Thursday, 24 April 2008

Two Timers - Bimodal climate change survivors

Alice's article about planting for climate change has been published in this months Garden Design Journal. Read more about bimodal plants that can ride the storm.

With changing weather patterns, designers may need to consider a palette of plants that are able to cope with more than one extreme.

The capricious weather we have been enduring over the last few years has highlighted the need for a new approach to our planting choices. In the past we could identify gardens as 'boggy' or 'dry' with some ease, and then plant them accordingly, but things are becoming less and less predictable. Weather extremes of flood and drought can both occur in the same garden within a short period: often without warning. If we can no longer rely on the accepted weather cycles of previous years, we must take action to adapt our planting designs to cope with the unpredictable impact our volatile weather may have on growing conditions.

In recent years we have all become experts in drought tolerant planting, but for the unpredictable weather challenges ahead we will need to develop a sophisticated arsenal of bimodal plants: that is, plants which can cope with more than one type of weather extreme.

Interestingly, plants that typically grow in moist habitats often survive suprisingly well in drier soils (so long as the soil is reasonably fertile) but the opposite is rarely true. Plants that are adapted to dry conditions often do not cope with flooding and waterlogged soils.

In fact, as a general rule, all grey and silver leaved plants hate flooding. This includes drought tolerant plants such as senecio, buddleja, echinops, eryngium, thyme and lavender. Therefore it is best to avoid using all of these mediterranean plants unless they are in planted in pots or raised beds, or have excellent drainage.

Beech and Cherry trees are known to be particularly vulnerable to flooding, so a sensible stormscaping choice would be to substitute them with storm proof alternatives that offer similar visual characteristics. Why not try using hornbeam as an alternative to beech for trees and hedges, as this tolerates wind, drought and flooding. As an alternative to the cherry tree, why not plant amelanchier: whose clouds of spring blossom are unrivalled in my garden. Amelanchiers also provide exquisite autumn colour – and if planted as a multi-stem specimen can act as a mini windbreak. An alternative source of storm proof autumn colour can be found in the form of the vibrantly coloured Liquidamber styraciflua or Acer saccharinum. – both of which tolerate extreme of flood and drought with some ease.

Few trees, except the large swamp Cyprus (taxodium distichum) can tolerate more than one month of submersion during the growing season but as a general rule, broad-leaved trees tolerate flooding better than conifer trees. Try alder (Alnus) river birch (Betula nigra) hornbeam (carpinus) willow (salix) and ash (fraxinus) for your best chance of success.

Most woody shrubs are not any better adapted to flooded conditions than trees. Common casualties of extreme weather conditions include Forsythia, Daphne, Camellias and Yew: who simply panic in waterlogged soil. These can be grown in a pot, or substituted for plants that can more effectively tolerate volatile weather conditions.

We can probably all think of a few ’hard as nails’ shrubs, such as Fatsia japonica and Viburnum tinus, that can tolerate almost anything you throw at them but wouldn’t it be boring if these were our only options for stormproof shrubs. Cercis Canadensis, one of my all time favourite planting choices, is surprisingly storm tolerant – and can cope with long term drought as well as a good few days of standing water. Hibiscus moschetos is another fantastically elegant shrub that is suspiciously hard to kill!

Cornus mas and Berberis thunbergii are two other shrubs commonly reported to be tolerant of flooding: even during the growing season. An ideal storm and flood tolerant plant partner for your cornus would be asplenium, Liriope, Aquilegia or dryopteris filix mas. Convallaria majalis is another woodland perennial that survives both drought and flooding but it is remarkably invasive and can be hard to control in a planting scheme so it is a less ideal choice on a domestic garden scale. Avoid hellebores and tiarella entirely as they so not cope well with waterlogged soil.

My ‘wonder plant’ award must go to the ornamental grasses – specifically Calamagrostis x acutiflora ‘Karl Foerster’. This wonderful grass is a deserved favourite amongst garden designers, and it is easy to see why. This plant seems to cope with almost anything I throw at it. It will happily grow in saturated soil and standing water . It copes effortlessly with drought and is tolerant of extremely acidic soils. It will return to a stiff upright position after having been bent flat to the ground by 80mph winds and it will stand strong through a soggy winter. My other stormproof favourite is Panicum virgatum: especially ‘Northwind’ and ‘Strictum’ which offer similar ‘superhero’ characteristics, but a softer effect.

Bulbs notoriously hate to be left standing in water, especially Eremurus, and are likely to rot away in waterlogged soil. In gardens where flooding is a prevalent threat, you may be better to plant your bulbs into pots in the autumn and treat them as seasonal bedding to get round this issue: planting them out into the border just a few weeks before they are due to flower. In gardens that only suffer from minor waterlogging, try Camassia - which originates from the moist meadows of North American prairies.

Although flooding is a natural event that can’t be prevented, you can be prepared for it. We all know that organic matter is the key to improving soil performance in drought conditions but did you realise that good soil preparation is also key to defending your garden against the ravages of flooding? Organic matter helps bind clay soils into crumbs, so that water infiltrates more quickly after a downpour. Improving the soil structure also makes water more easily available to plant roots, which can grow through gaps between the soil particles.

You can also help to prepare your garden for storms when you are planting and pruning specimens. Plant small and water deeply to encourage deep roots. These will not only anchor the plant and help it cope with strong winds, but will provide a vital connection to the water table deep underground. Choose multi-stem specimens where possible as these will act as a miniature shelterbelt and help to slow down racing winds.

There are other ways to cheat the weather with plants that are less tolerant of rough and volatile conditions. Drought tolerant plants which are not inherently wind tolerant can be pruned to make them tolerant of wind. Sedum, for example, can be cut back by half in early June – to about 4 inches, or pinched to control its height. I have found this very effective in my windy Midlands garden and my sedum stand strong and structural all through the winter where they used to flop and flail.

We are only just beginning to learn how to garden in a changing climate and I am sure we will continue to be surprised at just how quickly plants adapt to new environmental conditions. For the moment, if we just think a little harder about how we can plan our planting choices to stand up to the unpredictable - and experiment to push the boundaries of our existing knowledge.

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