I sort of know what hardiness means and how others define it. I know what a blasted flower pip looks like; a shriveled up blackened remnant of what could have been if not for a harsh combination of weather. I know what drought looks like, what winter burn looks like, bark split, Phytophthora, sun scald, leaf spot, stem die back, and frost damage. But is it hardy you ask? "That depends", I say.
Most rhododendron books define hardiness on the minimal temperature that a plant can endure and fully bloom. This definition works just fine if all one cares about is the bloom. Blooms are a bonus for 2 weeks out of a year. The plant has to look pretty darn impressive in bloom for that to be the only reason to keep it in my garden.
I consider plant hardiness as coming through the winter with healthy foliage and healthy roots. Cold hardiness requires that the plants have acclimated properly through the natural process of lengthening nights and cool temperatures to become dormant. This is an active metabolic process requiring adequate moisture and proper nutritional balance. Much more critical temperatures occur in spring after the ground has thawed and buds have swollen. At that point forward, the dormant temperature ratings are not relevant anymore. The temperature that a plant can endure without injury rises sharply. Even the hardiest R. dauricum will lose flower buds when a cold blast of Arctic air briefly descends in April. The wise plant knows to hold back from the urge to grow.
We have plants that are flower bud hardy for our winters and not foliage hardy. Case in point is the Hobbie hybrids with their R. forestii Repens Group background and lovely red tubular flowers. If only the foliage came through winter looking attractive. Most have moved on to other properties. 'Baden Baden' is the only one we have kept. If we had reliable snow cover I am sure it would be different.
On the other hand, we have plants that have never flowered and not only do we not care if they ever flower but we might even be disappointed if they do. R. pseudochrysanthum 'Blood Red' lost its interesting red pigment on the underside of the leaf when it flowered. The dense blue-green leaf forms of RR. impeditum, lysolepsis and litangense, often die back on the flowering branches creating an uneven, craggy habit. Many of the selections of R. yakushimanum x bureavii, such as 'Hatch's Small Clone' and 'B.L. Silver', have such lovely foliage and new growth that the flowers just seem to get in the way. Their foliage comes through winter like a champ without damage from snow loads or ice. R. williamsianum is a species listed in most books at -5°F but 2 different clones has flowered for us below -10°F since 1992. I think R. williamsianum is hardier than rated but the culprit for most New England gardens is the late spring freezing temperatures that damage the flower buds. Try the R. yakushimanum x williamsianum hybrids for a similar look with later bloom time. But then late spring freezes are rare on our property.
Living on Canobie Lake in Salem, New Hampshire gives us a distinct advantage or disadvantage depending on what someone's opinion of the cause of a rhododendron's demise. The majority of the tiny property is on a slope so there is good drainage both in soil and air: great for disease prevention, bad during drought. Mature white pines and hemlocks provide partial shade and natural mulch but also root competition. The winters are colder than the surrounding area when the ice covers the lake from mid-December to late March, but the summers are cooler, too. The January thaw rarely reopens enough of the lake to nudge plants out of dormancy. The spring-fed lake water modulates the temperatures so that late spring frosts and early autumn frosts are unusual. But spring doesn't start until the ice goes out and even then the spring is often downright cold. In contrast, the autumn is long and warm. It is difficult to generalize from my property to another about hardiness. We sit near the line for USDA zone 5b/6a just 30 miles north of Boston and 17 miles west of the cold Atlantic ocean. The rain/snow line often falls just north or just south of us which means the weatherman's guess is as good as mine and the snow cover is pretty unreliable. Our winter lows are normally -10° to -15°F.
Every year in the late fall, John and I will walk through the garden and put our predictive powers to the test. We take mental notes on the plants that are not doing well. The ones that will have a "rough go of it" through the long New England winter. For if a plant is not healthy going into winter it is a bad omen on it coming out alive. Often I think it is the unseen root system that is the problem. Rhododendrons can lose half their delicate fibrous root system over the winter. Freezing and thawing and the resultant heaving will wreak havoc on smaller plants. This will happen almost anywhere in the garden but is most likely in the sunnier locations or if the soil preparation has too much peat. An autumn drought is a dangerous precursor to winter stress. During the shortening days of the year a plant's vegetative and floral buds are triggered to go dormant and as long as there is moisture in the soil and the ground remains unfrozen the roots will continue to grow. Drought will put a stop to this all too important root growth at a critical time. Only the following spring will the destructive evidence of an autumn drought appear, as apparently healthy plants will fail to push growth. Did the roots die last fall? Did the plant fail to go dormant? Did the plant go dormant too early and dessicate over the long winter? Was it really not cold hardy?
