JARS v37n2 - A Note on the Germination of Vireya Rhododendron

A Note on the Germination of Vireya Rhododendron
Donald W. Paden, Urbana, IL

Members who grow vireya rhododendron from seed have more than an academic interest in the viability of the seed they plant. This note describes an experiment which provides fragmentary evidence on the germination of two such hybrids.

R. wrightianum
R. wrightianum
photo by Donald Paden

Successive batches of one hundred seeds of R. aurigeranum x R. zoelleri (selfed) were sown at intervals of ten days over a period of roughly three months. This was done both for seeds stored in the open and seeds stored in a frost-free refrigerator. Five days after harvest, the first seeds were planted on blotting paper in "dishes" (plastic covers from cans of tennis balls, punched to provide drainage). The planting dishes (with seeds) were put in trays and covered with plastic. They were then placed under artificial light 16 hours a day at roughly 70° (room temperature in an air-conditioned half-basement) and misted daily. Seedlings were counted at intervals of ten days, removed from the planting dishes (using a tweezers) and discarded. The results for the unrefrigerated seed are shown below:

Planting Schedule
(Days from Harvest)
5 56
15 35
25 42
35 25
45 17
55 13
95 0

As expected, the longer the time between harvest and planting the poorer the germination. Thus, for the seeds planted five days after harvest, slightly more than fifty percent of the seeds germinated. For every subsequent 10-day delay in planting, the rate of germination declined by roughly one third that of the previous period. After three months none of the seeds germinated.
The rate of germination of refrigerated seed in the first period or two after planting was much the same as for the unrefrigerated seed. The rate of germination was maintained, however, for several intervals thereafter, in contrast to the results shown in the table. At the end of the three-month period, thirteen percent of these seeds germinated, as contrasted to none of the unrefrigerated seed.
The performance of a second batch of seed from a hybrid of uncertain parentage treated in the same manner was similar to that shown above, although the decline in the rate of germination for unrefrigerated seed was somewhat less (roughly one-fourth per period). Moreover, a few of these seeds germinated 90 days after harvest. The germination rate of the refrigerated seed for this hybrid was no better (but no worse) than that of the unrefrigerated seed. (None of the seed was refrigerated during the first five days.)
Irregularity in the rate of germination from period to period was observed for both hybrids and both treatments. This was due, no doubt, to conditions which were not precisely the same from one period to the next: humidity, temperature, light intensity, possible contamination, a lack of the randomness with which the seeds were selected from what might have been a non-homogeneous seed population, and so on.

R. intranervatum
R. intranervatum
photo by Donald Paden

In an attempt to extend the experiment, several hundred seeds of R. wrightianum were planted immediately after harvest, but only one seed germinated. Either the parent plant was in poor health (as its subsequent demise after transplanting later suggested) or this seed has special germination requirements. Seeds from R. intranervatum , accidentally harvested early, produced no seedlings. Both of these misadventures were disappointing inasmuch as R. wrightianum and R. intranervatum probably deserve attention in breeding programs whose purpose is to produce plants of manageable size. The spectacular color of R. wrightianum and the grotesque foliage of R. intranervatum (shown in the color photographs) add to the charm of these two plants. (Of possible interest is the fact that R. intranervatum produced trusses with both two and four flowers, instead of only two as reported by H. Sleumer in An Account of Rhododendron in Malesia pp. 654-655.)
In summary, what all of this suggests is that germination depends importantly upon the inherent vigor of the parent plant, the timing of the harvest, the period between harvest and planting, and the external conditions associated with their care after planting. The rate of germination varied somewhat between the two hybrids. Germination for both hybrids ultimately deteriorated as planting was postponed. Regardless of when the seeds were planted, very few germinated before 20 days from planting and very few germinated after 40 days. For one hybrid, seeds stored in the refrigerator retained their viability longer than non-refrigerated seed, but not for the other hybrid. All of this is not very surprising. The observations, however, may help in coming to grips with what to expect from vireya seeds and in understanding poor germination from one seed lot to the next. In any event, it would be helpful to see the experiment repeated with other hybrids and other species.