We are profiling Heucherella 'Quicksilver' and H. 'Heart of Darkness.'
Origin of Heucherella
This little group of garden plants is unusual in that it is composed of varieties that are all hybrids between Heuchera and Tiarella. None of them exists in the wild. Generally the plants are intermediate to the two genera, but in the eastern United States Heucherella have cultural requirements more like those of Tiarella. Unfortunately, all Heucherella are sterile, so it has been impossible for plant breeders to move the traits of Heuchera into Tiarella and vice versa.
The first Heucherella to become well known in gardens was 'Bridget Bloom,' a hybrid between a pink Heuchera x brizoides and Tiarella wherryi. This was introduced by Blooms of Bressingham in 1955 and had the market to itself until 1983 when 'Rosalie' appeared. Both of these have pink flowers intermediate in form to Heuchera and Tiarella and green leaves with some maroon marking. In 1987 The Primrose Path tried a cross similar to that done by Blooms and in 1989 introduced a pink flowered variety, 'Tinian Pink,' and a white flowered sibling, 'Tinian White.' These were late renamed 'Pink Frost' and 'Snow White,' respectively, and resemble the earlier introductions. The great diversity of varieties of Heuchera and Tiarella that appeared in the 1990s has made possible a tremendous number of possible new combinations for crosses, and many Heucherella, some with bronzy and cut foliage, have been introduced by The Primrose Path and Terra Nova.
Part of the reason that Heuchera and Tiarella will produce hybrids is that they both have the same chromosome number (N = 7). The related genera Mitella and Tellima also have the same number of chromosomes and will probably produce hybrids, also. At The Primrose Path we found an unusual spontaneous seedling that seems to be a hybrid between Tiarella polyphylla and Tellima grandiflora . It is of botanical interest only, with small green flowers, and rounded hairy leaves.
Unfortunately for gardeners Heucherella have proven generally to have neither the wide adaptability to environmental extremes or the vigor of either Heuchera or Tiarella. When they are grown under pampered conditions as well-fed, protected potted plants or outside under relatively even climatic conditions like those in the Pacific Northwest, Heucherella grow well. However, where plants are grown under less than ideal conditions and given hot, humid summers like those in the eastern US, there is often a serious dieback ("meltdown") problem, with the plant reduced to small crowns surviving around a dead center by late summer. If these small crowns are divided and replanted, they will form new clumps for next year. We have not trialed all of the new Heucherella at The Primrose Path, but of those that we have grown, only 'Quicksilver' has been completely free of this dieback problem.
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