Some varieties of rhododendrons will make very satisfactory plant growth in full sun in many areas. However, a very hot day during bloom may cause the blossoms to wilt, possibly show burning around the edges, and last for a very short time. One of the principal reasons for growing rhododendrons in partial shade is to preserve the blooms for a maximum period.
After getting by last winter without a serious frost, we had one on May 12 which froze the flowers that were open, and the new growing shoots, some of which were several inches long. Temperatures reached 22° in nearby cranberry bogs. Plants under lath, or at a few feet higher elevation, were uninjured. This again emphasizes the importance of careful selection of planting site.
The importance of additional nitrogen where a sawdust mulch is used is frequently stressed. However, the amount will naturally depend somewhat on the fertility of the soil before it was mulched. On our very sandy soil, almost pure sand as a matter of fact, where sawdust has been rototilled in and a sawdust mulch added, very heavy nitrogen applications have been useful. In one case we applied, to young plants in the lath house, close to 1200 lbs. per acre of ammonium sulfate at one application. This is about four times as much as is usually recommended for an application on crop land. The ammonium sulfate was put on dry and flushed off of the leaves with a hose. Very satisfactory growth throughout the season was made after this application.
Many sources of nitrogen are available for use on rhododendrons, but it would seem that ammonium sulfate is about the most nearly ideal. The nitrogen is in the form of ammonium which is the way rhododendrons absorb it, rather than in the nitrate form. The residue left when the ammonium is absorbed is sulfate which has a rather strong acidifying action. A question might be raised as to whether this would be harmful on soils which are already strongly acid. The acidity of the soil in the lath house mentioned in the above paragraph was slightly below pH4, and no harmful results from excess acidity were observed. However, it does seem that some lime might be useful under such acid conditions.
Where plants growing in rather poor dry soil are mulched with sawdust, and presumably fertilized sufficiently, roots from nearby trees frequently find such a spot and luxuriate in the unusually favorable conditions. I have seen such areas very closely matted with roots of trees which have kept the soil dry, and presumably short of nitrogen, even though attempts to correct these conditions have been consistently made. Under such conditions it may be necessary occasionally to use a sharp spade and cut the roots around the mulched area. On very shallow soils where the roots are near the surface, this may not be very much of a problem, but it would be much more difficult where the soil is quite deep. However, where soil conditions favor deep rooting of the trees, there is less likelihood of their competing so seriously with the rhododendrons in the surface soil.
R. 'Radium' is a very attractive bright red rhododendron which is beginning to receive considerable notice in California. It blooms a little later than 'Vulcan,' is brighter red, and the flowers are not over-grown by the developing shoots as much as they are in 'Vulcan.'
Frank Kingdon-Ward, internationally known plant explorer, and one of the last of the active ones, died in London on April 8. He made his first exploration trip in 1909, into the interior of China. His last trips were made in the 1950's to Burma. He received many honors and medals from the Royal Horticultural Society, Royal Geographical Society and the Scottish Geographical Society. Several interesting books on rhododendrons and plant hunting were written by him. R. wardii, discovered and named for him is one of the most beautiful of species rhododendrons.
Two fine forms of R. tephropeplum bloomed in our garden this spring. One grown from cuttings from a neglected plant in an early-day Portland collection, was large flowered and a very clear pink. The second plant was a seedling from the 1952 Kingdon-Ward expedition to Burma. It was also a fine pink and the flowers were twice the size of the R. deleiense form of R. tephropeplum.
On June 22, when these notes are being written, 'Polar Bear' is in full bloom, also 'Margaret Dunn', 'Golden Bells', R. nitens, R. brachyanthum. Several of the Lapponicum series are showing a few scattered flowers; blooming for the second time this year. Many others are heavily budded and the buds are swelling and they are certain to bloom again in a month or two. In the garden of Mrs. A. C. U. Berry, R. diaprepes, a big 8 foot plant is loaded with flowers, wonderfully scented.