There is very little record in the literature regarding species hybrids in this genus. It seems likely that in the early decades of the nineteenth century crosses were made using P. officinalis or P. paradoxa as one of the parents, and that some of the forms offered in commerce as officinalis varieties originated in this way. There is also a group of horticultural varieties traceable to P. anomala or to P. tenuifolia which give indications of hybrid character. But of the means by which all these were produced we know little or nothing.
Towards the end of the nineteenth century, some important crosses were made by LEMOINE in France, using as parents P. Wittmanniana and the garden forms of P. albiflora. LEMOINE and HENRI also intercrossed P. lutea with the garden forms of P. moutan with strikingly successful results from a horticultural point of view; while in Germany ARENDS produced a few hybrids between P. Wittmanniana and P. peregrina.
One of us began work of a more comprehensive character about 14 years ago, and this is still in progress. The present paper gives a partial account of the results so far obtained.
1. P. albiflora X P. macrophylla: Various garden forms of P. albiflora were used in this cross. Of this strain about 400 mature plants are in being. These are almost all singles, though the Chinese peonies used in the crosses were in most cases double. The foliage characters are extremely variable in color, hairiness, etc., but in general the leaves are much larger than in the varieties of P. albiflora. The cross takes fairly well. The reciprocal cross on the other hand takes very rarely and has given only a few plants; almost all of them however are double flowered. So it would appear as if doubleness was perhaps a sex-linked character in this cross. All these hybrids are sterile or nearly so, and most of them have sterile pollens. They do however set an occasional seed, and from such seeds an F2 generation is now coming on, of which a very few plants have bloomed; in these the pollen appears to have come back to almost complete vitality.
2. P. officinalis X P. macrophylla: Various double garden forms of P. officinalis were used in this cross. The F1 generation are all single flowered. The strain includes about a dozen plants and they are all fertile and give viable pollen. The F2 generation so far as they have bloomed are also all singles and resemble either the F1 generation in general character or else go back to the type of P. macrophylla. No double-flowered forms have as yet appeared in the F2 generation, but not all of them have yet come to the blooming age.
3. P. officinalis X P. albiflora, and the reciprocal cross, using garden forms of both species: There are about 50 plants of this cross which have reached blooming age. Those which resulted from the cross. P. albiflora X P. officinalis are all singles where the male parent was a single officinalis variety whether the albiflora parent was single or double; but where albiflora pollen was used on double officinalis varieties the progeny tend to be doubles. Doubleness therefore appears again in this cross as probably sex-linked. The foliage of these hybrids is intermediate in character but in most plants is larger than in either of the parents. The flowers are usually very large and striking and they set seed only extremely rarely.
4. P. albiflora X P. Wittmanniana: Plants of this cross take a good deal after the male parent, are always single-flowered and sterile as to pollen. There are about a dozen plants of this strain that have reached blooming age and none of them has ever yet set seed.
5. P. albiflora, garden varieties, X P. lobata: This cross takes with great ease, and there is a group of about 1000 young plants as a result of it, none of which have yet come to the blooming age.
6. P. albiflora X P. Mlokosewitschi: It seems quite impossible to effect this or the reciprocal cross. Hundreds of pollinations have been made in both ways without ever producing a seed.
7. P. albiflora X P. tenuifolia: In very rare cases seed may be obtained from this cross, but of the half dozen plants so produced none have as yet reached blooming age.
8. P. tenuifolia crosses with ease with P. Veitchi, P. Woodwardi, P. Beresowskyi, these three being probably all forms of one variable species. The progeny are intermediate in character, usually almost sterile as to pollen and setting seed only extremely sparingly.
9. P. tenuifolia also crosses easily with P. anomala.
10. P. Mlokosewitschi crosses easily with P. tenuifolia either way. There are approximately 50 plants of this strain in being. Their character is intermediate, the flowers pink or light red; they have so far all been sterile as to pollen, and in the two years that they have bloomed there has been no seed set.
11. P. Mlokosewitschi crosses with P. Woodwardi and gives plants of an intermediate character whose flowers possess only minute or abortive stamens from which so far no pollen has been obtained. The flowers usually wither without expanding their petals and they are such mongrels in appearance that it is not likely they will be able to form seeds. There are about a dozen plants of this strain blooming ill 1932 and they are all closely alike.
12. P. Mlokosewitschi crosses with great ease with P. triternata and gives hybrids which have somewhat creamy whitish flowers. The pollen of those which have so far bloomed is as viable as a species pollen.
13. P. Mlokosewitschi crosses reluctantly with P. macrophylla. Two plants of this cross bloomed in 1932 and both of them gave very active pollen.
14. It seems quite useless to try to get crosses between the shrubby section and the Chinese peonies. Many pollinations have been made on the latter with pollen of tree peonies and also with pollen of P. lutea, and no success has been obtained. Messrs. VILMORIN report a probable cross between P. lutea and P. Veitchi. If this is a true cross it is the only one recorded between a shrubby peony and a herbaceous species.
15. The shrubby species intercross fairly well. P. moutan pollinated by P. lutea or by P. Delavayi gives striking hybrids which seem to have sterile pollens and to set no seed. The reciprocal cross has not been successfully made.
Many other hybrid strains than those reported on above have been produced and are now in being, but most of them only in very young plants; it seems scarcely worth while to mention them all here. Chromosome studies are in progress on some of these hybrid strains, and will be reported at the August meeting if the work can be brought far enough along by that time.
APS bulletin no 84, Sept 1941
Some Hybrid Peonies, by A P Saunders
..."Most strains of hybrid peonies are sterile during the earlier years of their growth, and this strain follows the usual rule. However, after the plants have attained to full maturity they begin to set an occasional seed.... these hybrids do acquire in time the habit of setting a few seeds...."