Perennial Gardening in the Mountains

Gardening with perennials in high altitudes.

Veronica oltensis
Item # 97350 		Veronica oltensis
Thyme-leaf Speedwell

each $5.49
3 to 6 $5.29
7 or more $4.99
Penstemon pinifolius 'Compactum'
Item # 75762 		Penstemon pinifolius 'Compactum'
Compact Pineleaf Penstemon

each $7.99
3 or more $7.79

30% off! $5.60
Thymus citriodorus 'Doone Valley'
Item # F5530 		Thymus citriodorus 'Doone Valley'
Doone Valley Lemon Thyme

per flat $140.68
Thymus serpyllum 'Minus'
Item # 95578 		Thymus serpyllum 'Minus'
Minus Creeping Thyme

each $5.49
3 to 6 plants $5.29
7 or more $4.99
Veronica x 'Blue Charm'
Item # 97390 		Veronica 'Blue Charm'
Blue Charm Veronica

each $7.99
3 to 6 plants $7.79
7 or more $7.59
  • Topic: High Altitude Gardening
  • Keywords: high altitude, perennials, climate, Garden Design, gardens
  • Date: February 2002

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With proper plant selection, a mountain perennial garden can be the envy of any lowland gardener. The short summers result in a concentrated blooming season with perennial flower combinations not typically seen at lower altitudes where the flowering times don’t overlap. The higher precipitation typical of mountain climates also increases the variety of perennials that can be grown without a lot of additional irrigation. As an added bonus, the intensity of sunlight at high altitudes also intensifies flower pigments creating a virtual riot of color in your garden.

For the purposes of this article, areas of the western US above 6,500 feet in elevation, are considered to be part of the mountain gardening arena. As you move further north, the equivalent conditions occur at lower elevations. This includes both foothill and mountain localities.

Within this mountain arena, four basic types of garden sites can be identified; the dry, hillside site (particularly in foothill locations), and the open meadow/house site will be discussed in this article. Refer to the booklet “Mountain Gardening” (Denver Botanic Gardens, 1998) for detailed information on the wooded garden, and the stream-side/damp meadow site. When planning gardens in these different areas, it is key that the cultural requirements of the perennial match its garden site.

Cold temperatures and short growing seasons are the dominant climatic variables that mountain plants must contend with. When considering perennials for the mountain garden several factors should be kept in mind.

Winter’s low temperatures affect a plant’s ability to survive; in general plants that are cold hardy to -20( or colder (USDA zone 4) should be selected. Although, in warmer, more protected microclimates (such as flower beds against the sides of buildings), Zone 5 (-10( to -20(F) plants will also be successful.

The short summer growing season also dictates plant choices. Frosts in late spring and early summer will trouble some perennials whose flowers are frost sensitive (e.g. Peonies). Conversely early fall frosts preclude use of late fall blooming perennials (e.g. Maximilian’s Sunflower).

Perennials native to areas with long, hot summers (Texas and the Southwestern US) are also not generally recommended in mountain gardens. The lack of summer heat, in combination with cold nights and potential summer frosts, may result in poor garden performance.

Soil preparation and mulching are of utmost importance when planting a perennial garden. Soil improvement should be done before the planting begins. The key ingredients to creating a healthy, nutrient-rich garden soil include ample compost (at a rate of 1 cu yd per 100sq ft), phosphate (soft rock phosphate is an excellent organic source), and PlantersII trace mineral fertilizer, known generically in organic gardening circles as “rock dust”.

Dig these additives into the soil to a depth of 12-15 inches. Mulch should be applied to new transplants once the soil has warmed in mid-June. A thick layer of mulch applied in April or May will keep the soil cold and slow rooting and top growth of new transplants.

For most perennials, a 1 to 2 inch thick layer of a coarse textured compost is recommended. Pine needles are also a suitable mulch. For established planting, mulch should be applied in early summer to conserve soil moisture during summer dry spells.

Organic mulches decompose and are absorbed by the soil over the growing season. Re-apply to established perennials in the fall as needed. This insures that the roots and crown are protected from the winter cold and keeps plants dormant until later in the spring.

For rock gardens, a one to two inch layer of 3/8” to 1/2” diameter crushed or “chipped” gravel mulch is best. Gravel mulch keeps the crowns of the plants dry during the spring thaw, holds heat around the plants in the summer and encourages many rock garden and native plant species to re-seed and spread.

