When I first moved to rural north Boulder, I saw the ten acre short grass prairie next to my land as “empty”. Almost nothing that grew there was over a foot tall. Where I came from in Eastern Iowa, we would call that “barren”. However in the last 20 years in Colorado, my tastes and perspective have changed. In Iowa, the weeds as well as the herbaceous perennials spring 2′-6′ high each year, then die back to the ground to hide in their roots under the plant debris and snow until spring. In sunny, dry Colorado, there are many natives as well as well-adapted foreigners from high desert and steppe regions that stay evergreen to make up for a shorter growing season and that stay short in accordance with leaner soils and fierce climate changes, wind and hail.
In my rock garden, I have grown fond of the shorter, tighter plant forms and of what renowned Colorado rock garden designer Gwen Kelaidis calls “the Persian Carpet effect”. Instead of the English perennial border with its vertical masses of flowers intermingling with each other, the “Persian Carpet” of smaller plants weaves horizontally, the foliage and evergreen appearance through the winter being as important as the flowers. Since my garden is only watered five times a year, many alpine plants that would have created a more traditional rock garden could not survive for me. And since I know that many of you may need or want to conserve water, but would like to grow rock garden plants, groundcovers or shorter plants for the front row of a border, I will share with you some of my successes. These plants are a foot or less in height, no more than two feet in diameter and have shown a good tolerance for growing in dry conditions.
Aster coloradoensis is a small gem 2″ tall and 9″ wide with blue-gray leaves in a tight evergreen clump. The pink, daisy-like flowers are on short stems, but are surprisingly showy. Discovering this little native beauty behind a rock can be as thrilling as finding a group of foxgloves gushing in their glory. And Aster coloradoensis is not hard to grow.
Almost everybody is familiar with Purple Rockcress, (Aubrieta), but few know that it requires little water. It makes a dense spreading mat of ever-gray-green leaves, 4—6″ high and 16—24″ wide, blooming in early spring. The flowers are in short clusters and range from the dark violet of Aubrieta deltoidea “Whitewell Gem”, to reddish purple in “Royal Red”, to purple in “Purple Gem”, to lavender pink in Aubrieta pinardii (which blooms for 8-10 weeks). Even though they survive on little water, siting them where they are shaded from late afternoon sun can improve quality or length of bloom, and shearing spent flowers right after blooming also helps it to more quickly recover from the expenditure of blooming.
The Aethionemas, or Persian Candytufts, are exceptionally fine plants for dry conditions. They love loose, gravelly, lean soils and the company of rocks, but will thrive in most soil conditions except heavy, wet clay. These are small shrubby plants with tiny blue-green to very blue leaves that are extremely attractive even when not in bloom. Aethionema caespitosa makes a tight mat only one inch high and 6—10″ wide; A. grandiflora is more shrub-like to 8—12″ tall and 8—14″ wide with clusters of lovely pink, very fragrant flowers that put on a long-lasting show; A. ‘Worley Rose’ has deeper rose-pink flowers; Aethionema cordifolium is the only one among several others that was very short-lived and aggressively self-seeding. All are deep tap-rooted, bloom in mid-spring to summer, self-sow a little where happy and live longer with dead-heading.
There are several good, drought tolerant Alyssums under a foot tall. Most have soft gray-green foliage with yellow flowers in spring, and prefer well-drained, alkaline soils. The familiar Basket of Gold which used to be called Alyssum saxatile (now Aurinia saxatile) is 8—12″ tall and 24″ wide, has strong gold flowers and looks great cascading over a wall or down a bank. Alyssum montanum is similar but has a neater growth habit with lemon yellow flowers. Other good Alyssums are: A. wulfenianum 8—-12″ high with attractive foliage and can self-sow a lot, A. markgraaffii 4″ high and 12″wide (16″ flower stems” and A. pateri which makes a tight mat 2—4″high and 8—12″ wide. All will rebloom in fall and live longer with dead-heading, and will look better if not watered or fertilized too much.
Greek Bladder Pod, Alyssoides graeca, has naturalized in a very dry wall in my garden where it gets 8″ high and has self-sown to 24″ wide. It makes a spectacular very early spring display of bright yellow flowers, and the inflated seed pods which follow are ornamental as well.
Not all alliums get tall and self-sow bountifully like Garlic Chives, Garlic and Leeks. Allium senescens glaucum only reaches 6—8″ tall and 12″ in diameter. It has flat, bluish leaves that swirl in the same direction, making for a very ornamental and sculptural effect. It blooms in the summer with lavender-pink flowers. Other Alliums a foot tall or shorter are A. cyaneum with purple-blue flowers, A. moly with golden yellow flowers, and A. flavum which blooms in summer with charming mop-head inflorescences of tawny yellow flowers.
A few of the yarrows are small plants. Achillea serbica is only 4—6″ high and 12—24″ wide. It has evergreen silvery-gray leaves and umbrella clusters of white flowers in spring. A. ageratifolia is very similar, perhaps a little taller in flower. Achillea kellereri is a wonderful plant, beautiful, tough and well-behaved. It has blue-green ferny foliage with large, white flower clusters that bloom for a long time. It only grows 10″ high and 12—15″ wide and does not spread aggressively.
As you may have noticed, I have only been discussing the small, low-water plants that begin with “A”, and I have not even mentioned Arenaria, Acantholimon, Anacyclus, Alchemilla or Arabis. A more complete list of short plants for xeriscapes can be found on my website under “Xeriscape Plants” @ www.harlequinsgardens.com.
Of course, for the best success in growing xeriscape plants: water when they really need it (like right after flowering), dead-head the spent flowers, mulch with fine gravel (”squeegee”) or groundcovers, plant next to a rock, add some compost to your soil and move any suffering in full sun to a place with some protection from the late afternoon sun.