Only later did I read that D. knappii was a diploid as opposed to other Dianthus, which are hexaploid. So among my dozen or so D. knappii there must be a polyploid lurking.

I had rather hoped it would be either yellow or double (hopefully both), but turned out to be neither. The seedling is pink, it has a wonderful fragrance and is a rather petite plant. It might not be worthy of cultivar status but I'll certainly be keeping it.

Not only will I be crossing my entire stock of D. knappii this year to find which one is the polyploid, I'll also be growing the seeds of this cross, provided it makes any.

Congratulations. It's pretty.

Incompatibility between species is not always related to differences in chromosome number. It is possible to cross diploid and hexaploid roses, for instance. It may be that your hybrid is a tetraploid -- one set of chromosomes from D. knappii and 3 from 'Helen'.

It might be worthwhile pollinating your hybrid by 'Helen'.

Karl

I've got some more knappii seedlings on the go now so am planning some more attempts this June.

The seedlings from the cross with the white plumarius were pale cream-yellow with some differences in intensity. Twenty crosses were made and four seedlings produced. Because the researchers found self-pollinations in early crosses they emasculated the flowers for subsequent crosses. D. knappii was the pollen parent in all crosses.

As a possible interest not all Dianthus allwoodii are hexaploid - a chromosome count of D. allwoodii 'Doris' found it to be 4n.