Drought-Tolerant Rhododendrons

by CJ Patterson

Rhododendron yakushimanum showing signs of drought stress
Picture by S & J Perkins
Rhododendron yakushimanum in the Case Estates Garden.

Eastern Massachusetts was a very dry place this summer (1997). Our good friend El Nino generally brings what we living near Cape Cod call 'tourist weather'- sunny, warm, cloudless skies with cooler nights and, relatively speaking, low relative humidity. Good for traveling, sports, and tourists - but not good for gardens, unless you own a well or a very small garden. Here on the South Shore and the Cape, if you have a large garden or use town water, you will sooner or later become interested in 'drought-tolerant' plants, and if you are a member of the ARS, you will very soon be thinking in terms of solid plantings of rhododendrons anywhere beyond the reach of your hose. This is because Rhododendrons in general are mostly very resistant to dry conditions once they are established. I cannot emphasize the word 'established' in that sentence enough. Most rhododendrons require at least twelve full calendar months of good growing season to become fully established, more if you are planting in poor soil, crowded conditions, or in competition with trees. Notice that I said growing season. Root growth cannot occur in frozen soil, in a drought, or in high soil temperatures, so you cannot count those months. In the all-or-nothing weather of New England, I can readily imagine a scenario where it might take a full decade for a plant to take hold. And there is one situation where the gardener may never relax- when a rhody is planted in direct competition with a mature specimen of one of our nastier-rooted trees, such as beech, hemlock, Norway or red maple. In the early '80's one of our best P4M donors, Johnny Gwen, lost a twenty-five year old plant of R. maximum, a plant that was maybe twelve feet tall and ten feet wide (he pruned a lot), a gorgeous sight in midsummer, killed stone dead by the eighty-foot tall red maple that shaded it. I wouldn't have thought it possible myself, and John was embarrassed to admit that he regarded R. maximum as so indestructible that he had not bothered to water it even though it showed stress.

Before we look at some plant suggestions, let me point out that 'drought-tolerant' can mean very different things in different parts of the country. In the South, they are less likely to have a total absence of significant rainfall (although it does happen), and more likely to go three weeks of no rain followed by a solid week of downpour. Also, nighttime temperatures often remain high along with killer humidity. Their drought-tolerance is closely tied to heat tolerance and resistance to root rot, thus the Southern emphasis on raised beds, air circulation, and very coarse substrate. Between the Mississippi River and the Rockies, summer humidities are low, and in the Midwest winter humidity is low and temperatures cold; here, desiccation of leaves and stems is the danger. Bearing all this in mind, let us look for rhodies that will weather the slings and arrows of our outrageous New England summers. (I am going to deliberately ignore azaleas for this article as drought-tolerance in azaleas is too interesting to deal with lightly; perhaps if there is enough interest, we can do another article to give them the respect they deserve!)

For my money, the most drought-tolerant rhody is R. carolinianum. It has, under laboratory conditions, been desiccated to less than 20% of its normal moisture and been resuscitated. I have seen a heavy dew revive it; it is so efficient at wringing the last drop of moisture from its environment. Many of its offspring are likewise survivors, premier among them 'PJM' and its ilk, which now come in so many colors and sizes that if you can't find one you like, you're too fussy.

R. maximum is usually the first plant that people think of when they think of drought-tolerance, but it must be remembered that this is a montane plant, and rather quickly looses quality under chronically droughty conditions, becoming either leggy and shy blooming in dry shade (giving the irresistible impression that it is trying to crawl to a more suitable spot), or wizened and lacebug-riddled in dry sun. R. maximum can be very resistant to the occasional drought, largely because it knows when to cut its losses, being capable of aborting flower buds or even curtailing vegetative growth, hunkering down, waiting for Ma Nature to settle down. This is one reason that R. maximum hybrids are not always tolerant of dry conditions- you must consider the other parent. 'Edmund Amateis' is a lovely hybrid with R. strigillosum, a Himalayan beauty, and not resistant to drought. On the other hand, 'Summertime', a chance seedling of R. maximum, is more drought-tolerant than its parent, so sweeping generalities are difficult.

The big guy itself, R. catawbiense, is remarkably drought-tolerant for a montane species, and a southern, low altitude form, var. littorale, is heat-tolerant also. This species has passed this quality on to the so-called ironclad hybrids (about which more later), as well as a fine series of hybrids between it and R. maximum. These hybrids are not well known in the trade, but are well worth seeking out for anyone needing bulletproof rhododendrons.

