QBARS - v8n1 Wintering Tender Rhododendrons in the Northeast

Wintering Tender Rhododendrons in the Northeast
G. G. Nearing

For about twenty years I have been growing tender rhododendrons in the intemperate climate of the Middle Atlantic States, and having them flower for me. Naturally they don't spend the entire year in the open air, nor yet are they carried in a hothouse during the winter. Rhododendrons resent heat as much as they do cold. The hardiness ratings which consider only how much cold each species or variety will endure, should be modified to account for their intolerance of heat, which is equally important. And their capacity to face both heat and cold is greatly reduced by high wind velocity, greatly increased by atmospheric moisture.
In the Atlantic coastal plain south of Baltimore there are few places where it is possible to grow evergreen rhododendrons at all, though Azaleas flourish. Misguided by hardiness ratings based entirely on minimum temperature figures, ambitious gardeners from Baltimore to Texas are now planting rhododendrons sure to die in the heat of summer. When the thermometer at my nursery in northern New Jersey reached 107 last summer, R. 'Vanguard' and R. 'Arthur Osborn', though almost completely shaded by large oaks, shriveled and appeared dead. A thorough soaking that evening saved both, but the end half of almost every leaf on R. 'Arthur Osborn' is still brown and shriveled. Another such day would have killed both plants. Such temperatures are not exceptional in the South.
Many have attempted to grow tender rhododendrons in heated greenhouses, and occasionally have met with some success for a time at least. However, it may be said generally that unless an inspired gardener is in charge, the rhododendrons eventually fail. Forcing hardy varieties for flower shows, and formerly for Easter sale, is entirely practicable, but the forced plants cannot usually endure a second season in the greenhouse, and are often unable to survive the following winter in the open.
I have no heated greenhouse and do not want one, yet I am growing, or have at some time grown for several years the following species:

Numerous hybrids involving these and other tender species grow equally well for me.
I use various types of wintering frames and cold pits, all of which perform these necessary functions. They keep out the wind. They are more or less insulated to prevent the penetration of extreme cold. Though admitting an appropriate amount of light, they keep out much direct sunlight and most of the sun's heat. They maintain a moist atmosphere. By these means my losses through all these years, except from a flood disaster, has been astonishingly few.
I am now building, and using in part where it is completed, what I consider the best form of cold pit thus far devised. Seven feet deep and six feet wide, enclosed by a 16-inch stone wall which continues a couple of feet above ground level, it runs exactly east and west, covered by hotbed sash sloping toward the north, and by lath shades made to fit over the sash. The bottom, except for a narrow path, is made up as a bed with about a foot of topsoil mixed with peat, and into this the plants are moved late in autumn from similar beds in the open shaded by large oak trees, to which they are returned after the last spring frosts.
The largest plants now in this pit are R. 'Loderi King George', about 4 feet high, and R. 'Lady Bessborough', nearly 5 feet. They will be moved into the pit every season until too tall, when they will be discarded in favor of younger plans now rooted from cuttings.
Small potted plants up to a foot high are wintered in a frame so easy to construct that 1 shall describe it in detail. The only preparation required is a piece of ground, preferably with top soil removed, the surface then carefully leveled and smoothed. On this surface, without any other foundation, is laid a rectangle a little less than 6 feet wide inside and of indefinite length (any multiple of 3 feet), of 8 x 8-inch cinder blocks. A second course is then laid on the south (long side), east and west sides. All this is capped with 4 x 8-inch solid cap blocks, closing the hollows in the 8 inch blocks, which thus become effective insulation. No mortar whatever is used, but care is taken to fit the blocks closely, and of course they must be bonded by laying them as bricks are laid, with the blocks of each course covering the cracks between the blocks below.
Because the blocks are not mortared, they remain almost as good after a number of years as when they were purchased, and could be sold at any time for only a little less than the cost. For this reason the cost of the frame is very much less than if it were built of lumber, though the original outlay is greater. If after two or three years some of the blocks become tilted a little by frost, they are removed, and after the ground has been leveled again, replaced with little expenditure of effort.
Standard sash are laid across, their slope toward the north. This is very important, since, if the frame face south, as cold frames usually do, the sun will overheat it for rhododendrons, which do not want the conditions that favor lettuce. Eight inches higher at the south end, the glass receives the sun's rays at so low an angle in winter that little of the heat penetrates. Even so, it is necessary to shade with lath. Shades are built 3 feet by 6 feet to fit the sash, the lath running north and south so as to throw a moving shadow. If they ran east and west, the shadow would be fixed, and the foliage would burn in bands. In summer the sash are removed, the shades remaining.
The pots are plunged in damp peat moss, and when one is removed, another pot is put in the hole immediately to save any necessity of ever rearranging the pots in the peat, which requires a considerable amount of labor. Every year or two a little loose peat is sprinkled around to fill any holes left by the sagging of the old peat. In warm weather the south ends of a few sash are raised by placing a brick or small block under them.
All the species mentioned pass the winter while young in these frames. Only after they outgrow 4-inch pots must they go into the pit. At one time I carried them on into larger pots, which in summer were brought out and stood in shallow pans of water under the trees. I now believe that transplanting is better.
While the pit was unfinished, it was necessary to winter a number of plants too large to go into the frame described. One more course of 8-inch blocks can be added without much danger of their being heaved out of place by the frost, but higher than that the wall might topple, and it had to be higher. In 1951 therefore I made a frame with a double wall of the 8x8-inch cinder blocks all around, leaving a little space so that the wall was about 17 inches thick. These were carried up about seven courses on the south, east and west, six on the north. Instead of using the cap blocks in the customary way, they were laid crosswise, binding the two walls together. Not only did the walls not topple, but the added insulation proved highly effective, giving almost the protection of a pit, though everything was above ground.
This frame has served remarkably well during the two mild winters it has been in use. The only damage was in a lower single-walled frame built against it the second season. There most of the buds on R. 'Bric-a-brac' and R. 'Cilpinense' were lost. No plants were lost in either frame except R. 'Elizabeth' and R. 'Carmen', which arrived in the fall and died almost immediately, R. 'Elizabeth' because it had previously been butchered for cuttings, R. 'Carmen' because very small and probably not well rooted. No other plants were injured in any way.
Many of the rhododendron, had been purchased from nurseries on the West Coast, and arrived with root, in a ball of pure peat. In this condition even the hardiest varieties usually winterkill if planted in the open. But for wintering in a frame or pit, the peat ball is ideal, for it makes transplanting simple and easy. The root are reluctant to leave the peat, hence in some cases even after two years the ball is hardly larger than when first received. In any case the transplanting twice a year keeps the ball small. It has only this disadvantage, that in dry weather extra care must he taken to give plenty of water.
In passing let me remark that the heavy losses in Oregon and Washington during the cold snap two years or so ago were probably due in large part to this custom of selling plants grown in a ball of peat. It is handy for the grower but hard on the customer.
In all about a hundred tender plants were obtained from various sources in 1951 and later, and of these I can remember only five that have died from any cause. This does not include the thousands I am raising from seed, most of which are still in small pots.
My purpose in carrying these tender rhododendrons is almost entirely for use in hybridizing. They could however be so grown for decorative planting or for indoor display. Outdoors only late blooming sorts such as R. 'Tally Ho' or R. 'Arthur Osborn' would be worth while, because the great majority flower while still under glass, and they must be kept there until all danger of frost is past, for with protection their growth starts early in spring, and any frost at all will injure them severely, perhaps kill them.