By Judith Phillips
Gardening is an extreme sport in the arid Southwest, a place of remarkable variety in native species and ecological niches. While not all native plants transition well to gardens, the most beautiful, resilient and adaptable natives are indispensable when designing gardens able to field the extremes of heat as it inches to new highs and occasional sudden cold events that plummet to numbing depths.
Resilient design relies on trees, shrubs and, in the low desert, architectural succulents. These plants are in effect structural and look and grow best when coordinated with soil contours, walls, and paving.
Consider the architecture of space: vines to drape arbors, trees to shade paths and patios, shrubs combined with walls to create privacy and buffer heat. Good design reconciles how the space is used, how it will look and feel, with how the hard surfaces serve the future planting and how the plants complete the built space.
TREES: DIVAS OF DESIGN
Trees as garden sculpture and architecture are shaped both by genetics and by forces in their environment. Arid ecosystems support fewer trees clustered where more moisture is available. Thick trunks or multiple shorter stems store energy and provide trees greater wind resistance; their broad rounded canopies balance food production and storage with evaporative loss.
In the hottest, driest desert ecosystems, trees are only found along perennial streams or in arroyos where groundwater is shallow. Even in those sweet spots small trees dominate unless water is abundant.
With a rise in elevation temperatures moderate, and even if there is only slightly more rain, lower evapotranspiration rates allow an increase in plant size and numbers. These natural patterns are well worth considering when deciding how many trees a garden can sustain and where they might grow best. The spaces surrounding buildings and along streets are the most seen and used; and also the shortest distance for directing rainwater to plants, ideal places to plant trees.
Trees offer shade and their network of roots keeps the soil from eroding as they shelter and feed wildlife, sequester carbon dioxide and oxygenate the air. Urban trees also reduce the need for air conditioning as moisture transpired from their leaves cools the air. They improve air quality by absorbing pollution through leaf pores and collect fine particulates on their surfaces.
Trees help create more temperate spaces beneath their outspread branches, sheltering tender plants, preventing damage from intense sun and heat as well as buffering hard frost. They share resources, especially moisture, with everything within rooting range. It makes good sense then to invest water in trees, especially in desert cities where rainwater captured from all the hard surfaces could provide much of the moisture they need.
Pavement limits where trees have space to grow, but at the same time rainwater harvesting from pavement is an opportunity to create urban oases. Loosening compaction and making soils pervious improves the conditions for the roots of trees. Contouring to capture rainwater and loosening the soil to absorb water are vital when planting new trees and shrubs.
Reestablishing soil porosity and permeability is much more beneficial than creating a humus-rich growing medium foreign to arid climates. Rain captured and stored either in cisterns or directly in the soil will green a home gardens and ripple through the neighborhood and the city. It makes places more comfortable, shades pavement to reduce urban heat islands, and coincidentally manages storm water as a precious resource.
Shrubs dominate desert ecology; super-resilient because they store most of their energy in their roots protected from climate extremes. With wildflowers and grasses, shrubs weave patterns through the landscape.
Clusters of large shrubs can provide cooling shade in places too extreme to support trees. Evergreen shrubs provide leaf color and cover year round. Deciduous shrubs add intense flower color seasonally then fade to skeletons in winter, adding contrast and depth to the garden.
The first rule of extreme gardening is don’t plant more than the conditions will support. Large shrubs are at their best when they aren’t crowded, spaced closely enough to fill their intended role in the design but not so close that as they mature they interfere with each other.
Prickly plants, useful as barriers and green security fences, can pose maintenance problems if they aren’t given enough space to allow routine maintenance. The barbed and sword-leafed succulents need tempering in numbers and placement to avoid the appearance of a hostile takeover, as well as the physical pain of an armed confrontation.
Succulents are exclamation points in design! Like that punctuation mark, succulents are most effective when well-placed and not overused! They are comparable to shrubs in the space they need but are much less subtle in personality!
Versatile mid-sized shrubs three to six feet tall with similar spread can be used to divide spaces, control access, partially enclose a patio, screen trash and recycling bins and in large gardens fill open space with color, texture and fragrance.
Small shrubs one to three feet tall and wide can separate spaces physically without doing so visually. They can make a small space feel more generous and avoid that shrinking feeling as the landscape matures. Small shrubs add diversity to courtyard spaces, pockets of color, fragrance and texture. Their mounding or sprawling silhouettes add layers and depth. Having the advantage of deep roots, they also benefit most from cool-season rainfall and snowmelt, which evaporates more slowly and penetrates the soil more deeply.
While in nature plants are at the mercy of climate and topography, in gardens it’s tempting to force rapid growth with water and fertilizer. But even small amounts of fertilizer and a little too much water can cause rapid growth that is vulnerable to wind breakage. This undermines plant health and the overall design, especially if the purpose of the shrubs is buffering the wind.
FLOWERS AND GRASSES
Wildflowers keep the soil and surrounding air cooler, provide seasonal color and add greater depth to the landscape. They complement shrubs and grasses and share water with trees, adding a greater degree of sophistication, subtlety and maintenance to the garden.
In all but the coolest areas, intense summer heat interrupts flowering. Since temperatures and rainfall patterns vary so much from place to place and year to year, some perennials may bloom all growing season when moisture is available. Others ebb and flow with temperature changes and summer rains.
Ecologically they are pioneer plants that quickly stabilize disturbed soil. In designed landscapes they are useful as infill while the slower growing, more structural plantings become established.
Wildflowers can be seeded directly where they will grow. Prolonged drought makes seeding riskier, but when conditions are right, their uninterrupted rooting gives seedlings a huge advantage and can be the most cost-effective way to cover large spaces.
While a thick carpet of lawn is not sustainable in the Southwest, masses of native grasses have great habitat value including soft green leaves — some strongly vertical, others gracefully mounding — that are forage for the caterpillars that morph into butterflies. The splendor and diversity of their seed heads, showiest from August well into winter earn grasses their garden space, dancing in the wind, shimmering when backlit.
There is so much to recommend them that it is wise to consider some of their drawbacks before turning the whole garden into grassland. Grass pollen is airborne, the hay of hayfever. Strongly seasonal, once the party is over there’s a major cleanup needed.
Grasses are best cut back to the ground before growth resumes. Wildfire is nature’s tool, but with plastic irrigation parts and the danger of regeneration becoming conflagration, power tools are our best alternatives. There is usually a period of a month or more when the glory of grasses resembles stubble, and sexy as that may be on some men, it leaves large gaps in the garden unless balanced by plants that take the limelight when grasses rest.
Garden design is a catalyst, ideas applied in time and space change the way that a place looks and feels. Plants are changeable; they have personalities; they make space both enticing and comfortable.
A design where the hard surfaces, roofs and paving, shed rainwater into planted space to support the growth of well-adapted plants creates a strong framework. As the garden matures, the environmental engineering is likely to disappear in the foliage. Change is inevitable, but strong design and well-adapted materials yield a garden that requires fewer outside resources as it matures.
Climate change is making the process more uncertain as plants are challenged by increasingly extreme conditions. Plants that are well suited to the conditions will be healthier and healthy plants are less vulnerable to insects and disease. Plants radiating vitality make people healthier by association. This is the great advantage of understanding where you are, how nature operates there and designing accordingly.
Judith Phillips is a landcape designer, author and horticulturists in Santa Fe, NM. She will be delivering the keynote address at Desert Green XX, October 20-21 at Texas Station, North Las Vegas, NV. For more information, visit www.desert-green.org.