JARS v53n3 - How Growing Rhododendrons in New England Builds Character

How Growing Rhododendrons in New England Builds Character
Neil Jorgensen
Kittery, Maine

There is a sumptuous variety about the New England weather that compels the stranger's admiration - and regret. The weather is always doing something there; always attending strictly to business; always getting up new designs and trying them on people.
Mark Twain

Mark Twain was right, but he didn't go far enough. When he was busy counting the kinds of weather occurring on a day in his hometown of Hartford, Connecticut, he was missing all of the weather that was happening across the rest of New England. The region, small as it is, encompasses four full hardiness zones, as well as a few thousand miles of coastline and several mile-high mountain tops. All kinds of weather is happening all the time.
Gardeners here face floods, blizzards, bare-ground winters, northeasters, heat waves, cold snaps, hurricanes, and even the odd tornado. Summer droughts, which contribute mightily to gardeners' woes, are often local; in one town the gardens may be parched while just a few miles away they may be lush jungles, the result of a somewhat random pattern of summer storms.
In recent years, the winter weather has also become less cooperative. A covering of snow is no longer reliable to protect small seedlings and liners: it seems that some of what used to fall as snow now falls as freezing rain, often with devastating effects to trees, garden plants, and electric lines.
Although the day to day weather is erratic and difficult to predict, it is possible to make some sense out of climatic conditions, those averages of the weather over many years. Climatic data gives us hope. If New England gardeners know that, on average, they are supposed to receive 42 inches (107 cm) of rain evenly distributed through the year, they may keep faith as they watch their rhododendrons wilt in a prolonged summer drought. Perhaps, they think, next year will be better.
Average minimum winter temperatures are of course vitally important in choosing trees and shrubs for a garden. The "banana belt": Cape Cod and the Connecticut and Rhode Island coast, enjoy minimum annual winter temperatures comparable to parts of Mississippi, while in New England's icebox: the northern parts of Vermont, New Hampshire, and Maine, winters rival those of North Dakota, Alberta, even some of the milder parts of Labrador.
Until the advent of the Northern Lights azaleas and the Tigerstedt and other recent super hardy hybrids, I used to think that somewhere, winding across northern New England, was a line north of which no rhododendrons would survive (save possibly the lapponicums and maybe a few other alpine species if they didn't succumb to a summer heat wave). Now I am not so sure. It may be possible that, with some care in siting, especially for winter shade, gardeners almost anywhere in New England could grow at least a few of these newer hardy rhododendrons.
It is instructive to note, also, that among the scattered stands of Rhododendron maximum growing wild in New England, a few hardy disjunct colonies survive into the semiarctic winter conditions of mountainous northern New Hampshire and Maine. In the White Mountains, R. maximum grows improbably, along with mountain laurel, on the slopes of Mt. Chocorua. In Maine an even more northerly stand of R. maximum , perhaps now consumed by deer, has been reported from the vicinity of Kingfield, where minimum average winter temperatures must reach close to -30°F (-34"C).

