Spring 2025 Rhododendron Damage in New England

Guest Post by Jim Connolly, MCH, MCLP, Weston Nurseries Garden Center Manager, Chelmsford and Joe Bruso, President of the Massachusetts Chapter of the American Rhododendron Society and owner of Boulderwoods Nursery, Hopkinton, MA. Photos by Jana Milbocker

Jim Connolly writes:

With the snow finally melting away and warmer temperatures starting to happen I have been seeing some foliage damage to Rhododendrons specifically, however this could also happen to many broad-leaved evergreens as well. Most of the damage I see is to the top of the plant and I believe it is due for a few reasons. 

Last summer and fall we had a very dry season and because of that many broad-leaved evergreens could have been stressed before the winter even started. All evergreens require having some kind of moisture in their stems to tolerate the winter conditions. With the drought occurring last year this could already put these plants in a position to fail. Why some plants are not affected versus others could be the age of the plant or the health of the soil. 

During the winter it is typical to have a Rhododendron curl their leaves to help protect them from the cold winter conditions. Typically, the colder it is, the tighter the roll. You almost don’t even need a thermometer, just look out your window and if the leaves are tightly curled you know it is freezing outside. Once the weather becomes warmer the leaves unfurl and are in their typical formation and all is good. 

During the growing season when a rhododendron curls their leaves it could be caused from a pathogen like phytophthora root rot or an insect like root weevils or rhododendron borer. These pests would be more likely to be active in May/June or in the summer, not in March and April when we have nighttime temperatures still in the 30s and 40s. 

So why is the top of your Rhododendron showing curling leaves and the bottom leaves are fine then? I will tell you. The bottom branches were protected a few weeks ago by the snow cover we had. The tops of the plants were exposed to the very cold conditions we had this past winter. With the cold temperatures comes winter burn or desiccation of the leaves or leaves and stems. Possibly some root damage occurred from the cold temperatures before the snow came as well. It was a double whammy for the plants – going into the winter dryer then they should be and the extended cold period. Also when we have snow around the plant the sun will reflect off the snow and warm up the branches during the day and then the cold nighttime temperatures freeze the stems and leaves causing damage too. 

So, what can you expect and what can you do? As the days get warmer the plants will react to what damage was done by either dropping all the leaves or stems totally dying back or just the tips dying back. Each plant will vary to the extent of what damage has been done. You would have to wait till late May and see what parts of the plant have new growth emerging. You can prune off any dead stems at that point and hopefully the new growth will replace some of the damaged winter killed growth that occurred this year. Don’t start cutting branches today. Yes, you can water if the soil is dry, but don’t do anything to the plant which could be alive and find out you cut those branches off by accident. 

IN THIS MIXED-SHRUB BED, SOME RHODIES FARED WELL WHILE OTHERS DIED

With plants that are stressed, horticulturist Trevor Smith has recommended using a combination of ¼ cup Neptunes Harvest and ¼ cup Espoma Bio-Tone Starter Plus in 5 gallons of water and drenching the soil. We have in Weston Nurseries Chelmsford a product called Bio-Pak plus and that works phenomenally on stressed plants too. I have also mulched with the Coast of Maine Lobster Compost and seen some great results too. 

If you mulched the plants heavier in the winter, watered more frequently during the drought, improved the health of the soil by incorporating compost or used foliage protection during the winter with Wilt Stop or Wilt Pruf, you may have seen less damage. 

Drought and winter damage on Leucothoe

Small-leaVED HOLLY AFFECTED BY THE WEATHER

We never know what the weather conditions will be like in the winter. Plants are resilient and you should see some positive results by the end of spring. If you don’t, my suggestion would be to remove the plant and look at installing a plant that can tolerate the winter conditions better in your landscape. 

Joe Bruso writes:

I would like to add one critical factor that was omitted from the article. We had colder than normal temps for longer than normal this winter, at least by the standards of recent winters. The ground froze deeply and was still frozen in a few places in our garden as of a few days ago. When the snow came, the ground was solidly frozen. Warm sunny days meant light was reflected off the snow onto the foliage, as you point out, adding to moisture demands on those leaves. Additionally, however, the stems/leaves could not draw water from the frozen soil, leading to leaf desiccation. These conditions occur about once every 10 years in my experience. 

