Tips for Beginners: Pruning of Rhododendrons and Azaleas
Sidney, British Columbia
Types of Pruning
There are three categories of rhododendron pruning: leaf bud pruning, woody pruning, and dead-heading. Most rhododendrons (excluding the evergreen azaleas) make new leaf in one sudden burst, immediately after flowering. In mild Falls, and if growing conditions are good, a second much smaller flush of leaves is sometimes produced. The evergreen azaleas make leaves over a period of about a month after blooming and so can be lightly sheared immediately after blooming. The elepidote rhododendrons, the most common garden rhododendrons, often do not break out into leaf freely from old wood that has been cut back. The other groups - the lepidotes, the deciduous azaleas, and the evergreen azaleas - usually break freely from pruned old wood.
Leaf Bud Pruning and Dead-heading of Elepidotes
Left to their own devices, elepidotes often become open and leggy. Once established, legginess is difficult to correct. Several factors contribute to legginess, but the principal one is that the plant has not been properly shaped in its early years. This is best done by removing terminal leaf buds.
The following procedure is applicable to elepidotes. (It is generally impractical in azaleas and small-leafed lepidote rhododendrons, because terminal buds are small and very numerous. For them there are more appropriate procedures.)
During summer, terminal leaf buds develop at the end of each new shoot. In young plants, all terminal buds are leaf buds, but in healthy mature plants most will be flower buds. Flower buds can be readily distinguished, being three to four times larger. The differentiation of leaf and flower buds is evident by Fall.
Beneath the terminal bud there is a whorl of leaves, each with a small axillary leaf bud. If the terminal bud is a flower bud, then, after flowering, two or more axillary buds are activated, each producing a shoot. (The vigour of these shoots is enhanced if flowers are promptly dead-headed.) Consequently, from a single blossoming shoot, two or three new blossoming shoots arise. Such is the normal healthy adult condition. However, if the terminal bud is a leaf bud (the common situation in young plants or in unhealthy older plants), then, as a rule, only it alone is activated and only one shoot is produced; branches extend rather than divide. Repeated over several years, the plant becomes leggy and blooms poorly.
Young plants from a good nursery have several branches. If, after planting, you do not leaf bud prune, few extra branches develop, and after several years the plant becomes leggy. However, if you remove terminal leaf buds, the axillary buds beneath are activated, and several branches are produced. A well dressed, sturdy bush develops, able to withstand wind and carry snow with minimal damage. As the plant matures, an ever increasing number of terminal buds are flower buds, and the need to leaf bud prune diminishes. In some varieties, several sub-equal terminal leaf buds are formed. If there are three, remove the central one. If there are two, retain both. Leaf bud pruning of very large-leafed species can give too crowded an effect, and I leaf bud prune them selectively every second year.
Woody Pruning of Elepidotes
Elepidotes often do not break readily from old wood, or do so in a rather unpredictable way. Modest annual trimming immediately after blooming of all plants I find helpful. Prune floppy branches back to sturdy lower shoots and take out crossing wood. Remove perhaps 5 to 10 percent of the superstructure. This helps to prevent the superstructure outgrowing the root and improves drought resistance. Do this woody pruning before dead-heading.
Long neglected plants that have got quite out of hand or that have been heavily damaged by snow often benefit by being cut down to ground (coppiced). Most will quickly regenerate into a large bush, which you can then shape as it grows back. It may not bloom well for some years.
For plants that are not quite out of hand but nevertheless are too leggy and need thickening, selective pruning of old wood should be tried. Prune back to a sturdy lower shoot or to an active bud. If this is not possible then prune to a dormant bud. Begin by pruning a few branches one year, a few more the following year, and so on. Somewhat surprisingly, this rather tentative procedure sometimes does not work as well as the brutal coppicing just described; elepidote rhododendrons, selectively pruned, often break from old wood less readily than if they had been cut to the ground.
Many elepidotes, when mature, have a tree-like form. They can be readily pruned to a single stem and the lower branches progressively removed as the plant ages. Under-plantings can then be added.
Woody Pruning and Dead-heading of Lepidotes
Lepidotes have smaller and more numerous flowers and leaves than elepidotes, and dead-heading of entire plants would be prohibitively laborious. They break freely from old wood, much more freely than elepidotes. As a consequence, small varieties can be lightly sheared followed by dead-heading of the few spent flowers that the shears have missed. Extensive dead-heading of larger varieties can be much reduced by modest pruning of last year's wood (cutting back last year's longer shoots by perhaps 20 percent), which removes most of the spent blooms. After this has been done, the rest of the bush can be dead-headed. Because of the readiness to break from old wood, major reshaping of large, old lepidotes can be freely and confidently undertaken. Remove crossing wood. As before, all this is done immediately after blooming, before new leaves appear.
Pruning Deciduous Azaleas
Deciduous azaleas break freely from old wood and so can be pruned in much the same way as lepidotes - cut back last year's wood (and so remove spent blooms) after blooming but before leaves develop. You may also wish to prune back stems that did not bear flowers the previous year. Deciduous azaleas sometimes throw long irregular shoots which are charming in the woodland setting and which you may wish to retain. If, however, you wish to contain growth in a more formal setting, then prune them down and they will break freely from the wound.
Pruning Evergreen Azaleas
Flowers on evergreen azaleas are far too numerous to dead-head. Instead shear the entire plant lightly after blooming. After shearing, pick over the plant, removing spent blooms missed by the shears. By shearing, some early leaves will be lost, but later leaves will soon clothe the bush and the plant is not weakened. Over the years you can shape the bush to your tastes. Evergreen azaleas, together with small lepidotes, are the easiest of all rhododendrons to prune.