There are some basic requirements for success when planting rhododendrons…
They must NEVER SIT IN STAGNANT WATER! Roots submerged in poorly drained soil will certainly die. The plant may survive through unaffected roots but will not flourish. Hot, wet conditions promote a type of root rot and are more dangerous than cool, wet conditions; that is why Rhodys do so well in our Pacific West Coast climate.
They must be grown in a COARSE, ACID SOIL. Rhododendrons have a very fine root system so they cannot be planted in an area where there is heavy clay or hard packed soil. Porous, well drained acidic soil can be made by incorporating one third fine bark mulch and one thirds COARSE grade peat moss with one third garden loam. One caution about peat moss; too often the only peat moss that is available is extremely fine. It is so fine that it contains no air spaces when wet. Particularly if this is mixed with heavy soil, the result will be a soil that is too dense for good rhododendron culture.
WHERE TO PLANT
Rhododendrons and azaleas need company. Mass them together (taller varieties in the back) or group them with companion plants, which are simply acid tolerant genera eg. Gaultheria, Pieris, Kalmia, Pernettya, Skimmia, Camellia, Dogwood, Magnolia. Plant them in a border or island bed and around the foundation of the house with other shrubs. Avoid planting under the roof overhang or where watering might be a problem. A planting site which has dense shade and trees with shallow aggressive root systems should be avoided. Even with sturdy varieties, plant performance will be improved with reasonable protection from drying winds; plant in a protected place, or cover with burlap or other protective material (never use plastic) during a severely cold winter.
In most situations part shade is preferred; in hot dry areas it is necessary. The novice, without any information on the requirements of each Rhody they may have bought would do no harm to plant it where it will receive some shade from mid-day on. If grown in full sun, many Rhodys will make a better shaped plant and will set many more flower buds. Some varieties will not stand full sun and announce that by displaying yellowish leaves.
WHEN TO PLANT
Rhododendrons are relatively easy to move because the root system is quite fibrous, compact and shallow. Transplant almost any time of the year, early spring and early fall are first choices.
For Rhododendrons planted in less fertile soils, like ours, a complete fertilizer designed for acid-loving plants may be applied in spring. A second application may be needed in the fall if bud set is consistently small. Do not over feed, however, Magnesium in the form of Epsom salts is sometimes recommended for yellowing leaves on Rhodys. Lack of iron produces yellow leaves as well. Rhododendrons need calcium from dolomite lime applied in the fall. Use about two small handfuls around each good-sized plant. If you wish, scatter a little acidic mulch over it to counteract the PH of the lime. If your soil is alkaline, (greater than PH6) use sulphur or ferrous sulfate to acidify the soil. Do not use aluminum sulphate, as over time, the accumulated aluminum residue may harm the plants.
HOW TO PLANT
First prepare or modify the soil, following the previous instructions. At least half of the planting mix should consist of coarse peat and fine bark mulch.
The plants will come in containers which are removed, or with burlapped roots. The burlap can stay, but cut away any poly twine and pull away.
If a root ball is dry it should be thoroughly soaked in a tub of water before planting.
Root bound plants or plants previously grown in heavy soil should have their root balls loosened or partly broken apart to encourage the fine roots to grow out into the new soil.
Rhododendrons and azaleas are easily damaged by planting too deeply. The top of the root ball should be at the surface of the ground or above it. In light sandy soils which are high in organic matter they may simply be planted in a hole a little larger than the root ball, pack the soil in lightly and water. Where the soil is clay and holds water in the bottom of a dug hole, it is better to make a raised bed and plant on top of the ground. The bed can be confined by landscape ties or planks or tapered off the soil at the edges. Such raised beds drain wonderfully during wet seasons but need special attention to watering in dry weather.
After planting, mulching with a coarse, acid organic matter is needed to conserve moisture, keep the roots cooler in the summer / warmer in the winter, and eliminate the need for weeding. Two to three inches of fresh leaves, pine needles or wood chips will practically eliminate weed growth.
Rhododendrons, because of their shallow roots, should not be cultivated.
Is seldom required. A moderate amount of shaping and trimming may be done in the spring or after blooming without danger of damage to the plant. Cut back to trunk or side branch. To encourage branching in young upright plants, break out the single terminal shout just as it starts to “push” in the spring. Old plants can be cut back severely and still recover although it may be a while before they bloom again. Most often all one needs to know is how to “deadhead”and pinch the tops of the new growth in June.