Rock garden plant selection

Now that Labor Day has come and gone, it is time for fall garden projects. Though nights are getting chilly and approaching the first frost, it is still a perfect time for garden design and construction in Colorado.

Is there some area or corner of your yard that presents a challenge to keep plants or grass growing? Maybe it is sloped or terraced. Perhaps it is a difficult area to water. Or maybe it’s faced with difficult exposures —— whether it’s a hot, dry southwest exposure or an area that faces northwest and gets the full brunt of our desiccating winds. These are ideal areas to convert to a small rock garden. Not only will it be an excellent use of these tough spots, but a well-planned rock garden can become a focal point and showstopper year round!

Peggy Burch, a Larimer County Master Gardener, wrote an article a few weeks ago on how to prepare a site and build a rock garden. Her article is a great reference, as well as Planttalk Script No. 1115 on rock garden design at www.planttalk.org.

Plants most suited for your rock garden are generally low-growing and have a clumping habit. Planttalk Script No. 1016 discusses “Perennials for the Rock Garden.”

Another resource is CSU Extension Fact Sheet No. 7.242, “Native Herbaceous Perennials for Colorado Landscapes,” found at www.ext.colostate.edu. Native plants are naturally adapted to Colorado’s climate, soil and weather. Consider attributes like water need, sun exposure and bloom time.

Pussytoes, wine cups, sundrops, primrose, mat penstemon, spreading vervain and prairie zinnia are all excellent native choices. Many non-natives also thrive in rock gardens: iceplant varieties, basket of gold, carpet sedums, creeping phlox, candytuft, rock cress and many others. An entire chapter on “Perennial Groundcovers” is found in Durable Plants for the Garden. This book is the most recent collaboration between CSU, Plant Select®, and the Denver Botanic Gardens. Each page features the plants by name with large color photos. Details such as plant height, sun exposure, flower color and bloom period are also listed.

It is getting a little late to plant your perennial rock garden. Perennials should be planted by mid-September so that roots have time to establish before winter. However, if you do plant, just remember to water throughout winter and consider adding an extra layer of mulch around the plants for protection. If you wait, use the winter months to plan which rock garden plants you wish to use when spring arrives.

Many rock gardens can be described as “miniature.” When picking out plants, choose diminutive relatives of your hardy favorites. Place these smaller plants so their size accentuates, yet softens, the large stones or boulders nearby.

Unless you live in the mountains, alpine plants are not the best choice. These plants have adapted to grow above treeline, at high elevations, with cool summer temperatures and continuous winter snow cover. Better choices for those who live in the foothills areas are “saxatile” plants, those that grow on or among rocks and thrive in meager, well-drained soils that allow roots to penetrate deeply in search of moisture and nutrients. Rich soils actually weaken rock plants; don’t make the mistake of using soil we would use in our vegetable gardens or perennial beds.

Plan a visit to the Gardens on Spring Creek (2145 Centre Ave. in Fort Collins) and view the newly opened rock garden. There, you will feel the harmony created among the winding paths of this miniature landscape that evokes images of the rugged terrain we enjoy on weekend hikes.