We have a rule that a plant is not declared dead until June 22nd at which point a post-mortem is performed. The "scratch test" of scraping the bark away with the fingernail to expose a healthy green cambium layer usually fails. The plant is then dug and roots are examined as well as the bark. Figuring out why a plant died can be helpful on correcting cultural conditions. Last year a mature 'Canary Island' looked wonderful coming out of winter but never put on any growth and died by summer. The plant was in my neighbor's yard and despite gentle reminders that deep infrequent watering was best during a drought situation the habit of watering lightly was hard to break. The deep roots were probably dead before the winter ever happened. On post-mortem, we were sure we would find signs of Phytophthora but alas, it was just dead roots. There were no tell tale signs of red or brown streaks in the stem or in the main roots.
If a young plant dies in its first winter it is often due to "bark split". This may be the result of freezing at the cambium layer which happens when the ground is not frozen either early in winter or early in spring when a sudden cold snap descends upon us. We have seen it only on small plants that have not established a thick or mature bark layer. Similarly, grafted plants are most prone to graft failure under the conditions that foster bark split. Sometimes we will see girdling by mice or voles that find the delicate cambium layer a tasty morsel. It amazes me how long a plant can look nice in the spring completely girdled. Obviously, protecting young plants throughout their first year or two helps to reduce losses.
We have tried a few interesting materials and techniques to coddle rooted cuttings planted out much too soon compared to prudent practices. Small plants such as rooted cuttings are simply covered with appropriate mulch material after the ground is frozen. Salt marsh hay makes a great mulch material as it does not pack down but needs to be secured from blowing away with evergreen boughs. A cut up Christmas tree is excellent for this but we do not mulch if we are lucky enough to have snow cover by Christmas. For a few years we tried cylinders of sonatubes which are waxed heavy cardboard tubes in diameters of 6-12" used in constructing poured cement columns. Using a hacksaw to cut the tubes to a height a few inches taller than the plant the tube is secured with wire prior to ground freezing. The tubes provide a windbreak and a sun shelter but allow natural precipitation of snow or rain to fall. Additional insulation of salt marsh hay or pine needles provides a cushion to our unreliable snow cover. Roofing tarpaper stapled to form a tube and secured with stiff wire does a similar job and can be cut to size in order to protect larger plants transplanted very late in the season. Burlap is just as ugly and more expensive. Old plastic pots with mesh bottoms work well too. The trick is to remove the mulch or windbreaks at the right time. Too soon and wide temperature fluctuations can still occur. Too late and the ground remains frozen as the warm March sun creates temperature and water stress.
Plants that are marginally cold hardy come out of winter looking pretty sad and struggle to bloom or to put on new growth. If they bloom heavily they may exhibit signs of drought stress from insufficient roots or may flag quickly during a growth flush. Assuming a decent growing season they will look best by the fall when they have reestablished a good root system.
Plants that are marginally heat tolerant, the alpine species in particular, will look great coming out of winter and start to look a little peaked by July and be truly ugly by September 1st. Heat tolerance is not as much of a problem here on the lake as summer highs rarely reach into the 90's. If you are lucky enough to have a summer home in one of these locations, go ahead and try some of the fussier alpine species. R. ferrugineum would be my first pick as it has handsome glossy leaves on a compact plant and blooms later than most and sometimes blooms sporadically in the summer. For the same look in a hybrid plant 'Tottenham' and 'Myrtifolium' are much more forgiving.
I think heat tolerance may also be more a matter of roots. Alpine rhododendrons often are described as being found where there is bright light but not necessarily direct sunlight, cool soil, excellent drainage, and reliable soil moisture. This is a demanding condition to match. Soil temperatures can build up with the dark bark mulch that we use extensively. Even the north side of my house gets direct sun in the long days of summer. Ground covers such as Cornus canadensis, Tiarella cordifolia, and Phlox stolonifera actually keep the soil temperatures lower by the air cooling properties of transpiration but only as long as the soil remains moist enough to satisfy the needs of the ground cover as well as the rhododendrons. In alpine gardens the use of white or light colored stone as mulch does a fine job of reflecting light and heat, slowly warming up during the day and slowly giving off heat during the night. Surface layers of stones effectively preserve the moisture underneath. Larger stones buried in the ground may be a hindrance to growing corn in New England but not to growing alpines as the roots may grow under the stone into its cool moist environment.
