First, let me say that this is what works for ME, in MY environment, with MY plants. Other growers, in other conditions, may have very different results. I don't suggest that I am an expert, only that I've been lucky for a short time now, and this is what worked for me. While I have shared this with other accomplished growers in the recent past, I have heard from only one person who was successful in producing seeds. There are others, who've had success, of course. Other well before I had luck. Mr. Nakayama, in Japan, in fact, produced the first Petrocosmea hybrid around a decade ago... P. 'Momo'...the result of a cross between P. nervosa and P. flaccida. He has since remade the same cross and produced two selections..P. 'Asa Blue' and P. 'Imperial Butterflies'. Mrs. Joyce Stork, here in the US, produced introduced two hybrids a couple of years ago, P. 'Fluffer Nutter' and P. 'Short'nin' Bread' both from a cross between P. forrestii and P. flaccida. And Mr. Jeff Foerderer of the Netherlands, had produced a number of crosses recently. He registered two of them with the Gesneriad Society....making these the first registered Petrocosmea crosses. Those were P. 'Lexi' and P. 'Milan'. So we are beginning to see some success with hybridizing Petrocosmea. To date, I have been successful in producing seeds from a total of 22 seperate hybrid crosses over the past three years. Most of those, 14 of them, produced during this past winter.
So what has worked for me? Well, the key factors for me, which I have tested and proven during the past two winters, have been a combination of high humidity during flower production and seedpod maturity, and cool to cold temperatures. If either of these factors was changed or missing, I have had total failure in the crosses. Furthermore, I must say that those same two factors have been proven critical to successful seed germination as well. But first, lets get the seeds and then we can discuss how to germinate them.
So here are the steps I follow to produce seeds:
1. The seedpod parent must be flowering in high humidity...100% if possible. As plants come into bud, I enclose the whole plant inside humidity domes over the nursery trays on my lightstand shelves. Also the area should be cool...around 40 degrees F. A bit cooler is fine...I've had successful pod formation in 38 degrees F and as warm as 50 degrees, but no warmer.
2. Select flowers for pollination that are at least four days old. Older is fine as long as the pistil and stigma are still in good condition. Around the time the flower is about to drop seems the best, but the stigma is usually not receptive until around four days.
3. Select pollen from the freshest flowers you can find...freshly opened up to three or four days seems optimal. Pollen should look "fresh and fluffy" dry dusty pollen does not work for me. Frozen or refrigerated pollen has also failed to work for me too. I use a simple process of collecting the pollen on my thumbnail from the anthers so that I can see it easily. "Pack" as much pollen on each stigma as you can get it to accept.
This photo, taken by my friend and master hybridizer Dale Martens, shows the reproductive structures of a Petrocosmea flower. Note that the stigma at the tip of the pistil is purple. The filaments supporting the anthers have furry white hairs.
4. Label each flower pollinated. I use a small square of paper with a hole in the center and a small slit on one side to allow it to be slipped onto each flower peduncle. I also record the parentage and date of the cross in a manual which I keep all of my hybridizing records in. This becomes a most valuable resource later on.
This photo shows seedpods forming on P. forrestii. Note that after successful pollination, the tip of the pistil dries to just below the stigma. the ovary swells quickly and pollination will be evident in around 8-10 days after making the cross if it has been successful. These seedpods are about one month out from pollination.
5. Wait. This is the hardest part. Ripening for me has taken 60-80 days with around 64 days being the most common. I have had successful germination from seeds taken from a pod that was only 49 days old, when the mother plant began to deteriorate and I was afraid of loosing the cross. Generally, though, I wait until the seedpod browns and is just beginning to split open. Seedpods will split from the base upwards along two sides of the pod. The seedpods appear to have two chambers.
Another excellent photo taken and shared with me by my friend Dale Martens, shows Petrocosmea seeds. Viable seeds appear plump while non-viable seeds appear more linear and smaller.
6. Once the seeds are ripe and harvested, I sow them immediately. I only sow a small amount of the seeds and freeze the rest. I sow seeds on moist, fine vermiculite or moist peat moss pellets. I enclose the seeds once planted and keep them moist with high humidity. In the same temperatures used for seed production...around 40 degrees F, I get germination in 14-28 days from fresh seeds. Once seeds are a bit older, I find germination takes longer, and it has taken as long as 3 months on older seed. Seeds sown in warmer temperatures have consistently failed to germinate for me and several other growers with whom I've shared seeds for testing.
Once seeds germinate, I begin to fertilize them with a weak solution of water soluble fertilizer at a rate of 1/8 - 1/16 teaspoon per gallon of water. I keep them under lights in the same conditions the mature plants grow in.
Once the seedlings are large enough, I transplant them into community pots and treat them just like the mature Petrocosmeas in every way. Of course, labeling and careful record keeping are maintained at every step of the way. The photo above shows seedlings at about three months from germination.
Again, cool temperatures and high humidity are what have made the critical differences for me. I hear often from growers who have tried repeatedly to cross Petrocosmea and failed. When I ask about the conditions in which they attempted the crosses, one or both of these factors always seems to be missing.
This year, I tested this several times by attempting crosses in lower humidity and in warmer conditions. I had NO success. Only when both factors were present did the crosses take. Even then, with the two critical factors in place, my success rate with total attempts remains under 50%. I make sure to pollinate around ten to twelve flowers each time I make a cross. Sometimes I get as many as 7 seedpods, other times, I may get only one. And of course, sometimes I get none. But, the challenges are what makes this so much fun and so rewarding once those seedlings finally do bloom. This is part of what fuels my passion for these magnificent creations of nature! One thing that alway occurs 100% of the time though, is that I learn something every time I try a cross. And I have fun 100% of the time too!