Tips on Cultivating Cold-hardy Natives
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Over the past decade, I have focused on finding cold-hardy selections of Southwestern natives. The objective of this work is to expand the usage of these spectacular species beyond the mild winter areas of Texas, New Mexico, Arizona and California. 'Wild Thing' and 'Furman's Red' are two Salvia greggii cultivars that have proven cold hardy in USDA zone 6. Salvia x Raspberry Delight® is a hybrid selection introduced by High Country Gardens in 2000. This ever-blooming hybrid is a cross between Salvia greggii 'Furman's Red' and a high altitude collection of Salvia microphylla from central Arizona.
The genus Agastache is rich with native ornamental species. These plants have highly aromatic foliage and flowers and are some of our best natives for attracting hummingbirds. Agastache x Desert Sunrise® is another High Country Gardens introduction. A cross between Agastache rupestris and Agatache cana, this large, long-blooming wildflower hybrid has become one of our most popular plants. Another fantastic and surprisingly cold-hardy hummingbird plant is Zauschneria arizonica. As this genus is typically not very cold hardy their use has traditionally been confined to the mild winter regions of California.
I have found that there are four key elements to the successful cultivation of Southwestern natives in cold climates. The first thing is to plant them from spring through late summer. Fall planting doesn't work for most of the Southwestern natives in zones 5 and 6. Secondly, plant in full sun locations where walls, pavement, and large rocks will provide reflected heat in the winter months. These hard surfaces provide a warm microclimate and help moderate wild temperature swings caused by cold fronts dropping in from Canada. Thirdly, don't cut these plants back in fall; wait until mid-spring before giving them their spring trimming. And fourth, plant in a lean, well-drained soil mulched with chipped gravel to help protect against excessive winter moisture and encourage re-seeding.
The southwest is home to some of our showiest Penstemon (Beardtongue) species. Penstemon pseudospectabilis, Penstemon cardinalis, Penstemon pinifolius (and its numerous cultivars) and Penstemon palmeri are all crowd pleasers. Be sure to plant Penstemon palmeri in sandy soils; loam and clay soils will kill this lightly fragrant species. I have found Penstemon pinifolius to be one of the most widely adaptable of all the Penstemon. 'Mersea Yellow' was originally discovered in England, where it has thrived in that country's maritime climate for many years.
Driving around Phoenix one is immediately struck by the importance of succulents in the landscapes there. Fortunately we're finding many succulents that survive cold winter climates. Hesperaloe (Texas Red Yucca), native to southern Texas and northeastern Mexico, has proven to be cold hardy into zones 5, 6 and 7. Previously this ever-blooming hummingbird magnet was thought to be quite cold tender, but experimentation by myself and other native plant gardeners in Colorado proved otherwise.
Agave (Century Plant) has numerous species from the Southwest and the Great Basin areas that grow in zones 5 & 6. Agave parryi and Agave neomexicana are two larger growing Southwestern species. Dasylirion wheeleri (Sotol or Desert Spoon) with its graceful evergreen foliage and enormous bloom spikes is another Southwesterner that is proving to be more cold-hardy than previously believed. Claret Cup (Echinocereus triglochidiatus) is the largest and showiest of our native cacti. It makes a superb companion plant to the other succulents mentioned above.
I have found that some Southwestern natives have expanded a genera's usage from colder climates into warmer ones. The genus Aquilegia (Columbine) is an excellent example. Arizona is home to several of my favorites, Aquilegia desertorum and Aquilegia species Swallowtail®. Both of these heat tolerant species were originally collected by Sally and Tim Walker of Tucson, Arizona. Unlike Aquilegia caerulea (Rocky Mountain Columbine) these two species have excellent tolerance to dry heat, expanding this genus' usefulness into warmer areas of the country.
The intermountain region of Colorado, Utah, Idaho, Wyoming and Montana, including both mountainous and high plains habitats, is home to numerous ornamental species. My interests in this region focus on more of the dry land species and don't include plants best suited to wet and boggy habitats.
The key to this ongoing effort will be to focus on selecting plants that adapt well to domestication and to develop improved methods of propagation and cultivation.
Penstemon are prominent on that list and include Penstemon strictus, Penstemon barbatus, Penstemon eatoni, Penstemon virens and an amazingly xeric species, Penstemon linariodes v. coloradensis. Colorado Narrowleaf Beardtongue, as it's known regionally, puts on an amazing display of lavender flowers in late spring. But even better is its bright blue evergreen foliage, which gives this little gem year-round appeal.
Campanula rotundifolia is another favorite, long blooming wildflower. Useful in both full sun and partial shade, this little bluebells species naturalizes (re-seeds) readily when happy. It is an excellent companion to mountain Aquilegia (Columbine) like Aquilegia formosa, a red and yellow flowered species, and Aquilegia caerulea (Rocky Mountain Columbine) that both enjoy cooler summer temperatures and moister conditions.
The high, cold desert regions of Nevada, western Utah, eastern California, Oregon and Washington are known as the Great Basin. Amazingly it has only been recently that many of its fantastic ornamental species have found their way into cultivation. Salvia pachyphylla (Giant Flowered Purple Sage) should be on top of everyone's list. The seeds of this fascinating species were originally supplied to me through the collecting efforts of Alan Bradshaw (Alplains Seed Company). Native to California, this species has proven itself cold-hardy in Colorado. Numerous native plant enthusiasts in Denver and elsewhere rave about this spectacular native shrub. It has large, pungently scented silver foliage and showy blue and rose-pink flower spikes that begin in mid-summer.
In writing about native plants, I have been reminded of how they are an indispensable part of any garden or xeriscape. I have not had the room to cover all the species we offer (including many grasses and shrubs), but I've tried to discuss some of the more interesting, lesser known varieties. Certainly, all of us native plant enthusiasts hope to continue to bring more species and selections (cultivars) into cultivation. The key to this ongoing effort will be to focus on selecting plants that adapt well to domestication and to develop improved methods of propagation and cultivation. Keeping these criteria in mind promises that both experienced and beginning gardeners will enjoy growing natives in their yards and gardens.