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Machaeranthera coloradensis “Colorado Tansy-Aster”
by Kristl WalekIf you have sought out daisies suitable for the rock garden, you have undoubtedly tried the ubiquitous (and not particularly small) Aster alpinus. Then there are the worthwhile mounding and rock-hugging species of Erigeron, as well as members of the genus Townsendia. The latter are small and beautiful as a clan, but generally monocarpic. T. parryi is the only species that has persisted through self-sowing in my garden.
One could name western daisies. There are choice forms of Heterotheca, such as H. jonesii, perhaps the best small yellow daisy for the rock garden. It has fantastic silver foliage and blooms all season. I have also experimented with Hymenoxys but many of the choice endemics of the American Rockies do poorly or are short-lived in my eastern North-American garden.
That is why it was such a pleasant surprise to find a beautiful, small, western daisy that is so happy here. Machaeranthera coloradensis (formerly Aster coloradoensis) is considered by some to be the finest American aster. It is at home in Colorado and Wyoming growing in sparsely populated patches on gravelly or rocky slopes in full sun. It has recently taken on threatened status in both states.
The plant is a small gem 4-5cm (1.6 – 2.0”) tall to 15cm (5.9”) wide, forming lovely open mats of toothed, silvery foliage. Comparatively large, luminous bright pink flowers are produced on short stems all season. I have tried it in the rock garden, sand bed and in scree; it has been happy everywhere.
While it is a bonus to have long-blooming plants in the garden generally, we do not have as many choices among rock plants. The Colorado Tansy-Aster will begin flowering soon after the rush of the spring alpines finishes and continues blooming until fall. Although I would surely welcome it, there has been no self-sowing in my garden. However, it is easy to collect the fluffy seed once ripe and to sow it that winter to increase one’s collection. It germinates easily without needing any cold treatment.