New England temperature zones map

The accompanying portion of the USDA Hardiness Zone Map shows, in a very general way, the range of winter temperatures across New England and how latitude, elevation, and proximity to the ocean determine what plants can be grown where. Latitude and elevation are straightforward. Travel north and the winters become colder; travel upward in elevation and they become colder. All other factors being equal, a 400-foot (122 m) increase in elevation produces the same winter temperatures as 100 miles (161 km) farther north.
The map also shows the effect of the ocean on the climate, though what actually happens is more complex than meets the eye. The Gulf Stream flowing along the Eastern Seaboard moderates the coastal climate along much of its course. North of Cape Cod, however, the Gulf Stream swings farther out to sea, replaced by the cold Labrador Current flowing southward just off shore. ARS Annual Convention visitors who plan to go whale-watching in Cape Cod bay need not worry about a repetition of the Titanic disaster, but the water there and on up into the Gulf of Maine will be distinctly chilly. This cool water, along with the fog it generates, slows down the onset of spring along the coast by as much as two weeks.
As summer progresses, the ocean warms up a little. Though New England has its share of beaches, I suspect that most of the people who actually swim in the ocean north of Cape Cod are descendants of the Puritans, who used immersion in cold water as atonement of their sins rather than for pleasure.
By fall, the water has grown almost warm. Its tempering effect on the coastal climate produces gorgeous mild autumn weather that often lasts for as much as two or three weeks after there has become a nip in the air just a few miles inland.
The ocean stays warm relative to the air throughout the winter, and, as the map illustrates, the average minimum temperatures along the coast remain a good deal higher than inland. Gardeners in coastal sites therefore can grow an impressive number of species and hybrids, especially if they are out of the wind.
A small-scale hardiness map such as the one shown, at least for hilly areas like New England, only provides the most general information about what plants can be grown where. What the map does not show is the important, sometimes crucial, effect of microclimate, those substantial climatic differences that occur near the ground, often over very short distances.
Sloping ground, provided it doesn't slope to the south, produces a favorable microclimate for rhododendron growing. On cold, still winter nights, when the danger of frost and low temperatures are at their greatest, the heavier cold air near the ground slowly flows down slope and, on the way, stays riled up and mixed with the warmer layers above. Where barriers across the slope, such as a stone wall, or a hollow in the terrain, interrupt this downward flow, the cold air stops moving and accumulates like a puddle of water. The coldest air, being the heaviest, sinks to the bottom of the pool forming frost pockets that are much colder than the surrounding land.
My garden in central Massachusetts, for example, enjoys a particularly favorable microclimate. A high canopy of oak trees holds in some of the warmth of the day, and the air drainage on the sloping site is so good that in some years the tomatoes in the vegetable garden die from lack of sun before they are killed by frost. In the 30 or so years I have been gardening there, the lowest temperature that I have recorded is -11 °F (-24°C). That same night, a friend whose garden is only a mile away but at the bottom of a frost pocket recorded a temperature of -26°F (-32°C).
Fifteen degrees of cold will of course make a big difference in the variety of rhododendrons a person can grow. Although most of the 6a hybrids do fine for me, my friend had to be content with only a few rock-hardy varieties, even though, according to the USDA map, his garden and mine are both situated in Zone 6a.
Another microclimatic factor, the exposure of a garden, can have a profound effect on what will grow. Garden exposure, the compass direction of its sloping orientation, defies conventional logic. We think of a southern exposure as the warmest orientation, but in New England, as indeed in other parts of the country, a sloping southern exposure is probably the harshest, most difficult of all sites on which to grow rhododendrons.
Bright winters are the rule here. By March the sun shines with the same intensity as in September. On a south-facing slope, the worst-case March scenario would be twelve hours of strong sun, coupled with cold, drying winds, deeply frozen ground, and an absence of an insulating layer of snow. Life would be difficult for broadleaved shrubs under these conditions.
In summer, the southern exposure is no kinder. Plants that survive the winter must then face intense sun, often coupled with drying southwest winds. If a south facing orientation is the only possibility for rhododendron growing, shade, especially afternoon shade, both winter and summer, is highly desirable if not a necessity.
By contrast, a northern exposure, often considered to be cold, windy, and inhospitable, is usually a far more favorable setting for rhododendrons. Because a north-facing slope receives less direct sunlight, it is always cooler and moisten If there is a snow cover, it will remains longer on north slopes, affording more winter protection to the plants growing there and keeping the frost from penetrating too deeply in the ground. All other things being equal, gardeners will have the best chance of success with their tender varieties on a northern, or better still, a northeastern exposure than on any other. But it cannot be stressed too often that even on these more favorable exposures some shade is probably the greatest single factor for success, at least with the elepidotes.