Notice anything special about all of these species? They are all natives of our own Eastern North America, which should give all of you complaining gardeners an idea of what the weather is SUPPOSED to be like on the East Coast in the summer. If we look at other areas of the globe that are cold in winter and frequently sunny, warm, and dry in summer, we can hope to find other species that will thrive in our environment.

First stop, the Caucasus Mountains, home of Rhododendron smirnowii. This is a very handsome species, with long tapering, English Racing Green leaves that are fairly heavily indumented with a very light beige close-packed fur and warm pink and white flowers in a somewhat loose truss. It is easy to distinguish it when it is out of bloom from oriental lookalikes by the thin raised edge in the indumentum around the edge of the underside of the leaf. This species is very hardy as well as drought-tolerant, although you will begin to lose flower pips at -25 F (I saw an eight-foot specimen in central Vermont whose owner claimed had never shown any plant damage). It is also the parent of some remarkably tough hybrids. Although smiryaks are not always as tough as we would like, Weldon Delp in western Pennsylvania has produced a heat-resistant smiryak (#2) that he has tested outdoors, bud hardy to -35 F, which has come through some memorable droughts. I suspect the R. yakushimanum influence would also have a calming effect in its performance Down South (R. smirnowii needs cold winters to flower properly and does not always do well down there).

Next stop, China. R. fortunei and its hybrids have worked their way into the heart and soul of eastern Massachusetts' gardeners, and they are so successful in our poor acid sandy soil, resistant to heat waves, cold snaps, and droughts, that we frequently forget that the species comes to us from monsoon territory. How can that be? R. fortunei comes from the low mountains of southeast China, where it gets quite hot in the summer. These mountains are not high enough to consistently wring moisture out of the monsoonal flow; in years that the monsoon is late, or is not strong, spectacular droughts can occur (ask any Chinese peasant!). In addition, the same physiological mechanisms that allow R. fortunei to survive seasonally dry winters help it to survive the occasional failed monsoon and the occasional Massachusetts dry summer. However, it more closely resembles R. maximum in its toleration of dry conditions; repeated drought, compromised root systems, or poor growing conditions will quickly cramp its style (It should not be forgotten that many of the Dexter hybrids that do so well for us were hybridized, grown, and selected right here in eastern Massachusetts after surviving our weather for many, many years- any plant that WASN'T suited to tourist weather was gone long before the Dexter committee showed up!) And if you cross R fortunei with one of our other drought-tolerant species like R. smirnowii you can get something like 'Katherine Dalton', a handsome, reliable plant, very floriferous under all conditions, sun or shade. The flowers are a very good light pink, slightly fragrant, in loose trusses. A shapely shrub, it can get quite large, but will not lose its shape under heavy snow load. Those clever Canadians have remade this cross in 'Fundy', with more fragrance and more color in the truss. I have not had this plant long enough to tell if it will retain the same good habit with age, but I am assuming that anything that comes from Canada will be able to handle snow!

Our next logical species would be R. dauricum, from the cold, dry northern forests of northern China, Mongolia, and eastern Siberia. We have already looked at this species as a parent, mated with our own R. carolinianum to produce the aforementioned 'PJM'-ish group, but we should not overlook the species itself, which is often damned with faint praise as a "useful" plant. We now have many excellent selections of this super plant, in both evergreen and deciduous forms, and a wide range of dwarf forms (which I have a particular fondness for). My favorite for the last several seasons has been Weldon Delp's dwarf form, the only form I have which seems to have no faults, except that it does not come in true red, pure white, apple blossom pink or daffodil yellow. (Daydreams. All of us hybridizers have daydreams.)