We have thousands of rhododendrons. Some have already recovered from (apparent) desiccation once the ground thawed out, with leaves returning to normal or near normal. Others look permanently desiccated. In some cases the stems are also desiccated (they are crinkled) and will have to be cut back. In other cases it is just the leaves that are toasted and will be replaced when the vegetative buds begin to grow. I wait until July to prune as oftentimes dormant buds erupt along the stems of otherwise dead-looking branches. 

I have not noticed a distinct pattern among which plants are affected, and which are not. About 5-10% are affected either a little or a lot. It seems to be somewhat random among species and hybrids in our garden, and also not related to the size/age of the plant. I will be doing a more detailed analysis to see if I can detect any kind of pattern.  

Big Rhodys

heritage museums and gardens, photo by jana milbocker

By CJ Patterson

A number of years ago, I was selling a rhododendron to a citizen, and he was thrilled with it.  It was Calsap (Catalgla X Sappho), a large frilled white with a dynamite purple-black blotch, very showy. He had seen it in our flower show and was smitten.  He had asked about it, and was told “it will be hard to find”. As he picked it up to head for the cash register, I remarked “it gets big, so be careful where you site it.” He froze in mid-lift and turned.  “What do you mean, big?”

calsap, photo by jana milbocker

And so it begins. Breeding with R. yakushimanum, R. brachycarpum, and other compact, slow-growing hybrids has changed our aesthetics in the garden. Nowadays, if it isn’t a little green toadstool, no one knows what to do with it. We really do need to cultivate some creativity in our garden design. One of the most fun things about Rhododendrons is their sheer variety. Every natural plant form from hedging, groundcovers, and foliage accent to small trees can be found amongst the rhodys right along with the standard rounded green shrub. All it takes is a little imagination to use them.

photo by jana milbocker

My Calsap loving customer had a small suburban garden, and I think the term “gets big” suggested a giant shambling mound that would eat the dog. I admitted that Calsap could indeed fill that description, but that it would take it a number of years to make it to the dog-eating stage, and in the meantime, if he gave it a good site with a half day of sun, and regular deadheading, he could look forward to being the envy of the neighborhood for two weeks every year for quite a while. Personally, I like a really big rhododendron. Something about a tree-form rhododendron that you can stroll under is irresistible to me. Like an evergreen magnolia, only better because it’s a rhododendron. And then once a year, it blooms gloriously, not a flower here and there like the magnolia, but great masses of bloom. Anyone who owns a mature “Andy Paton” will know what I  mean. 

heritage museums and gardens, photo by jana milbocker

And they can be so useful.  Big rhododendrons can be used as accent plants, or hedging, or blocking a nasty view.  They are good for anchoring beds, be they wildflower, perennial, or rhody collection.  I speak here of the varieties that have a naturally upright growth habit, with a solid scaffolding of branches that can be pruned up a bit to allow for air circulation and “head space” for underplantings. But which to choose?  There are so many fine varieties to choose from!  In no particular order, here are some suggestions for a “big” rhody.  All are dependable healthy solid citizens, hardy to at least USDA zone 6, and several to zone 5 or lower.

cadis, photo courtesy cornell university

Cadis (Caroline X R. discolor) is an old Gable hybrid that has stood the test of time, and is now considered a standard. A sturdy upright grower with strong crotches, it will reach 6-8 feet in ten years and bears copious masses of candy pink flowers in late midseason, as shown in the picture to the right. It takes good disease and drought resistance from the pod parent Caroline (an earlier hybrid of Gable which is famous for its ability to turn away the slings and arrows of outrageous weather) and an upright tree form habit and later bloom period from the pollen parent, R. discolor.

Wyandanch Pink, courtesy ARS

Wyandanch Pink is one of the fastest and largest growing Dexter hybrids.  A mature specimen can have multiple upright trunks 4” or more in diameter. Add to this hardiness to at least USDA zone 5, and you have a good candidate for the colder garden.  It is one of the hardiest Dexters we can recommend for western Massachusetts and southern New Hampshire. Its main flaw is that because it is such a fast grower and grows so large, that it can have brittle wood.  For this reason I do not recommend planting it under white pines or where it will catch the wind.