Planting season for mountain gardeners begins as the snow melts and the soils thaw and dry out somewhat. Very late spring (early June) through August are the best times to plant. Fall planting can be somewhat hit or miss. Perennials need 6-8 weeks of growing time before the soil freezes for the winter.

Sunny hillside settings are well suited to rock gardening. These sites typically have dry, well drained soil conditions. The Rock Alpine Garden at the Denver Botanic Gardens in Denver and the Vail Alpine Garden in Vail, CO are two fine examples of how this gardening style reaches its greatest potential in the inter-mountain West.

Here are a few of the vast numbers of short growing native and adapted species that can be used in the rock garden:

  • Prickly Dianthus (Acantholimon species)
  • Persian Rockcress (Aethionema schistosum)
  • Basket of Gold (Alyssum); “Goldkugel”, A. caespitosa
  • Dwarf Pussytoes (Antenaria “Mc Clintock”)
  • Sea Pinks (Armeria); A. maritima “Compactum”, A. juniperifolia “Victor Reiter”
  • Alpine Aster (Aster alpinus)
  • Rockcress (Aubrieta “Whitewell Gem”)
  • Hardy Ice Plant (Delosperma); D.congestum “Gold Nugget”, D.sphalmanthoides
  • Garden Pinks (Dianthus); “Tiny Rubies”
  • Fleabane (Erigeron); “Grand Ridge”, E.compositus
  • Gentian (Gentiana septemfida)
  • Dwarf Candytuft (Iberis.sempervirons “Little Gem”)
  • Flax (Linum); L. flavum “Compactum”
  • Beardtongue (Penstemon); P. caespitosus “Claude Barr”, P.virens, P. linariodes v coloradensis, P.pinifolius “Compactum”
  • Creeping Phlox (Phlox); P.subulata “Compact Pink”, x douglasii hybrids
  • Alpine Soapwort (Saponaria caespitosa)
  • Thyme (Thymus); T. serpyllum “Minus”, T.serpyllum “Elfin”
  • Groundcover Speedwells (Veronica); V. oltensis, V. liwanensis
  • Hummingbird Trumpet (Zauschneria garrettii “Orange Carpet”)

The “cottage style” perennial garden with its informal design and collage of colors is perfect for use in sunny, open areas around the house and in surrounding meadows. Be sure to include winding paths through the flower beds. This gives the garden a wonderful informality. Avoid the temptation to install Kentucky Blue Grass in the pathways, as blue grass can be an aggressive weed among the flowers. Instead use crushed gravel; it makes a superior pathway surface.

The plant choices for the sunny garden are huge. Listed below are some “essential” perennial groups with recommended species and cultivars:

Border Perennials

  • Yarrow (Achillea); “Moonshine”, x kelleri, “Paprika”
  • Columbine (Aquilegia); Red Spur (A. formosa), Rocky Mountain (A. caerulea), Golden Spur ( A. chrysantha)
  • Bellflower (Campanula); “Blue Clips”, Mountain Harebell (C. rotundifolia)
  • Delphinium (Delphinium); D. grandiflora “Butterfly Blue”, other varieties
  • Blue Flax (Linum lewisii)
  • Lupine (Lupinus); “Russell Stain Hybrids”
  • Catmint (Nepeta); Blue Catmint (N. x faassenii), “Six Hills Giant”
  • Poppy (Papaver); Oriental Poppy (P. orientalis)
  • Beardtongue (Penstemon); Scarlet Bugler (P. barbatus), Rocky Mountain (P. strictus)
  • Sage (Salvia); “Maynight”, “Blue Hill”, “Rose Queen”
  • Speedwell (Veronica); “Red Fox”, “Blue Charm”, Compact Blue (V. allioni)

Groundcovers

  • Hardy Ice Plant (Delosperma); Hardy Yellow (D. nubigenum)
  • Garden Pinks (Dianthus); “Horatio”
  • Candytuft (Iberis sempervirons “Snowflake”)
  • Creeping Phlox (Phlox subulata); “Emerald Blue”, “Red Wing”, “Emerald White”,
  • Soapwort (Saponaria); Pink Soapwort (S. ocymoides), Giant Flowered (S. x lempergii)
  • Creeping Speedwell (Veronica); Turkish (V. liwanensis), “Blue Reflection”
  • Partridge Feather (T. densum v. amanum)
  • Creeping Thyme (Thymus); “Reiter”, “Coccineus”, “Doone Valley”, Wooly, “Pink Chintz”