Those of us who are trying rhododendrons varieties that, according to the hardiness map, we should have no business growing, must always heed Lincoln Foster's warning, "Some day a winter will come along that will make honest people out of us all." In the 40 years that he experimented with rhododendrons not considered hardy in his northwestern Connecticut garden, he himself became an "honest man" more than once, almost always during winters when the snow was sparse or late in coming. But through these trial and error experiments, Foster discovered a number of species and hybrids more cold hardy than hitherto thought.
Although the unpredictable New England weather and extremes of climate cause gardeners endless worry and torment, there is one feature that, on the whole, is a great boon to rhododendron growers here. That is the abundance of trees. After widespread clearing in the 19th century, much of New England has returned to woods. Trees cover an astounding 85 percent of the land.
More specifically, it is the abundance of deciduous trees that helps the rhododendron grower. Deciduous trees make up the bulk of the woods in the southern three states, but even in the north, where conifers are more common, deciduous species cover extensive areas.
Conifers are usually unsatisfactory as close companions for rhododendrons. Their deep shade and aggressive, shallow root systems make it almost impossible to grow anything in their vicinity. A rhododendron gardener facing a conifer forest has only one option: gradually thin the trees until enough light can reach the ground and enough room exists beyond the tree roots for planting.
In the Chandler and Brooks gardens, convention goers choosing the west of Boston garden tour will see at least two stunning exceptions to the conifer dilemma. In both gardens widely spaced groves of huge old white pines let in enough light to support wonderful plantings of rhododendrons, Japanese maples, and other woodland species. In good soils, white pines are often deep rooted and therefore offer less root competition than other conifers.
More often, however, deciduous trees make a better setting for rhododendrons. The benefits of the summer shade cast by deciduous trees is clear to anyone who has stood a 90°F (32°C) day in an open field; less obvious is the amount of protective shade cast by the bare branches of these trees in winter. Surprising also is the amount of winter wind protection afforded by deciduous trees.
Unfortunately, not all deciduous trees coexist well with rhododendrons and other shrubs. When they are mature, maple, ash, and beech all have such shallow, aggressive root systems that planting much of anything in their vicinity is a waste of time. Though the three can grow to be magnificent specimens, the ground beneath them is usually dry as a desert.
Hickory, another common southern New England species, is also an unsuitable companion for rhododendrons. Like walnuts, to which they are related, the roots of hickory secrete a poison that stunts or kills outright rhododendrons and other ericaceous plants that grow too close.
Of the deciduous species, oak trees are probably the best canopy trees for rhododendrons and other shrubs. Oaks, except for pin oak, have deep, less competitive root systems which allow plantings to grow happily almost up to their trunks. Most of the gardens on the convention tours will be set in oak woods.
The stately Liriodendron , common in Connecticut and on down the Eastern Seaboard, is also deep-rooted, but as a garden tree, it is often plagued by aphids which leave a black sticky substance over whatever grows beneath them.
Though oaks still win the root competition prize, birches also may have a deep root system. With birches, as indeed with other tree species, the depth of their root systems often depends upon the quality of the soil in which they grow.
Whatever the variety of trees a gardener has to face, there will probably be too many of them. On almost any unmanaged woodland site, the summer shade will be so heavy that tree removal is nearly always necessary if a woodland planting is contemplated. Fewer trees, and therefore fewer roots, also mean more soil moisture available to the plantings.
Thinning and pruning are ongoing tasks in a woodland garden. After thinning, the remaining trees all too quickly grow to fill in any holes in the canopy. Light levels in the understory soon begin to diminish and plants grow leggy and shy about flowering. Then it is necessary to do more cutting. It is no wonder that chainsaws are common items in the tool sheds of most New England rhododendron growers.
Convention goers will be lucky to see New England in the month of May. The weather then is usually cooperative; the mild May climate is between the extremes of winter and summer, the flowers are out, and the trees are decked with their lemon-green spring foliage. Although October is the other great time to visit New England, for gardeners, of course, May is the clear winner.
When Mark Twain claimed that New Englanders "kill a lot of poets for writing about the beautiful spring," he was probably thinking about March and April. It is true that those last two weeks of March usually leave everything to be desired, and it is also true that T. S. Eliot's April is sometimes extremely cruel. But May in New England...ah, May! Dazzling, intoxicating, glorious, magnificent...most of the time.

Neil Jorgensen is a teacher, writer, lecturer, landscape designer, plant enthusiast, and long-time member of the ARS Massachusetts Chapter. He has recently moved from Harvard, Massachusetts, to Kittery on the coast of Maine.

by Chase Dooley