Some Pointers on the Production and Care of Rhododendron Seed
J. A. Witt, Seattle, Washington
Raising rhododendron species from seed can be a most frustrating method of propagation, as anyone who has ever received seeds of species collected from cultivated plants can testify. All too often seedlings raised from an open pollinated plant turn out to be obvious hybrids and not the true species that ardent rhododendron collectors desire.
This has been most graphically demonstrated in the University of Washington Arboretum where literally hundreds of rhododendrons have been raised from seed collected both from our own plants and from other similar gardens in England and Europe. Many fine rhododendrons have been raised but all too few really qualify as good species.
American Rhododendron Society members who have authentic rhododendron species in their gardens can, by taking the little time and effort necessary for self-pollinating a few of their flowers, do a great deal to promote the distribution and culture of truly fine species. The actual process of selfing a rhododendron is very simple and normally can be done with very little equipment. A pair of small forceps, or tweezers, a few tags and minimal knowledge of floral anatomy will be enough, but a small hand lens and some method of covering a flower truss can be helpful in insuring pure seed. Ideally, the same care taken in making a cross should be used in selfing a species. That is, unopened trusses are covered with insect-proof bags to keep stray pollen out and pollen taken from protected florets used. However, since many growers object to disfiguring their plants with bags it is possible to self-pollinate most rhododendrons in a simpler manner. Use only newly opened florets and examine the stigma to see if any stray pollen grains are evident - if the stigma appears clean then a ripe anther from the same flower on which the pollen is shedding can be brushed lightly across the stigmatic surface. This should leave enough pollen to insure a good seed set in the fall. The corolla may then be cut back about half way or taken off altogether so as to remove the "landing field" for bees and other insects carrying foreign pollen. Normally only several flowers of a truss will be in a suitable condition to hand-pollinate, others will be too far advanced or too tight in bud to work with. It is well to destroy the ovary of those florets by squeezing them with the forceps; this leaves the corolla intact but removes the chance for contaminated seed in other capsules. The whole truss can then be labeled and when it is harvested there will be no capsules from open pollinated flowers. Labeling selfed flowers is as important a step as any in the process and should not be overlooked. A small paper tag hung around the base of the truss marked "self" and with the date is usually enough and will help to locate the selfed capsule when they are ripe. Not more than ten to twenty florets need be treated this way unless the species is unusually fine or very rare, since each capsule may produce enough for several packets of seed.
The hand pollinated capsules should be watched carefully and harvested when the pedicels begin to turn brown or when their tips begin to show signs of splitting open. This will vary with the species―R. macrophyllum is often ripe, under Seattle conditions, by mid September while R. racemosum may be safely left until December or later. The Arboretum makes its major harvest of Rhododendrons in late November and early December.
Cleaning the seed is as easy as selfing the flowers, in most cases. Often it is only necessary to leave the nearly ripe capsules in a box or sack in a warm room. Very shortly the capsules split open and the seeds fall out by themselves, and by shaking the container a few times then removing the empty capsules, the seed is cleaned. There may be a certain amount of debris left with the seeds but this can be removed with the aid of a kitchen sieve with a mesh large enough to pass the seeds through but retain the chaff. More recalcitrant capsules such as those of R. racemosum or those picked somewhat green may need force to get them to free their seeds. These can be broken open by gently pounding or beating with a flat stick, or by rolling them with a rolling pin. Naturally the seeds released by these strong-arm methods contain much more dirt and chaff but most can be removed again by sifting, as above, although a much more thorough job can be clone by using several screens of graded mesh sizes.
The resulting seeds should be placed in seed-tight envelopes and carefully labeled with the species name, any comments about uniqueness of the parent plant, and marked "hand pollinated." A few milligrams of species seed from hand-pollinated flowers is worth much more than a kilogram of open pollinated seed from the same plant, and will do much to improve the standard of rhododendron culture in America.