Rhododendrons for the Midwest Landscape

Springtime visitors to the Northeast and the Pacific Coast are always entranced by the magnificent, flourishing rhododendrons which they often find to be the most impressive feature of the landscape. Gardeners living in the Deep South and places west admire these superb evergreen shrubs and frequently comment with keen regret that they cannot grow them back home.

There is good news now for these envious visitors. In the last few years advances in our understanding of rhododendrons and the conditions they need in order to thrive have made it possible for us to grow them successfully in uncongenial climates where hot, dry weather prevails.

Many nurseries specializing in rhododendrons and azaleas have sprung up in Midwest. Recent information on the nutrition of rhododendrons has simplified cultural methods in soils not to their liking.

Site Selection

In selecting a site on which to grow rhododendrons in the less favorable climates, a northern exposure is always best. Such a situation misses the intense sunlight, both direct and reflected, and the searing winds which are so hard on rhododendrons in midsummer. A site facing north is all but a necessity in such extreme climates as that of Memphis, Tennessee, and Austin, Texas, where the direct sunlight is so brilliant.

Here it is best to plant rhododendrons in good light but with no direct sun whatsoever. Elsewhere, if it is impossible to place them on the north side of a house or of shrub planting, a situation sheltered by trees is almost as good. High overhead shade from deep-rooting hardwood trees is ideal. In the Midwest honey locusts are especially desirable as an overhead cover for rhododendrons.

In the South tall pines and oaks, their lowest branches at least 12 feet from the ground, are preferred. The aim should be to provide about 30 per cent sun, with the quota of sunlight striking the plants in the morning and as little as possible in the afternoon. In most Midwestern states the plants should be shielded from the prevailing hot, drying winds from the southwest.

After an appropriate site has been selected, the soil must be altered to meet the needs of rhododendrons. It is this particular phase of culture that has been the source of so many past failures.

Rhododendrons must have acid soil to thrive, and the usual practice in areas with alkaline soil has been to excavate a planting hole and fill it with a prepared, acid soil mixture. The fact is, however, that it is impossible to maintain the proper acidity in such a small volume as is contained in an isolated planting hole. Rhododendrons in areas with alkaline soil must be planted in beds; even if only one specimen is to be planted, an area no less than 4 feet square should be prepared.

Southern coastal regions are seldom troubled by poor drainage but Midwestern gardeners are often plagued by an underlayer of dense clay which obstructs the movement of rainwater through the subsoil. It is a waste of money and effort to plant rhododendrons in such a location because their roots must have a loose, well-aerated growing medium. If the soil becomes waterlogged, the plants quickly die from lack of air.

Simple Drainage Test

A simple test can be made at any site by digging a hole 2 feet in diameter and flat at the bottom and filling it with water to a depth of 2 inches. If the water seeps away within 6 or 8 hours, the drainage is satisfactory for rhododendrons. If water still remains, the drainage must be improved by sloping the bottom of the bed toward an area filled with coarse rocks. This “well” should be large enough to hold a 2-inch layer of water that drains from the bed.

Another method of correcting faulty drainage is to raise the bed 18 inches above the ground level, using a low retaining wall to hold the soil. In no case should a planting site be chosen which is lower than the surrounding ground lest alkaline surface water flow into the bed to cause a rapid reduction of acidity.

Naturally Alkaline Soil

Where the soil is naturally alkaline it should be replaced to a depth of 2 feet with a prepared acid mixture. A good width is 4 1/2 feet. If the plants are to be placed in a single row, the bed should be about 4 1/2 feet wide. An 8-foot width allows a more pleasing and informal arrangement, supplementing rhododendrons with ferns, lilies, columbines, bluebells and other wild flowers which thrive in loose, acid soil.

At the bottom of the excavation, place 4 inches of coarse stone and on top of this place the soil mixture. In Indiana, one successful gardener uses 50 per cent coarse peat, 25 per cent rotted leaves and 25 per cent sandy topsoil. An enthusiast in North Carolina, who has a large flourishing collection of rhododendrons, uses a mixture of 35 per cent leaf mold, 20 per cent peat, 15 per cent sand and 30 per cent topsoil to replace the heavy clay soil.

