Sunday, September 6, 2009

Zippers and Offsets - Problems Affecting Symmetry in Petrocosmeas

I was happy, and relieved at a recent Gesneriad Society show, to see that the judges has given a blue ribbon to a Petrocosmea that had "zippered". Zippering is a term sometimes applied to a phenomenon seen among some Pet species as they mature, where the normal circular central growth point begins to elongate and become linear, creating the appearance of a "zipper" down the center of the plant. It is often felt that this ruins the symmetry of the plant, which we've come to expect, and perhaps even demand be a perfect, symmetrical circular rosette of leaves. I have heard judges comment that they "don't like" this appearance, however it is "what the plant does", and therefore should not be penalized substantially, when judging.

A plant of Petrocosmea 'Short'nin' Bread' that has "zippered", causing the central growth point to elongate. P. 'Short'nin' Bread' is a hybrid between P. flaccida and P. forrestii. P. forrestii is prone to zippering, and is likely the reason this hybrid is also prone to this phenomenon.

A plant of P. 'Short'nin' Bread' showing the more typical, and aesthetically pleasing round growth point in the center. With maturity, this plant could still go on to zipper.
Zippering is one of those qualities that is not common among all Petrocosmea species but is typical of a few. Those species that are prone to zippering are P. rosettifolia P. forrestii, and P. species 'China-2005'. One of the goals of my hybridizing program is to select cultivars that are free of this trait. This will require growing the plants until they are mature up to three or four years of age, as this trait does not often show up until the plants are a few years old. P. forrestii is a prolific parent for me, and I have used it in a number of crosses, so these will have to be evaluated for a while in order to check them for this trait. As P. forrestii contributes a smaller plant size, floriferousness, and a pleasing leaf shape and rosette shape to it's progeny, it is still worthwhile to utilize the species as a parent in spite of the tendency to zipper.
Why plants zipper is a mystery, however, I have a theory. As the zipppered plants continue to mature, they bloom much more heavily and will eventually form multiple crowns, which will eventually seperate on their own and become seperate plants. I feel that zippering may be an evolutionary tactic to increase the species by forming new plants as the crowns seperate, and perhaps, with the heavy flowering, making the plant more likely to produce more seed for propagation of the species. Of course, I have not idea if any of this is true, but it is my own theory of why this might happen in some species.

An example of a plant that never zippers for me ( at least it hasn't yet on my seven year old plants). Petrocosmea sericea forms and maintains a beautiful symmetrical flat round rosette.

Another bothersome habit of some Petrocosmea species is that of abundant offset production. As some plants mature, and near flowering size, the flat round rosette suddenly will begin to become "crumpled" and the orderly leaf arrangement becomes jumbled and messy. Soon after, offsets can be seen enlarging underneath the leaf crown. As they mature, the plant begins to look more like a mass of jumbled leaves. This ruins the symmetry of a plant that might otherwise have been a perfect showplant. Of course, the offsets can be removed and rooted for instant plants, or left on the plant if a flat rosette is not that important to you. The offsets will bloom too, creating a large mass of flowers .
A four-year-old plant of P. rosettifolia #3, in a five inch pan pot, showing this year's crop of offsets. I have chosen to leave them on the plant to let it mature and see what continues to happen. Offset production seems to be triggered by stressing the plant from underwatering or as the plant nears flowering size.
Again, offset production is a problem from the standpoint of selecting future showplants. It is another trait I have chosen to select against in my hybridizing program. Species prone to offset production are P. nervosa, P. flaccida, P. barbata, P. rosettifolia, P. sp. 'China-2005', and P. forrestii. P. duclouxii and P. sp. 'Yumebutai' have been rare producers of one or two offsets per plant. P. minor, P. sericea, and P. begoniifolia have never produced offsets for me in my conditions.

A three year old plant of P. forrestii showing a splitting of the crown into three seperate crowns. This particular plant did not zipper, but just formed three crowns. As the plant continued to mature, these three crowns eventually "fell apart" in the pot and continued to grow as seperate crowns. This plant flowered very heavily, with in excess of two hundred flowers produced last season.

Since most Petrocosmeas entered in shows are entered "not in flower", as foliage plants, the plants must have a high degree of ornamental value to score well. Ornamental value, to many judges, seems to demand a Petrocosmea have a perfectly round, flat rosette, with the leaves arranged in an orderly spiral extending out from a single crown in the center of the plant. Again, this is most, but not all judges. Therefore, I feel that selection of clones and new hybrid cultivars that maintain a rosette free of offsets and zippering is a trait hybridizers should be striving for.