Rhododendrons in the Cool Greenhouse
Dr. J. A. Bradley
Some of the finest rhododendron species yet discovered either are too tender to be grown in the open in this country or are so frequently damaged by cold spells or early frosts as to prove discouraging to even the most enthusiastic growers.
With greenhouses now within the financial means of the average gardener, and their numbers increasing rapidly, it was felt that an article dealing with the species and hybrids which do well in the shelter of a glasshouse would be of interest, particularly since so little on the subject has appeared in the American literature on rhododendrons.
It is true that species such as R. crassum , leucaspis , ciliatum , burmanicum , johnstoneanum and hybrids such as 'Fragrantissimum' and 'Cilpinense' to name a few, have survived in the gardens of the Pacific Northwest for years, but usually with the loss of some foliage in colder winters, or of the bloom by frosts. Yet these same plants are far more beautiful and satisfactory to grow in the cool greenhouse every year, and furthermore can be grown in many localities where their out-of-door growth is out of the question.
Even the smallest greenhouse has room for a few plants since many are dwarfs, and make excellent pot plants. For the average gardener a house 8x10 or larger is desirable. My plants do well in a house 15 x 24, but as interest grows so do the plants and crowded conditions will probably lead to expansion. The smaller greenhouses are easy to heat but it is difficult to control the temperature in summer, so the pot or tub plants are best moved to the protection of a lath house or the shaded garden in hot weather. A minimum night temperature of 32 to 40 degrees is desirable, but an occasional drop below this will do no harm. During the past winter when the heating system failed temporarily, a temperature of 22 degrees for hours did no apparent damage.
The soil mixture is important in plants grown in pots or tubs, and should contain more humus and sand than needed by outdoor growers, and also very sharp drainage provided by broken pots or gravel in the container is a necessity. This is particularly important with many species of the Boothii, Edgeworthii, Moupinense, and Maddenii series since many of them are epiphytic in nature; in fact some of them seem to do well in a mixture of only peat moss and sand. For the average plants a mixture of one-sixth soil, one-sixth oak leaf mold, one-third peat, and one-third sharp sand has proved excellent. This mixture has given a pH reading of 4.5 to 5 as tested with nitrazine test papers. The only fertilizer used has been an occasional light feeding of cotton-seed meal.
Watering is another important item in successful culture. In the dark days of winter when the plants are dormant anyway, they are best kept on the dry side, with an occasional watering on a bright morning. When active growth starts they will stand more moisture, and as the weather warms up they enjoy a spray on the foliage. A misty spray on a hot day will also tend to keep the temperature in the greenhouse within bounds.
The Maddenii series take very kindly to pot or tub culture in the greenhouse. R. taggianum , described and pictured in the July 1949 Bulletin by Mr. D. W. James is undoubtedly the finest. Several of the plants bloomed locally this year and were magnificent. R. burmanicum which has been a rather indifferent garden plant, was a gorgeous greenhouse shrub this year. In March, my four year old, two foot high plant had thirteen buds which on opening changed from chartreuse, to a greenish-yellow when open, then faded to a cream-yellow, and with seven to eleven flowers on a truss. Others in the series which have bloomed for us locally have been ciliatum , ciliicalyx , crassum , iteophyllum , johnstoneanum , rhabdotum , and veitchianum and all have been most beautiful. We are looking forward in the near future to bloom of the following which have been growing well: calophytum , carneum , cubittii , dalhousiae , formosum , lindleyi , maddenii , megacalyx , nuttallii , parryae , polyandrum , scottianum , supranubium , and valentinianum Of the Edgeworthii series, both bullatum and edgeworthii bloomed profusely, filled the greenhouse with their fragrance and were well worth observing at any time because of their interesting foliage. The Boothii series represented by aureum , boothii , leucaspis , and tephropeplum bloomed beautifully and were well worth going miles to see. Others of the series which we are looking forward to seeing bloom are auritum , megeratum and sulfureum . Both the pink and white forms of moupinense were a mass of bloom this year, and with no frost or rain damage far surpassed any plants seen blooming out-of-doors.
Last but not least are the tender hybrids. The most outstanding was 'Grierdal' with its truss of large scarlet flowers looking like an over-sized R. griersonianum . 'Lady Alice Fitz-William' was covered with bloom as usual, and is one of the best because of its delicious fragrance which persists even after the flowers have completely faded. There are many other hybrids too numerous to mention which are well worth growing, and many new ones are in the making as the result of an extensive hybridizing program with many of the mentioned species.
It should be noted that many of the above rhododendrons particularly those with epiphytic tendencies, do as well or even better than when on their own roots, if grafted. The article in the October 1949 Bulletin by Mr. C. P. Raffill tells of the exceptionally fine inflorescence of several of the Maddenii series when grafted on R. ponticum . Several which do not take kindly to ponticum root-stock seem to be doing well on the hybrid R. 'Fragrantissimum'. It should also be noted that most of the above mentioned rhododendrons are readily rooted from cuttings.
It is hoped that this article will stimulate others to grow at least a few rhododendrons in their greenhouses, and so write of their experiences for the benefit of all of us, since there is so little in the available literature on the subject.