Worse case scenarios are the marginally cold hardy plants that are not heat tolerant. They are usually dead by September 1st or it would be a blessing if they were.
So what would I recommend
to try? Here are a few of my favorite ones.
'April White' clear white and R. mucronulatum 'Cornell Pink' clear pink, are a lovely combination for early bloom with good contrasting fall color, too. 'Manitou' with its interesting color change from bud to full bloom has a better habit than 'Windbeam.' Of course I love lots of different variations of R. minus hybrids like 'Pioneer Silvery Pink,' 'Weston's Pink Diamond,' 'Dora Amateis,' 'April Snow,' and 'Grandma Matilda' which are all tough reliable performers. The species itself R. minus Carolinianum Group 'Epoch' or 'Gable's Album' are tough plants for sun and good drainage. The pale yellow-flowered dwarfs R. keiskei 'Yaku Fairy' and form cordifolia want a little shade. The R. keiskei hybrid 'Too Bee' with the most adorable dark pink flowers with red spotting is just too cute. 'Southland' with is salmon pink flowers and compact habit can fit into most any garden along with 'Ginny Gee.' And how can you go wrong with any of those charming R. keiskei x racemosum hybrids?
R. degronianum ssp. yakushimanum named forms such as 'Phetteplace,' 'Mist Maiden,' 'Ken Janeck,' 'Yaku Angel,' and King's dwarf are excellent foliage plants in order of plant size.
'Scarlet Romance' Mehlquist's strong red is good for its reliable bloom, dark foliage and contrasting brown buds. 'Henry's Red' will give you a big sprawling plant with that unusually deep red color but "nothing special" foliage. 'Hello Dolly,' 'Percy Wiseman,' and 'Vinecrest' are in decreasing order of interesting flower and foliage in the yellow/orange category.
R. degronianum var. tsukushimanium has the most wonderful pink flowers on top of dark green shiny leaves complete with a shiny brown indumentum on the showy underside. One can't go wrong with named smir-yaks like 'Dorothy Swift,' 'Ruth Davis,' 'Crete' and 'Today and Tomorrow' for great indumented foliage, habit, and reliable bloom. R. bureavii 'Lem's Form' and most R. bureavii hybrids do not need to bloom to earn their place in my garden with their rich brown indumentum but would like to stay out of full sun.
I do not avidly collect evergreen azaleas so my recommendations are limited. Not all the North Tisbury azaleas perform well but 'Michael Hill' blooms cascades down a hillside.
R. yedoense var. poukhanense and named formsThis species has been used extensively to develop hardier evergreen azaleas but it is nice in its own right.
R. kiusianumI have had no problem with hardiness in any of the 7 different cultivars I grow.
'Beni Suzume'A late double orange-red Satsuki azalea that should not be hardy but it didn't read the book.
R. nakaharaeBoth Polly Hill's low growing 'Mt. Seven Stars" and the taller 'Bovee Form' have strong orange-red color in June.
The Schroeder azaleas
'Holly's Late Pink,' 'Dr. James Dipple,' and 'Hoosier Peach' seem
to have enough hardiness to bloom reliably for me.
These can have problems with green worms, azalea borer, rust and powdery mildew but the following are my recommendations.
'My Mary'The multicolor orange and yellow flowers in a tall good foliaged upright shrub looks like the species R. austrinum.
'Marydel'Very fragrant with a low stoloniferous growth habit makes this one of my favorite recommendations.
'Golden Lights'My favorite Northern Lights azalea but recently I have been impressed with 'Mandarin Lights' and 'Northern Lights' with their healthy fall color.
'Weston's Innocence', 'Lollipop' and other fragrant Weston's introductions are perfect for summer at the lake.
R. cumberlandense 'Camp's Red'Hard to beat for July color and I would recommend hybrids of the species too.
R. vaseyi If you only have room for one deciduous azalea choose this one for its airy pink bloom or 'White Find' for pure white. Both have incredible multicolor fall foliage and are free of any of the insect or disease problems. If late spring frost is not a problem, R. schlippenbachii will reward you with even larger flowers but site it away from the afternoon sun.The Rosebay Volume XXVI Spring 1999 official journal of the American Rhododendron Society Massachusetts Chapter