Japan would be next on our tour, and although true drought conditions there are rare, dry sunny summer weather is fairly common on the main island of Honshu and Hokkaido to the north. R. yakushimanum, R. metternichii and its several varieties, and R. brachycarpum are all capable of weathering drought conditions, although bud set and leaf size will suffer in that years' growth. Likewise primary hybrids of these species, but the further you get from these parents the less drought tolerant they tend to be. Yak X reds backcrossed to red and ditto yellow especially lose this quality. But there is one species from the Japan Alps that is worthy of sitting at the right hand of R. carolinianum, as it were. R. makinoi seems impervious to any amount of drought; plants here on our rocky, sandy, woodsy soil received less than one inch of rain for each of the five months of the growing season this year, with no supplemental water. They are in shade, but in competition with oaks and pines; they are there precisely because this species is absolutely intolerant of excess wet at the roots, and must be placed in a situation with perfect drainage. I suspect that this species in its mountain home may live nearly epiphytically in a layer of duff and organic matter overlaying a rocky subsoil which would dry out very quickly with any interruption of regular moisture; this would make it necessary for this species to be able to tolerate complete drying out at the roots. R. makinoi has not been used extensively for hybridizing, but we are very fond of this species, and have gone out of our way to collect what we could find. An early hybrid was Umbrella (X R. yakushimanum), an interesting mix of the two parents. The leaf is more like an elongated R. yak., but the habit is definitely influenced by R. makinoi. A young plant is almost V-shaped instead of the low mound that a Yak would make, and older plants tend to be shaped like an oversized bird's-nest spruce (we get a great deal of snow here, which squishes habits, and I suspect it would be even more upright in areas of lighter snowfall). In addition, the overall look is layered, especially in younger plants. Very oriental. It is shy flowering as a youngster, but reliable with age, with rounded trusses of apple blossom pink. We have others of this same cross from other hybridizers, but they are less interesting, except for a dwarf R. yak. X makinoi that we have raised from seed, which might be nice if the caterpillars would ever leave it in peace. We have a very small R. makinoi from wild collected seed that we would like to use to remake this cross with a very dwarf Yak to make a miniature version of 'Umbrella', but it will be a while, as R. makinoi is often slow to attain its maturity, and a dwarf will likely take even longer!

Another worthwhile plant is R. makinoi X pachysanthum, which we acquired from Gloria Berube after her husband Ernie's death, and which we have donated to P4M in the past. It is a terrific foliage plant, with very handsome indumented and tomented leaves, but unfortunately, upright habit plus upright habit equals extremely upright habit, and this hybrid, left to itself, will form a rather narrow V-shape in shade. Pruning will ameliorate but not solve the problem. Flowers are typical for this cross, apple blossom pink fading to white, with a more cupped flower shape and more lax truss than one could wish for. I can also say that R.maximum X makinoi is very drought-tolerant, but I would NOT say that the cross is any improvement of either parent. Sorry. (Some things sound good on paper, but...)

A few words on R. yakushimanum as a drought-tolerant plant. It is by no means impervious to drought, but if you know that drought is a problem in your garden, there are a few things you can do to keep your Yaks in good condition. First of all, the more shade you can give them, the better they will tolerate both heat and drought. Full sun will keep tight habits, of course, but it can also give you leaf scorch, aborted flower buds, and wizened new growth. It can also lead to the bolting of new growth in the fall when cooler weather returns; such growth rarely hardens off enough in the North to survive the winter, and these frost-blackened blind growths sometimes lead to fungus infection or worse. Also, drought seems to cause Yaks to "relax" their habit; repeated or severe drought can cause quite a frowsy look to a plant. Cure this by avoiding the larger, more upright selections of R. yakushimanum, like Phetteplace tall form - stick to the dwarf or semi-dwarf types.

And now we return to the good old U.S. of A., to our problem spot, to see if we can make anything of what we have collected. Most of the "ironclad" hybrids that are mainly R. catawbiense and R. maximum will ignore small droughts and endure without obvious damage even very large droughts. Some of them are even worth having in the smaller garden as a reliable screen or backdrop. 'Edith Pride', an advanced generation of these two species, and 'Henrietta Sargent', an old Waterer cross, are both smaller ironclads that will stay in scale for a long time in the small garden. The former is a lively pink with a very full truss, the latter a paler pink with a yellow blotch, rather frilly, blooming later in the season. 'Homer' is a Sidel hybrid from the turn of the century, with heroic sized pink trusses in very fine form. It has been known to carry off the trophy for Best Ironclad. Another good Sidel is 'Viola', a fairly rare hybrid that can get quite tall with age, but responds well to pruning. 'Viola 'is an ethereal white with just a tinge of lilac at the edges of the corolla, only enough to give definition to the individual flower in the truss. Very cool on a hot day. The most drought resistant ironclad I know of, though, is 'Lady Grey Edgerton', a very fine white with long dark green leaves. This was remarked upon by no less a plantsman than Joe Gable, who wrote, "...to us its most unusual trait is its resistance to hot weather and summer drought. When the whole planting is wilted and looking like it is on its way out, Lady Grey is as crisp and fresh as if on an April morning after an evening shower. Why?" I couldn't have said it better.