Various other formulas can be used incorporating old pine needles, decayed sawdust and similar organic materials which may be available locally, but all successful growers use a mixture which contains at least 50 per cent vegetable matter. If fresh sawdust or other undecayed organic materials are used to supply the humus, it is necessary to broadcast over the surface of the bed 1 pound of ammonium sulfate for each wheelbarrow load used; this material will supply nitrogen, replacing that used by bacteria in breaking down the organic matter. In providing the proper growing medium, aim for a porous, aerated medium which will remain loose, yet retain both moisture and acidity.

Where alkalinity is a problem, trace elements should be incorporated as a part of every soil mixture. The use of this material allows considerable variation in acidity without adversely affecting the plants.

The bed should be built up 8 inches above the ground level so that it will remain several inches above grade after the prepared mixture has settled. This practice prevents nearby alkaline surface water from draining into the bed.

After the bed has been thus completed the acidity of the soil is tested. It should test between pH 4.0 and 4.5.

When the plants are received from the nursery they are often balled and burlapped. In unfavorable climates it is important to expose 2 or 3 inches of the root ends by washing away the soil on the outside of the rootball. Then when the plants are placed in the bed, the roots will be in contact with the prepared soil mixture and the plant will become established quickly. Rhododendrons must be planted at the same depth as they grew in the nursery.

After they are in place the plants are mulched with 2 or 3 inches of organic material such as pine needles, leaves of hardwoods, rotted sawdust, wood chips or peat, your choice depending upon cost and convenience. A good mulch will retain moisture and reduce soil temperature so is extremely important in uncongenial climates. It should be renewed each fall because it gradually disintegrates, adding vital humus to the soil. Even the best coarse imported peat lasts but about eight years in the soil. Leaf mold lasts only a year.

In the Chicago area and in similar severe Midwestern climates, expert gardeners cover the permanent mulch with an 8-inch mulch of leaves (preferably oak) in the late fall for extra winter protection. and then remove 3/4 of leaf layer the next spring.

Hot Days Watch the Water

The rhododendrons should now be watered well. Your careful initial preparation of the beds will be fully repaid, when you find your rhododendrons neither drowned in the wet weather nor destroyed by drought in midsummer. Many rhododendrons have been unwittingly killed during the severe droughts, when they were watered to the point where the soil became waterlogged and the plants smothered.

If rhododendrons wilt badly when the soil is already moist, do not flood the roots with additional water, but, instead, refresh the foliage with a light sprinkling in the evening. Most gardeners are overly alarmed when they see the leaves of their rhododendrons curl and droop in the heat and low humidity of midsummer. The plants will recover completely as temperatures moderate and rains return.

A very successful Missouri gardener soaks their beds thoroughly every two weeks and sprays the foliage lightly several times a week in time of severe drought. In the North routine watering must be stopped in mid-August so the plants will start to mature and harden their tissues to withstand the subzero winter weather. However, the soil should be moist when it freezes for the winter.

In areas where the soil is naturally alkaline there will be a gradual neutralization of the acidity in the planting beds. Also, the water supply usually is non-acid and the more it is used for irrigation, the faster the acidity will be lost. In one instance where rhododendrons were watered twice a month from May to October using alkaline city water (pH 9:0), it was necessary to apply aluminum sulfate in both spring and autumn. All who live in a area of normally alkaline soils should test the rhododendron beds twice a year so that aluminum sulfate can be used promptly whenever the acidity declines.

Stay Away for Grafted Plants

It is important to buy own-root rhododendrons, not grafted plants, for planting in uncongenial climates. They are conspicuously superior. The best size to buy is 15 to 18-inch specimens, well branched and with dense foliage to shade the ground beneath them. Those in the South and the Midwest who have never grown rhododendrons before would be wise to start with the hybrid Rhododendron ‘Roseum Elegans’ which has proved outstanding in its ability to thrive despite heat and low humidity. It has fine lavender pink flowers, grows compactly and is available from almost every rhododendron nursery.

It would be misleading to give the impression that rhododendrons can be grown in the South and Midwest with the same casual attention that they receive in more favored climates. But neither do they require the fussy watchfulness and constant concern which are usually associated with plants in a foreign environment. The initial planting may be laborious but once in place, rhododendrons will give a gratifying account of themselves.

Contributed by D. Leach

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