Choose the right plant for the right space

Why is my plant dying?" is a variation of many of the questions answered by local master gardeners.

A leading cause of these plant losses began long ago at the point of inappropriate plant selection.

The choices you make selecting major landscape plants will determine whether your garden will be easy to maintain or will take a lot of effort or money for upkeep.

So how does a property owner choose suitable plants? Commonsense plant selection is the process of matching plant species to site conditions. Conditions include available growing space, local environment, such as sunlight, climate and soils, and the availability and cost of water.

Available

growing space

The start to your easy-maintenance landscape is to realize how much growing space is, or more likely is not, available. For each plant, restrictions in height and width and available root space all need to be considered. Woody plants - trees, shrubs and vines - can share spaces with each other and with perennial or annual flowers and grasses.

The most common mistake we see is a plant that quickly outgrows its available space. The property owner drastically prunes branches that rub against the house or grow into power lines or over driveways. The plant reacts with quick re-growth to replace what it lost in the pruning process. The homeowner again drastically prunes. This re-growth and aggressive pruning weakens the plant. In addition, each pruning cut becomes an open invitation to pests and disease.

The first step is to decide how much growing space is available above and below ground. You can then determine which plants will grow to fit into that space when mature. Height restrictions can include utility lines, roof overhangs, patio roofs and taller plants. Width restrictions include walls, pathways, sidewalks, driveways and other plants.

Roots of healthy mature trees and shrubs can extend 1-1/2 to 4 times the width of the plant canopy. For example, the roots of a 30 foot diameter mature Afghan pine tree can spread from 45 to 120 feet. That's expecting a lot from the space in a 20-foot front setback.

Root space should only include soil or mulched areas. Space under an asphalt surface generally is too hot for proper root growth. Cement areas limit the amount of oxygen in the soil so are limited as a healthy space for root growth. Patio or pathway spaces covered with light colored loose brick or paving squares on a permeable base are acceptable for root space.

Width restrictions remind us that shrubs and trees should never be planted closer to a wall than one-half its mature diameter. For example, if you want to plant an Arizona rosewood, which has a mature canopy diameter of 10 feet, you'll need to plant it 5 feet or more from any vertical wall.

Sunlight

and climate

Plant lists show whether a plant needs full sun, partial sun or full shade. Ideal lists also show USDA winter hardiness zones or the "Sunset Western Garden Book" western climate zones. Some plant lists will also show water needs. Woody landscape plants must be well-adapted to our mid- and high-desert summer temperatures, and low freezing winter temperatures. Each of our yards also has microclimates depending on exposure and elevation changes and proximity to buildings and walls. North and east exposures are generally cooler than south and west exposures.

Soils

Most trees and large shrubs need a soil depth of 18-24 inches. Learning what soils you have helps to make plant choices. Desert soils are alkaline and have little, if any, organic material. Many areas around Kingman have poor soils, caliche or under-lying rocks. In Golden Valley, you may find sandy soils or clay layers. If you are lucky to have undisturbed desert soil, native plants are ideal choices.

Soils in recent construction areas may be compacted by equipment and the top soils stripped or moved to another location. Compacted soil can limit water penetration, and air exchange, which in turn inhibits healthy root growth.

Your soil may determine your plant choices. Some plants require well drained soils and will not thrive when roots sit in wet or damp soil for a week or two. Other native plants that grow along washes use every drop of water they can get.

While sandy soil is well drained and well aerated, it will need more frequent irrigation. On the other hand, clay soils do not drain well, are poorly aerated and are easily compacted but require less frequent irrigation. Either type of soil can be amended with some compost or other organic materials, and heavy compacted soils should be loosened. You will want to encourage roots to grow outward from the trunk, so adding amendments and loosening the soil during subsequent years in a gradually increasing radius will support roots that spread.

To help with drainage, use a post hole digger to drill through a caliche layer. A good amendment to add as you loosen and dig into our alkaline soils is soil sulphur.

Water

Thanks in part to growth in major cities like Phoenix and Las Vegas, xeriscape and low-water-use plants are becoming more and more available for use in our desert yards.

To remain attractive, native and low-water-use plants still require supplemental irrigation beyond rainfall. They will grow to maturity faster and once full grown will require larger amounts of water than when first planted. If you have a very limited water supply, keep this in mind and limit the number of large mature plants you will have, or choose smaller maturing trees.

The growth of trees and shrubs necessitates the addition of irrigation emitters to encourage roots to spread outward and to provide sufficient water for the larger area of plant branches and leaves. If a root system is not developed, a tree can blow over in strong winds.

The grouping of plants together can simplify irrigation and create beneficial humidity zones. Plants grouped together should have similar water needs. Plant water-loving plants together, and plant low-water-use plants together to make irrigation easier. The basic desert theme is to plant higher-water-use plants closer to your home or outdoor living space and the lower-water groupings further away, blending toward native desert space.

Mulching, whether with plant materials or gravel mulch, helps to save water by creating a barrier between the soil and the dry desert air. Most native plants prefer inorganic mulches similar to their native desert habitat.

Under no circumstance should you use a solid plastic barrier beneath any mulch. It inhibits rain water penetration, and does not allow needed oxygen to get into the earth. Most weed roots will not grow through a six-inch layer of mulch. Plastic also encourages roach populations to breed in the dark moist area just under the plastic. If you feel you must use a barrier, porous landscape cloth is a much better choice.

Resources

There is a new resource for choosing low-water-use landscape plants for your Kingman, Golden Valley or Dolan Springs gardens. A second one is available for the river city areas of Bullhead City and Lake Havasu City. The University of Arizona, Mohave County Cooperative Extension office now has available booklets, in pdf format on the internet at cals.arizona.edu/mohave/water/landscapingkingmangoldenvalleydolan.pdf or cals.arizona.edu/mohave/water/landscapebullheadcitylakehavasu.pdf and hard copies at their office at 101 E. Beale Street.

Other resources are the Web site plant database of Mountain States Wholesale Nursery at www.mswn.com and the "Sunset Western Garden Book."