Spellbinder, photo by CJ patterson

Spellbinder is a very large growing hybrid by Leach {(R. maximum X Russell Harmon) X (R. calophytum X sutchuenense)} that was bred for a tree form plant. It has plenty of hardiness from the pod parent, and a solid tree form from the pollen parent to give a sturdy tall upright plant.  It does not root easily from cuttings, so it may take a little searching for, but I have seen it in nurseries. It is hardy in zone 5 and should have at least a few hours a day of sun to bud up well.

katherine Dalton, courtesy Cornell university

Katherine Dalton is a Canadian hybrid of R. smirnowii and R. fortunei and taking the best from each parent.  A healthy hardy strong growing plant, with good clean foliage and well clothed, a dense upright plant with a shared leader, very resistant to snow load and windstorms.  It does not set many seedpods and so does not need much deadheading, a happy characteristic in a plant you need a stepladder to deadhead.

Now we reach the varieties that, while splendid, may take a bit of searching, but they are totally worth the trouble.

Babylon, photo by CJ patterson

Babylon (R. calophytum X praevernum). This is a famous plant, found in many collections, but almost unknown in the standard nursery trade, probably because it takes a while to mature to a flowering specimen. To become a convert, try looking at the specimen plant at Sakonnet Gardens in RI shown in the picture to the right. It needs a sheltered position as it blooms very early, with enormous trusses of white with a large jewel red blotch cascading over the plant. It is hardier than you would expect from the parentage, and I have seen trusses from west of I-495 and from southern NH, but their growers have been careful to provide both light shade and good air drainage. Even so, late frosts may ruin your show, although it seldom injures the plant.

babylon, photo by CJ patterson

Atroflo I and II (R. atrosanguineum X floccigerum) Another antique from Gable that is an ideal beginner’s plant for a tall rhododendron. It is an excellent foliage plant with long slender leaves of dark green with fairly thick indumentum beneath. It flowers freely with at least a half day of sun, bright rose in a medium sized truss, and the petals look like they were made of crinkly tissue paper. Habit is upright to a fault, growing up before filling out.  Branches that touch ground layer easily and then reach for the sky, forming a separate leader.  An Atroflo neglected in a large planting will form a little woods of its own, shading out its neighbors. Unfortunately, mine never seem to get much more than 15 feet or so tall, but perhaps they are not ideally situated. Atroflo I was the first to be selected from the seedlot for floral characteristics, but later Atroflo II was selected as being not quite as tall, but hardier.  Other than that, they are nearly identical in the garden.

hardy giant, photo by cj patterson

Hardy Giant (R. fortunei X fictolacteum) One of the earliest attempts to produce a hardy tree rhododendron, this uses R. fortunei as a pod parent for hardiness and fictolacteum as a pollen parent for size, as it is a true tree species related to R. rex. Unfortunately, the enormous leaves of the pollen parent did not come through, but it did yield a handsome sturdy upright plant with good foliage (though not overlarge) and a growth habit that grows up about twice as fast as it grows sideways. It blooms in early midseason with ample trusses of white flowers tinged pink. Solidly hardy to zone 5b and probably more, given a sheltered position with good air drainage. Our first plant grew to 12 feet in 20 years from a cutting, almost twice as tall as wide.

Russell Harmon, photo by cj patterson

Russell Harmon is a hybrid of our two native broadleaves, R. catawbiense and maximum and was introduced by La Bar’s nursery in the 1950s. It is about as hardy as you will get in a large rhododendron, down to -25F or better, and once established, is pretty bulletproof for a tall rhododendron. It roots easily and does not seem to be much discouraged by drought, once established. Personally, I think this rhody would be the poster child for the term “abundanza”, growing ten feet tall and fifteen feet wide, and once it reaches its maximum height it will continue to grow sideways. When it reaches its allotted space, it should be pruned.  It will reward good light with very large trusses of magenta tinged flowers, not individually large, but plenty of them. It blooms late in the season, too late for most shows which is unfortunate, as its tall triangular trusses are much loved by judges.

