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Selecting a The Arboretum at Flagstaff Extension Bulletin, 1990 - No. 4 Robert Wilson, Horticulturist Webmaster's Note: To create a list of Arboretum plants with low water demands, click here and type in "Low" under "Water Needs". Due to recent droughts, most homeowners are acutely aware how much their landscape can cost to water, and many would like to find alternatives that would reduce this cost. In many of the drier parts of the United States, people are learning about the Xeriscape concept which aims to promote suitable and appropriate landscaping to reduce water needs. There are seven principles to the Xeriscape philosophy, one of which is to use low water demand plants. There is much misunderstanding about the concept of a low water use plant, however. The water needs of a plant must be viewed in relation to how much rainfall occurs naturally in an area. The relative water use is the additional amount that a homeowner must apply to achieve the desired growth and health of the plant in the landscape. In other words, a plant that can do well with the natural rainfall of a climate is a low water user, even if it does need a lot of moisture at times. Put simply, a plant does not need to be able to live in a desert to be considered a good Xeriscape plant. In a climate such as ours that has natural periods of low rainfall (May -June, September -November), drought tolerance is an important quality in a Xeriscape plant. The ability of a plant to be drought tolerant is primarily due to the plant's ability to limit water loss through the leaves and acquire more water through the roots. Plants have developed several mechanisms to reduce foliar water loss, many of which can be recognized in native and cultivated plants. When shopping for garden plants, the presence of the following characteristics is a good indication that they will be drought tolerant. Look for:
Pinnate leaves, also called compound leaves, are larger leaves which are divided into smaller leaflets and appear like many small leaves. In some plants this serves to break up the leaf surface and reduce air flow around the leaves, thus reducing water loss. ‘This can be seen in Siberian Pea (Caragana) Blue Flax (Linum lewisii), Shrubby Cinquefoil (Potentilla fruiticosa), and California Poppy (Eschscholzia californica). The presence of numerous hairs on a leaf has a similar effect in reducing air flow across the leaf surface. In extreme cases it can make the leaf appear fuzzy or velvety such as in Stachvs byzantina, Lamb’s Ears. More commonly it gives the leaf a silvery-grey appearance which can be seen in many species of Artemisia (Wormwood, Silvermound) and Russian Olive. Plants which produce oils and resins are quite aromatic and often quite drought tolerant. Many herbs and other fragrant plants have this characteristic, especially Sage (Salvia officinalis), Rosemary (Rosmarinus offlcinalis), and English Lavender (Lavandula angustifolia). Many arid land plants have thickened leaves which in mild forms makes the leaf stiff and leathery, and in extreme forms makes the leaf thick and fleshy and is called succulence. Both reduce water loss by reducing the ratio of surface area to leaf tissue volume. Plants with leathery leaves also have a thickened, often waxy cuticle and include Manzanita (Arctostaphylos spp.), Kinnikinnick (Arctostaphylos uvaursi), and Buckthorn (Ceanothus spp.). Succulence comes in many forms. .Many are strictly desert plants but some are suitable for our area. Stonecrop (Sedum spp.) and Hen-n-Chicks (Sempervivum tectorum) have succulent leaves. Cacti have lost their leaves altogether and have thickened, photosynthetic stems. When looking for low water use plants, do not limit yourself to plants you know. Finding one or more or these characteristics in new plants is a good indication of drought tolerance. Even within a species or variety there may be some variation. Select plants that exhibit the qualities the most strongly to obtain those which will require the least supplemental water.
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