Gardening: Drought-tolerant design

Despite repeated deluges this winter, my typically contrarian instincts would have me bet on a hot summer. What better way therefore to banish the depression brought on by seemingly endless low-pressure weather fronts than by discussing the sensible use of drought-tolerant plants in the garden?

Gardening X-ratedRegardless of living in a region that thankfully delivers water with only rare restrictions on its use, the unavoidable truth is that good water management is becoming an issue of ever-increasing importance. Chances are that just because we haven’t seen much in the way of hosepipe bans here, doesn't mean we won't in the future. I was struck, during a recent visit to several of our growers in Italy, by the intensification of their efforts to collect and recycle their water runoff. One of the largest plant producers in Northern Italy aims to increase their efficiency at water collection from 98% (up from their current 95%) by 2015. Notably, this has several advantages for them, outside of simply being environmentally aware: the water they collect is already loaded with nutrients from the plant pots, and with the correct adjustment, allows them to reduce not only their net water consumption from sources, but also the net application of fertiliser. This last issue is one that here on the Riviera we would do well to take heed: recent reports indicate rising levels of nitrate and pesticide pollution in the groundwater throughout the region, and this cannot simply be fobbed off on agricultural practices. Some of the worst affected areas for pesticide pollution are densely habituated touristic regions. A case of effluent from the affluent?

So is it possible to have a beautiful garden that is also sensitive to the amount of water it requires? In the past I have been slightly dismissive of drought-tolerant garden design, having seen so many poor examples. However, while some things such as rolling expanses of lush green lawns are obviously going to have to be limited (or excluded), there is no reason why a garden cannot be stunning and yet still adhere to these principles if well designed. Indeed, there is a growing movement that believes adamantly in the principles of paying keen attention to the use of water in the garden to the extent that it heavily influences landscape design. There is even a specific word to describe it: xeriscaping (derived from xeros, the Greek word for “dry”). This was coined originally by employees from the department of water in Denver, Colorado, who have also gone to the trouble of registering a logo for trademark – so they really must be serious!

Many US garden centres have adopted an “X” rated scale for plants, from X to XXX, designed to indicate the water needs of a particular plant – something that I do not think will be long before it is adopted here in Europe. Note that there is a subtle but important difference between the natural landscaping and xeriscaping in that the former only selects plants based on those that are native to the local area, while the latter just selects plants with the aim of water conservation. While both of these will have an impact on reducing the net water requirements of the garden, as Denver water states, xeriscaping is by definition “designing an area with drought conditions and reduced water consumption as the focus” and covers several basic principles, which are applied to the garden as a whole.

The process starts with the planning and design, crucial for success when trying to achieve these goals and combine them for a beautiful result. Careful planning allows many more plants to be deployed outside of the most obvious drought-tolerant ones, simply by careful examination in advance of the site conditions and aspect, widening the palette of plants than can be used. Select your plants by first dividing your initial choices into groups and the garden into zones: different areas in a landscape receive different amounts of light, wind and moisture. To minimise water waste induced by the need to water for the thirstiest in a particular area when required, group plants together with similar light and water requirements, and place them in areas that match these needs.

Returning to the subject of lawn, this is typically one of the highest water users, so a decision must be made if this is to be exonerated from this vetting for aesthetic and practical reasons, modified or removed entirely. Several alternatives to lawns can be used, but if lawns are to be retained, the alternative to the classic turf grass is Bermuda grass (also know as “warm season” grass). This is much more resistant to drought and will remain green through the summer season with perhaps half the water used on classic turf grass. The downside however is that it will be brown while dormant, and this is nearly half of the year.

Preparation of the soil is also crucial, as our soils here are poor and calcareous. While some plants will always grow well on natural soils (notably the natives), others will need good amounts of organic amendment in the form of compost to ensure water is retained and that the plants root deeply as quickly as possible.

The subject of watering I have discussed extensively in several previous articles: in summary, avoid spray sprinklers in the beds at all costs as this uses copious amount of water, is not efficiently absorbed and can waste very large amounts to evaporation. Likewise, never water during the day to reduce water lost to the same effect. Evaporation is a major factor in water loss in the landscape, and astute application of mulches and composts not only minimise moisture loss, but also provide a buffer that absorbs nutrients that would otherwise be leached away, to provide later for hungry plants. Organic mulch can take numerous forms, from cocoa shells to shredded wood and bark, but several other materials such as leaves or straw can be used although these are perhaps less practical or less aesthetic. Be sure to spread sufficient depth when applying a mulch, as too thin a layer has little effect and will just wash away: generally 5 cms is recommended. That said, several prominent proponents of dry gardening recommend gravel in place of organic mulch since they find the organic much too rich for drought-tolerant plants, and this practice is gaining in popularity. The most important tip of all though: if and when you when you do water, do so deeply and infrequently to encourage the plants to develop extensive and deep roots. I cannot understate this and I visit gardens time and time again where the watering controller is set to drip the beds for 15 minutes every day. With an automatic sprinkling system in use, be sure to adjust the controller monthly to accommodate weather conditions, and the addition of a rain sensor to shut off the device when it rains is usually a very inexpensive add-on that can be fitted to most controllers to reduce water wastage.

Additionally, there are several areas that are not always discussed when dealing with drought-tolerant design, and this starts at the nursery, and most of these are based on good practices. Take roots for instance: plants that are naturally drought-resistant will often have roots that exceed the 1:1 root to shoot ratio, meaning that having good sized containers at the nursery will give these plants a much better chance of a good establishment as they are not stressed on arrival. Additionally, nursery workers should keep these trimmed at all times (which goes against the commercial practice of the biggest plant equalling value for money) for the same reasons. Once the plant is in the garden it is the responsibility of the gardener to plant correctly, in prepared soils and irrigate with a suitable regime. Overwatered drought-resistant plants are far more likely to develop fungal problems: I think almost everyone will have experienced dieback in Lavender and Rosemary at some point, so this, although obvious, should not come as a shock. Lastly there is the issue of expectations, since a second trim mid-year will control growth and reduce transpiration. This, though, will reduce flowering, so a balance must be struck between one's desire for drought tolerance and the demand for flowers and lush growth.

Refraining from using the plants with high water demands will keep your garden healthy and your water bill down. If you aren’t sure about what plants to use there is a dearth of information on the internet but an even simpler first step is to just look for plants that thrive naturally in the local environment, since these grow throughout the year without extra watering. Outside of the obvious cacti and palms, many more shrubs are surprisingly autonomous and ubiquitous in the region. Just a small selection is Oleander, Arbutus, Berberis, Ceanothus, Potentilla, Cotoneaster, Artemisia, Yucca, Rhus, Pistacia, Osmanthus, Cistus and Juniperus. A wide list of perennials can be found to survive well on minimal water, including Lavender, Rosemary, Santolina, Sage, Nepeta, Penstemon and Perovskia, to name but the smallest of selections.

By James Hartley
Director, English Garden Group