duke of york, photo by CJ Patterson

Duke of York (R. fortunei X Scipio) Last but not least, one of my all-time favorite rhodys, an antique from the early days of hybridizing.  Large flowers of pink with a slight tinge of magenta in lax but copious trusses, on a taller than wide plant, it is a healthy and long-lived vigorous variety bred in England but completely hardy in zone 5b here. When I was still working at the Arnold Arboretum, I found three of them at the base of Hemlock Hill that had been planted nearly 100 years earlier and were still in wonderful shape, at least twelve feet high and still blooming prolifically. I was thrilled to find that it roots easily and grows like a weed, and brought cuttings to P4M, where others shared my enthusiasm. Unfortunately, I made the major error of mentioning the plants to a garden designer, and she enthusiastically rerouted a major footpath right over them. In vain did I point out that rhododendrons do not like having their roots pummeled by traffic and that a stream of callous visitors would pull the plants down and rip off their stems (“there are so many flowers, no one will mind if I take a few” and rip them off they will do. I never mentioned a plant to upper echelons again.) I have not been back in many years, so I do not know if they are still there, but if they are, they are worth the extra hike when you visit the arboretum. The variety is so tall growing that I decided to experiment with it. I planted four rooted cuttings as a little grove, and then stood back to see how they would grow. The good news is that they grew into a wonderful grove that I could walk unimpeded under as though it were a grove of trees. The bad news is the closed canopy is very dense, allowing little light to filter down, and you cannot even tell when the plants are flowering except from a distance. And, of course no wildflowers or companion plants will grow under them.

scintillation, photo by jana milbocker

So I recommend tall rhodys to you as an accent plant, a boundary fence, or a visual screen from the neighbors. Use your imagination. A friend and chapter member, Berta Atwater had a superb garden and grew many unusual rhododendrons, but she also grew many standards, but with extra flair. She grew a row of Scintillation and limbed them up when they got big enough, and planted a group of clipped white R. kaempferi underneath. Come bloom time, the effect was stunning, and out of bloom was very elegant. Unfortunately, eventually a hurricane came and trashed the planting, but it was glorious while it lasted.

CJ Patterson is Vice President of the American Rhododendron Society, Massachusetts Chapter, and District 6 Director of the American Rhododendron Society. She and her husband have been hybridizing rhododendrons since 1986.

Boulderwoods: A Celebration of Rhododendrons

Boulderwoods in its spring glory. All photos by Joe Bruso.

Joe Bruso picked up a brochure from the Massachusetts Chapter of the American Rhododendron Society (ARS) at the Boston Flower Show 30 years ago. Rhodies were plants that he knew very little about. Today his rhododendron collection exceeds 1,500 plants. He is president of the Massachusetts Chapter, develops his own hybrids, lectures about rhododendrons, and sells plants from his home nursery.

When Joe first saw the three-acre building lot in Hopkinton, Mass., that would become his current home and garden, he realized that it would be the perfect setting for his gardening hobby. Glaciers had carved hillocks and valleys and deposited huge boulders throughout three acres of woodlands. After building their house, Joe and his wife set about clearing the lot in small chunks—digging out understory shrubs and saplings and culling trees that were diseased, short-lived, or tough competitors for other plants. The remaining red and white oaks, hickories, and white pines provide a perfect canopy for Joe’s collections of rhododendrons, azaleas, magnolias, and other ornamental trees that flourish in groves among the boulders. They are underplanted with drifts of trilliums, Jeffersonia, bloodroot, epimediums, jack-in-the-pulpits, and other shade lovers.

Joe’s interest in hybridizing rhododendrons grew out of his lifelong curiosity about the natural world and his education in biology and genetics. It was fueled by the American Rhododendron Society where kindred spirits engage in learning and experimentation. He joined the Species Study Group, which focuses on the 800+ rhododendron species that have been discovered worldwide. Thanks to rapid infrastructure development in Asia and advancements in genetics, new species of rhododendrons continue to be identified and added to the genera. 

Emerging rhododendron foliage in all of its variety

Joe’s particular hybridizing interest lies in big-leaf rhodies with unusual foliage—bright red leaf bracts, pigmented leaves, silvery tomentum (wooly fuzz on top of leaves), and cinnamon-colored indumentum (velvety underside). You will see hundreds of these plants throughout the garden, some as full-grown specimens, others as year-old shrublets in nursery beds. His collection also includes more than 100 straight species, with some blooming as early as March and others as late as August.

Joe Bruso, hybridizing in his garden.

Boulderwoods is open for visiting by appointment (see information below.) Large-leaf rhododendrons are always available for sale as well as a smaller selection of deciduous and evergreen azaleas, magnolias, and other woody ornamentals. I have visited many times and purchased more than a dozen of Joe’s hybrids that are thriving in my garden. This spring I also joined the Mass. Rhododendron Society to learn more about these wonderful shrubs. Why not join us? You can find more information about the Massachusetts Chapter of ARS at MassRhododendron.org.

A cross of Janet Blair x Sappho Kalmia Hill

Boulderwoods is located in Hopkinton, Mass., and open by appointment only. Contact Joe at (508) 435-8217, jpbruso@aol.com