Selection and Culture of Rhododendrons in North Alabama
From the March 1970 Newsletter of the Birmingham Chapter A.R.S.
In a porous, swell drained acid soil rhododendrons are not difficult shrubs to grow, provided suitable varieties are chosen. For the most part, suitable varieties will come from the group rated hardy to -15 degrees F. or lower, by the American Rhododendron Society. A temperature of 0 degrees F. in Alabama can produce flower bud damage equal to about -15 degrees in northern states. The Rhododendron Society's hardiness ratings are based on experiences of gardeners in the North.
In the twelve year period from February, 1958, through January, 1970, temperatures near zero visited much of North Alabama six times. As recently as January, 1970, a temperature of 2 degrees completely destroyed flower buds on several varieties in a local garden, some of them rated hardy to -10 degrees by the A.R.S. In the same garden in January, 1966. -5 degrees caused total bud damage on varieties rated hardy to -20 degrees.
One of the objects of the Birmingham Chapter of the A.R.S. is to try to keep local gardeners from having the same bitter experiences with rhododendrons that so many of us have had with Camellia japonica during the past fifteen years. It is for this reason rhododendrons planted in our area should be selected from varieties that do well in the cold climates of such states as New York, Ohio, and Pennsylvania. Thanks to such breeders as Nearing, Gable, Shammanello, Dexter and Leach there are many fine modern hybrids available; and we need not confine our selections to the old "Ironclads" to get hardy plants.
We suggest the following hybrids as ones that show a great deal of promise for our area: 'Albert Close' (rose); 'Album Elegans' (white); 'America' (red); 'Annie Dalton' (pink); 'Atroflo' (rose); 'Boule de Neige' (white); 'Blue Peter' (lavender blue);'Cadis' (pink); 'Caroline' (lavender); 'Catawbiense Album' (white); 'Catawbiense Boursault' (lavender pink): 'Charles Dickens' (red); 'County of York' (white): 'David Gable' (pink); 'Gomer Waterer' (blush - white); 'Elie' (pink); 'Henrietta Sargent' (rose pink); 'Holden' (rosy red); 'Janet Blair' (pink); 'King Tut' (pink); 'Mrs. Charles S. Sargent' (carmine rose); 'Mrs. Furnival (pink)'; 'Nova Zembla' (red); 'Pink Cameo' (pink); 'Pink Flair' (pink); 'Pinnacle' (pink); 'Purple Splendor' (dark purple); 'Red Head' (red); 'Robert Allison' (pink); 'Scintillation' (bright pink); 'Skyglow' (peach pink); 'Rocket (coral pink); 'Roseum Elegans' (lavender pink); 'Warwick' (mauve pink); and 'Windbeam' (light pink). 'Vulcan' is a little tender when young, but usually does well when four or five years old, and its bright red color makes it worth trying. 'Anna Rose Whitney', a massive deep pink is very promising. Both of these are very disease resistant.
Those that come in shades of yellow and salmon should be grown only if you are willing to risk freeze damage from time-to-time. 'Azor', 'Butterfly', 'Evening Glow', 'Mrs. Betty Robertson' and 'Autumn Gold' are worth trying from this group.
R. catawbiense, R. fortunei, R. minus, R. carolinianum and R. yakushimanum are species that will grow well here.
After a rhododendron is set, if water will puddle around its roots after a heavy downpour of rain or a fast, heavy watering with a hose, it should be assumed that drainage is inadequate. Whenever steps are necessary to assure fast drainage must be taken, even if it means planting them on top of the ground in a bed built up of a suitable soil mix.
A mixture of 25-50 per cent acid peat moss and sandy top soil make a good growing medium. If the soil that is to be used is a little on the heavy side discard it and use a sand-peat mix. When the mix has been properly prepared, it should feel rather light and fluffy in your hands.
When planted in the ground, a hole should be dug about 12 inches deep and three times as wide as the rootball. Fill the hole slightly less than full. Set the plants in the peat-soil mixture so that the top of the rootball is at least level with the soil around the hole: the rootball should never be covered with soil. In later years it may be necessary to widen the hole and add the proper mixture where soil is removed.
Mulching rhododendrons is usually helpful if the correct material is used. Peat moss and sawdust are not suitable. Leaves of hardwood trees are good, but tend to blow about during periods of high winds. Pine needles are an ideal mulching material.
Rhododendrons are easy to injure or even kill with fertilizer. With care. water soluble and liquid fertilizers can be used, but most gardeners will find it easier to use one of the slow-acting organic fertilizers once in the spring. The azalea and camellia specials can be used if used sparingly. You might wish to sprinkle super phosphate around the roots of young plants in the late fall to induce heavier bud set the next summer.
Rhododendrons need some shade from bright sunshine. The north side of a house is fine; so are the east and west sides. Light to medium shade from trees is good. Pine trees make the ideal shade.
Check often during the warm months to see if they need watering during the first couple of years. However, they should not be watered unless they need it. If new growth wilts on warm. sunny days, you might wish to sprinkle the foliage.
Leaf spot can be a problem the first few years. Zineb sprayed on the upper and lower surfaces according to the manufacturer's directions gives fairly good control.
Root-rot diseases are the worst enemy to attack rhododendrons. Soil drenches of Dexon and Terraclor, used during the growing season according to the manufacturers' directions can help control these diseases. Selection of disease resistant varieties and providing fast drainage is the best approach to the root-rot problem, however.
Dieback sometimes affects rhododendrons; usually it attacks one limb at a time. The infected branch should be cut with sterilized shears or pruning saw, to a point where healthy wood is found and the plant then should be sprayed with Ferbam and Zineb, using the manufacturers' directions.