Tips for Beginners: The Art of Pruning
Brookville, New York
Reprinted from the 2001 ARS Northeast Regional Conference program
Yes, pruning any plant is an art and not a science. Everyone has a different picture in their mind's eye as to what they want their plant to look like in their garden. There is no hard and fast rule that governs this. In some people, the thought of taking a pruning clipper to one of their plants strikes terror in their hearts, and some of these are otherwise very knowledgeable gardeners. This is true of some rhododendron growers as well as growers of other plants.
I will never forget what Mr. Edwin Costich, the very knowledgeable nurseryman and manager of the Hicks Nursery in the 1950s told me: "You will never hurt a tree or shrub by judicious pruning, but you could do it more harm by not pruning it, especially after transplanting it."
Let's start our pruning discussion with pruning mature elepidote rhododendrons, as this is where the majority of growers are faced with a dilemma.
One thing I dislike in a rhododendron is seeing a skeleton with many long, ill-defined branches. I know that there are varieties (i.e., 'Sappho') that love to grow that way. Many people do not like to grow such plants, even thought their trusses are truly beautiful. Answer, don't let them get that way. Regular pruning takes time, but with that problem plant, let it know who is boss.
When I have a mature plant that I want to "contain" for as long as I can, I have found that I am never satisfied with the first cutting. Therefore, I don't even try to do a finished pruning in one pass. I start by pruning one or two years' growth on the longest branches bottom to top. Now, step back and see what can be pruned further, and do it. At this point, I frequently realize that I don't miss what I have just pruned, so I step back again, make the final assessment of what I want the finished plant to look like and do it.
I have found that when I am even cutting some branches below the last flush of growth, latent* growth buds on a majority of the leafless branches can be counted on to fill in with new growth, if at least one third of the branches hold viable leaves to produce food to sustain the plant.
It is also a good idea at this time to check for "inside" limbs that have been "shaded out" and weak. These should be pruned out to increase air circulation.
Now, let's consider when best to do our pruning. If one is going to do superficial trimming, this can be done right after the plant is over bloom. However, if really drastic plant reduction is planned, it should be done the latter part of March. True, you are going to lose flowers, since rhododendron flower buds are produced the previous late summer/fall.
Let's look into the physiology of why late winter is the proper time. It is important to know that food stored in the top of the plant is highest during the growing season, but lowest from December to March. Therefore, it follows that heavy pruning late in the growing season depletes the amount of food reserves available to be translocated to the root system for overwintering and root growth. If this food is depleted, this will inhibit normal root growth and cause possible dwarfed top growth the following spring or possible loss of the plant. Conversely, March pruning will, therefore, allow for a greater quantity of food available to fewer buds to encourage quick recovery and stronger growth. Also, it provides for new growth starting before hot weather to give it a better chance to develop without deformities.
Another consideration! I have very tall rhodos lining the side of my driveway and all around the periphery of my front lawn. By nature, they want to keep spreading, so I go through and cut them all back so they can't "take over." I take particular care to cut the bottom branch shoots that drag on the ground just so I can get my lawn mower to cut under the branches without the mower blades tearing the bottom growth as I cut, but still leave the branches essentially intact. This keeps the illusion of the plant being branched right to the ground. I usually get a couple garden carts of trimmings by the time I finish.
Next, there are new plants that we buy from a nursery or garden center, or get from a fellow rhododendron grower that may need some attention. At times, I find a plant that has started branching too high to suit me. This past spring, I had such a plant with the most prominent set of five beautiful growth trusses, which were too high on the stem. It also had a side branch of three growth trusses slightly lower on the stem. If I planted this plant as it was, I know that I would never be happy with the bare bottom. So, I gulped hard and cut the full top growth of five off, knowing that the leaves on the remaining three would easily sustain the plant and that I felt I could count on some latent buds to grow lower down on the stem. It didn't take long, and it didn't disappoint me.
Finally, let's consider the grower that grows from seed, and determine when to first start thinking of pruning in a rhododendron's growth cycle. I do that as I go through my seedlings after they are about 3 inches tall. If they are growing single stems, I pinch off the top of the growth back to the next lower small pair of leaves. This usually forces at least a pair of new growth breaks. Then, as the seedling develops, I continue to pinch to develop a well-branched plant. Some varieties branch more easily than others and need little or no help.
I recall the 1993 Tacoma Convention where we visited the Briggs Nursery. In the seedling and tissue culture house (where they grow thousands of these transplanted fast growing "babies"), they had a set of tracks built into the raised concrete bench walls. These tracks were used to guide a hedge-trimmer type motorized machine to cut all of these plants to the same height to force branching.
As the seedlings grow, some continued pruning is necessary to force the plants to develop well shaped and compact. If this is done properly, it will reduce the need for extensive pruning later on in the plant's life, and will increase the overall beauty of the plant. What is "done properly"? When a plant is developing "straight up" with single growth, it will look more like a tree than a shrub if not helped along. There are three methods that I am aware of that are used to try to force multiple growths on elepidote rhododendrons.
In the first method, when the next single terminal growth bud starts to develop, the most widely used technique is to remove it to induce a multiple break. This is not completely foolproof, since at times, on some varieties, only another single growth point develops. In the second method, allow the new growth to start and before the leaves fully develop, cut it off about half way up the stem, leaving a stub. This is a technique growing in popularity and some strongly feel it is a more reliable way to force multiple growth breaks. I have tried this approach this spring on a 'Noble Mountain', 'Consolini's Windmill' and a 'Janet Blair' and found a majority of multiple new shoots with a few up to four and five multiples, but I must say that some were still single.
There is one last system that I have used many times. That is, on a larger fast growing plant that I want to contain, in the first week of June I cut off all new growth. A second flush is produced by the plant rather quickly and with a good number of multiple shoots. Some varieties regularly produce flower buds on these new shoots while others do not.
All of the above discussion relates to large leaf (elepidote) rhododendrons. It should be said that azalea and small leaf (lepidote) rhododendrons are much easier to prune, since they may be cut anywhere along the stem. The reason for this is, even thought they may not be seen, they have many more latent growth buds located all along the stem. Evergreen azaleas, therefore, can even be sheared to create a hedge (as I have done with good results) or other dense shapes (square or rounded).
Lastly, I must say I realize that many experienced growers have their own way of pruning and growing and I respect their knowledge. However, the above follows the old gardener's adage, "this is what works for me."
* latent defined (Funk and Wagnalls): "Not visible or apparent; hidden; dormant. Undeveloped, as a concealed bud."