Which rhododendrons can be relied upon to curl up and look miserable in dry weather? Most yellow species, alas, are pretty wimpy, so that any predominance of yellow, orange, or cream must be looked on with suspicion, as must those wonderful Taliense section species. In addition, they can be mighty sensitive to high soil temperatures, which usually accompany drought. Many reds are sensitive, too, especially those using genes from the Neriiflora subsection. Also the wonderful lepidote dwarfs from the alpine slopes of the Himalayas are mostly very easily damaged or even killed by persistent hot, dry weather. Any of these, constituting one-half or more of a hybrid, should be viewed as guilty until proven innocent.

If you must grow these beyond the reach of your hose, there ARE things that you can do to help. I must warn you, though, they are all work, and many are just good common sense once you think about it.

First of all, encourage root growth. Plant in beds, not holes in the woods. Dig your beds deep, at least ten inches, a foot is better. If your soil is too rocky, dig them six inches deep and add 2 X 8's to make a raised bed. Add organic soil amendments, the coarser the better, up to point, of course. Rough peat is better than fine milled stuff, chopped leaves are better than ground leaves. Don't use raw wood chips, let them sit for 4-6 months in a pile, and if they get too dry, water them (if you get a pile in the fall, by spring they will be fine). When you feed, use fertilizer that is low in nitrogen, and has trace elements. (Fertilizer protocols can be pretty intricate, and are best tailored to the individual site and plants) NEVER feed during a drought, even if you water. Rhododendrons will send roots down as far as they have air and moisture and good soil, even twelve inches deep, if you let them. Mulch with something relatively light and fluffy. Composted bark mulch is fine in clement weather, but in a drought, it will steal moisture from the soil, and heat up in sun, causing soil temperatures to rise. Bark nuggets are better (in not too large a size chunk), coarsely chopped leaves best. If you have oak trees for shade, you can actually reserve a pile in some out-of-the-way corner of the yard, where they can be run over with the lawn mower (NOT in mulching mode) and added when you know you will have a problem right on top of your regular bark mulch. Then just tidy up the leaves to the compost pile when the rains return. The added shade to the root zone and extra mulch to retain water can work wonders. When you water, water thoroughly, for at least four hours. In the U.S. of A., it is all right to water at dusk, dawn, or even the middle of the night, when the water Nazis can't see you. And it is perfectly legal to use temporary shade in a drought. Shade cloth is probably the easiest to use, unless you have trouble with wind, but lattice and snow fencing works just fine. Remay is probably cheapest and quickest, but remember you must provide air circulation.

I want to leave this discussion with three varieties that do not seem to fit into any of our pigeonholes that I heartily recommend for the droughty garden. The first is 'Atroflo', a hybrid of 'Atrosanguineum' by R. floccigerum, a Himalayan species. This hybrid can get tall, but it has year-round interest in the indumented leaves and extremely ornamental flower buds. There are two clones in circulation, 'Atroflo #1' has larger crinkly tissue-paper rose flowers but a ganglier habit. 'Atroflo #2' is marginally hardier, with tidier habit but smaller flowers. A hybrid of Joe Gable's, it will take heat as well as drought. 'Russell Harmon', a natural hybrid of R. catawbiense and R. maximum found and named by LaBar and introduced through his nursery (come to think of it, I guess this DOES fit a pigeonhole!). This has great huge trusses of bright pink on a VERY strong growing plant. If you can give it room, or tame it with pruning, it is as close to bulletproof as any rhododendron I know. If you do not have the room for either of these aggressive plants, you can try 'Albert Close', a hybrid of R.maximum and R.macrophyllum from Joe Gable again. This is a very attractive hybrid with fine rounded foliage and beautiful tight rounded trusses of rich rose with a lighter center, heavily spotted with chocolate on the upper lobe. Flowers are of good substance, slightly ruffled, and stand sun very well. Its only flaw is a tendency to legginess in too much shade, but this is easily curbed with more exposure. I have seen Albert blooming beautifully and fully while doing battle with a seventy-foot hemlock whose branches brushed its trusses- and there are not many rhodies you can name that can do that!

Good luck to my fellow gardeners, and may all your summers be rainy!

This article first appeared in the Rosebay, a publication of the American Rhododendron Society, Massachusetts Chapter.

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