C. florida var. sieboldiana has been in the European trade for over a century and a half, never very numerous nor easy to find, never very predictable, but perennially interesting. The creamy-white blossoms, usually not more than about three inches across, come in pairs, swaying at the ends of long stems. The feature that dominates them (present in only a very few clematis) is the showy central tuft of purple staminodes, which can stay in place for months, long after the tepals fall away. The foliage, too, remains green and fresh for a long season.
It is well to know from the start that the gardener who forms an attachment to the bicolor had better have a little sporting blood. It is a capricious plant, and may do well in one year and not another. (This is one reason why it is sometimes hard to find; garden merchants do not enjoy carrying plants that they fear customers may want to bring back for a replacement.) But the fact is while ‚Sieboldiana‘ can be quirky (schrullig), it is far more robust than it looks. Even after a year or so of quiescence (Ruhe) it can start up unexpectedly like a phoenix from its ashes and put on a splendid bloom. So a gardener should not feel betrayed if it does little or nothing for a season. Anyone who has seen a mature plant of it climb into a tree and send down long streams of its unique paired flowers should be willing to weigh that against the possibility of an occasional lean year. With a little protection from wind and marauding puppies, there should be few lean years, or none.
There is something else about this plant that fascinates students of clematis. Its exact relation to the parental C. florida is unknown; it is presumably a garden hybrid, originally from Japan or perhaps China. But what is extraordinary is that it exists in two (if not more) forms that can sport back and forth. The dominant form is the Sieboldiana just described; but growers who raise it in large quantity have grown accustomed to spotting random volunteers of the form known as C. florida var. flore-pleno (alias ‚Alba Plena‘) in the midst of the crop. Almost lost for a while, this cultivar is now in commerce on its own—like var. sieboldiana in its flower except for a greeny-white color throughout, even to the central tuft. Almost unbelievably, the two forms do not have the same period of bloom, the flore-pleno variant often starting up well ahead of its alter ego. (Both, however, bear their flowers on wood of the current season, and hence profit from hard pruning late in the year if they do not die back of their own accord.)