1) ... Similarly, 'Champney's Pink Cluster' has a long bloom because the repeat-blooming 'Old Blush' parent didn't suppress initiation of bloom later in the season, as is typical for R. moschata.
I have pictures of Rosa moschata
flowering happily into October.
http://bulbnrose.x10.mx/Roses/Rose_Pict ... chata.html
For many years, people thought that the once-blooming Rosa brunonii
was the Musk Rose parent to the Champneyana roses. It was not.
The influence of the real R. moschata
can be seen most clearly in the Tea-Noisettes. Each flower is borne on a very short lateral stem, rather than than at the end of every cane or shoot, as in the Teas and Chinas.
This is actually a dominant trait, as we learn from a few descendants of 'Marechal Niel', such as 'Souv. de Pierre Notting' that are bushy Teas with none of the Noisette climbing habit.
Interestingly, the triploid Wichurana Ramblers derived from the other parent being a tetraploid Bourbon, Hybrid Perpetual, or Hybrid Tea don't seem to as commonly (based on what I read) present some later blooms.
There is another possibility that I call "Elective Expression", though it has picked up a few other names in the past couple of centuries. It is known that, sometimes, it is not always a matter of dominance when two alleles are brought together. It has been observed in corn (maize) that when two inbred lines are crossed in the same way, year after, the results are not always the same for a given locus. That is, the maternal allele is suppressed some years, the paternal allele silenced in others, and in still other years both alleles are expressed.
Most of these cases involved are hidden from sight, though they are still important. But the same principle has been observed many times over the years. For instance, when the Trifoliate orange (Poncirus trifoliata
) is crossed with the common orange, some offspring are trifoliate, some are unifoliate, and some try to take an intermediate position. Hybrids of the Trifoliate orange with the lemon are even weirder.
http://bulbnrose.x10.mx/Heredity/Swingl ... e1911.html
It is useful to consider that the pattern of silencing/expression that was establish in the seedling, is sometimes altered, especially when the plant is forced into embryogenesis, as occurs when an adventitious shoot emerges from a root. This may explain how the reblooming 'New Dawn' came up as a root-sport of the once-blooming 'Dr. W. Van Fleet'.
http://bulbnrose.x10.mx/Roses/Rose_Pict ... wdawn.html
3) I think some species are easier to "nudge" into reblooming than others.
No doubt about it. Way back in 1825, Sarah Mackie listed the 'Scotch Perpetual'. No description, sadly, but at least we do have 'Doorenbos Selection'. However, I think the Damasks have the advantage.
I tried to learn when the "Autumn Damask" first came into existence. Eventually I ran into a wall: older writers were concerned with the medical virtues of plants, rather than with their gardening uses. Lobel and Pena (1571) wrote that garden roses are "also surprisingly fruitful, often twice-bearing, sometimes you may see thrice-flowering." No named varieties.
Ferrari (1633) mentioned only a Rosa "Italica flore pleno perpetua".
Then, in England, Austen (1657) wrote, "As for Rose-trees, some damask Roses, and some Provosts beare a second time, the same yeare, though but few, if cut soone after the first bearing in the full Moone. But besides there is a Rose-tree, called the Monthly Rose, which beares Roses untill the coldness of the winter stop it, about November.
I don't know how to dig further into this, but I will venture to guess that someone plucked hips, and was pleased to observe that some of the bushes opened another crop of flowers. Suckers and cuttings of these favored bushed will preserve the slight tendency to rebloom. And some of the seedlings from these plants may show the tendency more strongly.
And by the way, there was not just one "Autumn Damask". Loddiges (1820) listed the following:
103 four seasons
165 blush monthly
264 red monthly
276 bifera carnea
280 white monthly
617 perpetuelle rouge vif
660 tout les mois coeur gris
There was also a 'Striped Monthly' at the time, but Loddiges didn't mention it.