By Susan Fernalld, Master Gardener
Understanding and exploiting microclimates within the garden enables high altitude gardeners cope with desiccating winds, intense sunlight, extreme temperature differences between daylight and nighttime, shorter growing seasons, poor soils, and our inherently dry climate.
Microclimates are created by the interaction of prevailing temperature, precipitation, sunlight and soil type in relation to buildings, fences, large boulders, and the direction of slope.
Challenge and Opportunity
Broadly speaking, southern exposures tend to be hot and dry, whereas northern exposures are relatively cool and moist. Eastern exposures are moderately cool and protected from wind. Western exposures often expose plants to intense afternoon heat and drying winds, but they can also provide an extended growing season.
A plant growing in the shade of a large rock might as well be in a completely different climate than the same plant growing on the sunny side of the rock, just as gardeners on opposite sides of a valley can be in a completely different climate, despite being at the same elevation.
Rock gardens can be a way to provide the moisture and temperature conditions for growing ground-hugging plants adapted to the sub-alpine and tundra.
Near a foundation, against a brick or stone wall or in rock mulch, ambient heat can be extreme. Many mountain species simply can’t take this combination of sun and heat. To increase your options in those areas, you might use bark mulch instead of rock and plant a shade tree. Once established, “xeriscape” trees, shrubs and perennials can thrive with minimal irrigation needs. This is also a place to try growing some of the more cold-tender plants that prefer warmer climates (you can experiment with those zone 5 plants that you just have to grow in this location).
Take advantage of cooler eastern exposures to reduce evaporation or use the drip line of a roof to harvest water. If you don’t already have a natural feature where winter wind eddies and piles up snow, put up a perforated snow fence (40-60% density) to accomplish the same effect. In the spring columbine can bloom lushly year after year in that moist microclimate you created. The snow blanket will also help to insulate plants from winter dessication. A place where the snow lies thick all winter is a good place to try more tender plants.
If you live in a mountain valley where the heavier cool air shortens your growing season, consider planting annuals in containers that can be moved to follow the path of the sun over the seasons. Sheltering walls and overhangs offer plant havens in cooler periods, as do sunny open spaces during warmer weather.
Expanding the options
Light-colored decomposed granite soils, common in many mountainous areas, are less apt to absorb heat. Amending mountain soil with organic material (such as compost or manure) darkens the soil so that it absorbs heat plus helps shallow, rocky soils hold the water and air needed for root growth. You may not need to amend your soil if you garden exclusively with native plants, whereas you probably will need to amend your soil to expand your options to include non-native plants. (A soil test is needed to know for sure if amendments are necessary. Sample each of your microclimates.)
Plant Choices for high country microclimates
Drought-tolerant plants are generally better suited to our well-drained mountain soils, intense sunlight and severe exposures, but microclimates will provide some flexibility in plant choice. Choose plants that are hardy within USDA zones 2 through 4, and then group those with similar moisture and sun exposure requirements into corresponding microclimate areas.
Examples of drought-tolerant mainstays for dry areas include the deep blue-flowered mountain bluet, orange-yellow blanket flower and native white woolly yarrow. Mauve-flowered beebalm and white pussytoes grow well on dry hillsides. Taller perennials such as oriental poppy, coral and yellow hybrid yarrows, and delphinium will be better protected from wind on the east side of your house or walls. For the shadier north side of the house, try white or blue bellflowers, coral bells, and Jacob’s ladder. Daffodils and Johnny jump-ups tolerate some shade, like moisture, and grow successfully in north- and east-facing situations. For the rock garden, consider stonecrop, creeping veronica, moss sandwort, maiden pinks, and shrubby cinquefoil.
For further information about plant choices for mountain gardens, see Mountain Plant recommendations.