngium (e.g., sea holly) have silvery leaves that reflect light, while Phormium have tough-coated leaves that hold on to moisture.

Another water-conserving strategy is to go into a dormant state to save energy when the going gets tough, as bulbous plants do. Diascia will sit flowerless during dry spells.

Beth Chatto lives in Essex, a very dry part of England, with usually about 500 mm (20") of rain per year. Contrast that with our area on the south-east side of Vancouver Island, British Columbia, where we might have half that amount during a normal 4-5 month summer season (some years only 1/4 that amount), but we also have about a meter (30-40") of rain during the winter. Beth has a "gravel garden" which has not been irrigated for 14 years. How does she grow plants in it?

This part of her garden started out to be a horticultural experiment, to see which plants would survive when hosepipe watering was banned. She explained: "when temperatures soar and there has been only the merest dribble of rain for weeks or even months [this sounds like our last summer in coastal BC], it takes courage to stand and watch. If plants look stressed, we go in with secateurs rather than the hose."

Yet by choosing plants adapted to dry conditions, the garden is a rich tapestry of shape, texture and colour in every season. Even in January, with hardly a plant in flower, it is furnished with grasses, seed heads and a variety of grey and silver plants, all set off by contrasting clumps of Bergenia foliage of shades of wine red, crimson and mahogany.

"I'm surprised and delighted by the woody plants such as Juniperus scopulorum 'Skyrocket' and the Juneberry, Amelanchier lamarckii that have tolerated the conditions," she says. "Over the years the picture has evolved and dominant plants mature. The foreground consists of contrasting shapes and textures to cover the ground, and give an overall effect of simplicity." Plants that looked absolutely happy and comfortable on a September morning included Tufted Hair Grass ( Deschampsia caespitose 'Goldschleier'), Sedum 'Herbst-freude', grey-leaved Cistus and Caryopteris xclandonensis 'Heavenly Blue'.

The land had been used as a car park for 25 years and was 0.1 ha (¼ acre) of heavily compacted gravelly soil, irregularly shaped and backed by a tall hedge. In the winter of 1991-92 it was cleared of tatty grass, and the ground broken up to a depth of 0.3 m (12"). "After leveling and a light roll, I used hosepipes to indicate where the beds should be," explained Beth. "Once the planting areas were mapped out, as much home-made and mushroom compost and bonfire waste as possible was added to give the plants a good start. All were soaked before planting, and then watered in thoroughly. By the time roots reach the underlying sand and gravel, the plants have become established and the poor conditions promote tough and wiry growth, making them more resistant to both drought and winter cold."

We have to remember that most of Britain is Zone 8 or 9 all around the edges and only a small area around Perth, in Scotland, is Zone 7. Our BC area is partly Zone 7, partly Zone 8, and a few lucky members live closer to Zone 9. This means that several plants in Beth Chatto's garden are not hardy enough to be left outside all winter here on Vancouver Island. If you want to try a gravel garden, do a little substituting, or plant a particularly tender specimen in a pot, lift it for the winter, and fill the hole with a hardy dwarf cedar, for instance. The plants Beth Chatto recommends for a gravel or scree garden include Agapanthus , Allium flavum , Diascia , Gypsophila , Lavender , Nepeta nervosa , Nerene bowdenii , Oregano, Phlox 'Kelly's Eye', Salvia , thyme and Sisyrinchium .

There are many varieties of the plants listed that would be suitable. Points to remember - remove the top layer of soil if it is infested with weeds. Surround the bed with dry-stone walling, and place a layer of stones at the base of the bed, to ensure good drainage. Then fill the bed with compost-enriched soil mixed with washed gravel. In the spring, plant groups of small accent plants (3 or 5) such as box or tiny junipers, shapes that will complement each other. Plant compact low-growing specimens such as sedums and sempervivums. In the fall, add some species such as crocus or tulips for colour in the spring. Gypsolpila repens or thyme can be planted so they will spill over the edges of the stone walls. Keep the bed weeded and groomed, and the following spring add a thick gravel mulch.