All too frequently, during the past 30 years, seed production from rhododendron pollinations made outdoors during the cool spring and early summer weather of Western Pennsylvania has been either poor (few seeds) or non-existent. More recently, on plants pollinated in the greenhouse under high heat and humidity conditions, seed production is much improved. This phenomenon of improved seed production with high temperatures and humidity has been utilized to produce viable seed from hitherto "impossible" crosses, including a number of cold-climate rhododendrons with Malaysians. The more difficult crosses can be made with the use of a special high temperature and humidity cabinet described later in this article.
For a number of years now I have been letting my plants go dormant outside and then bringing them into my cold house in January where hybridizing can be performed under controlled conditions. The temperature is 40°F at the time of entry and each week I raise it by 10° until a 65°F night temperature is reached. During the daytime hours the temperature reaches a 90°F high when the sun is shining. The warm temperatures plus supplementary light to lengthen "daylight" hours are ideal to make the buds swell and blooming soon begins. This gives me the advantage of early crossing, while avoiding contamination from outdoor sources since there are no insects present nor any severe wind activity. Air circulation in the cool house is minimal and the chances of accidental pollination are quite low. The controlled temperature and high humidity give ideal conditions for hybridizing and I have been very successful in obtaining large seed capsules containing excellent viable seed. Another advantage of this procedure is that this hybridizing can be done when snow is still on the ground and there is more time available.
The good seed production obtained by following this procedure has also inspired me to experiment further with using heat in hybridizing. After exchanging ideas on this subject with the late Dr. E. A. Hollowell, an excellent authority on heat in hybridizing, and with other breeders who think along these lines, I began 17 years ago to try other crosses and temperature conditions. I want to mention here that I also read all available material on the subject, and a few of the important facts dug up during this research are as follows: When hybridizing Lilium, Dr. S. Emsweller heated the pistil to 120°F in water dried it and pollinated.1 Dr. W. Ackerman made crosses on Camellia, but when he dropped the temperature more than 8°F the same crosses failed to set seed. In 1932, L. F. Randolph used high temperature on polyploidy and other variations in maize (as printed in American Society Agronomy Journal, 28:990-996).
I began my serious experimental work by constructing a growth-temperature chamber of 3/4-inch plywood, painted white inside to provide supplemental light and to protect the wood from moisture. The top is plate glass, with fluorescent fixtures on the outside. There are six fluorescent tubes to enable me to give red light, white light, or a full spectrum of light to the plants. The cabinet has a glass front to allow observation of plants. On each end there is a small fan, and on the one end there is a hot plate installed above the fan. The combination of fan and heater supplies heated air throughout the chamber. On the inside at the bottom is another hot plate on which I can place a pan of water to produce steam and raise the humidity (at will). The heat and humidity systems are personally monitored for a 2-hour period, making adjustments as necessary to keep the temperature in the effective ranges. The aim is to have as much heat and humidity as possible without dehydrating the foliage on the plants. This is the principle I use when hybridizing plants that have proven difficult or impossible to produce seed by more conventional techniques. I have found that the temperature range which plants will withstand is 110°F to 114°F (for a maximum of about 3 hours) with the humidity as close to 100% as possible. At this temperature I found the plants remained in good active condition with no wilting or other detrimental effects on them. Van Huff's rule states: "For every ten degrees rise in temperature, the speed of chemical reaction is doubled."1 With this in mind, I decided that if I could hold this temperature for around 2 hours, without any ill effects, it would be the same as having the pollen on the plant for a period of 32 hours, which should be sufficient time for the pollen to act. As a matter of fact, I am sure the pollen reacted in a much shorter period of time and was more effective with the increased heat and humidity. A comparison of the amount and size of seed of selfed R. yakushimanum produced by this method to that by pollination in the greenhouse reveals three times more seed and larger seeds. Relative size of the capsules is more than double.
In 1974, I crossed County of York with R. laetum and successfully produced hybrid seed. Thirty-six plants have been grown from this cross. These were kept inside the cold house year round. Some bloomed in 1977, others in 1978, and a few in 1979. Some of the blooms resembled those of R. spinulierum (never fully opened — the pistil protruded), some opened halfway, but there was no pollen. I had no success whatsoever in making further crosses with these using R. laetum, macgregoriae, aurigerianum, and zollerii as the pollen parents. The pistils seemed to be active and I kept trying with the conventional method (no excess heat and humidity) but no seeds were produced. I felt the problem might be related to the fact that they had been kept indoors, so this year, after they budded, I put them outdoors. If there are any survivors I will test further with them.
Another cross I am closely watching is [((Mrs. Yates x R. yakushimanum) x Maletta)) x R. macgregoriae]. There are nine plants from this cross. Two budded and bloomed in 1979 inside the cold house. The florets were fully opened, but the stamens did not develop pollen. When these bloomed, I crossed them again (no excess heat or humidity) with the same set of Malaysians I used on the County of York cross. None made any effort to develop a seed capsule.
My greatest success in the heat chamber came within the lepidotes. I had been trying for years to cross my good, extremely hardy, pink carolinianum with P.J.M., which some have reported as pollen sterile. My purpose was to get a later blooming plant and still capture the good traits of P.J.M., or even an improved combination of the two. My pink carolinianum blooms late in May or early in June. I used the heat treatment and put P.J.M. pollen on my pink carolinianum and now have many seedlings as a result of this cross. I know definitely that the cross is true, for when I make a cutting or snap a leaf of the seedlings, there is the same odor that is present in P.J.M. This odor does not exist in my pink carolinianum. To date I have selected and numbered nine of these seedlings that I like best. The number nine I selfed last year under normal conditions in the greenhouse and it set good seed. These plants are now growing and the results are awaited with much interest. When dormant, as they now are, the foliage has a bronze/red rather than a dark red/purple fall color, as does P.J.M.
Using the heat treatment I also put P.J.M. pollen on Pioneer and definitely realized hybrids since these plants haven't the characteristics of either plant in foliage. Some are deciduous, being completely defoliated outside at present after several hard freezes.
Based on my experience, rhododendron crosses made under controlled conditions of high temperatures and humidity produce larger and more seeds than those made at cooler temperatures. In certain instances the use of high temperatures and humidity produces seed from wide crosses where normal techniques have been unsuccessful. This new technique seems to have potential for combining the drought resistance and brilliant colors of Malaysian rhododendrons with the cold resistance of hardy hybrids and species.
1Proceedings of Breeders Roundtable, 1973, American Rhododendron Society.