Lucky are the Maine gardeners who can count among their garden niches a granitic rock outcrop, who can step gingerly between patches of crusty lichen for a closer view of the life that colonizes shallow pockets of newly formed soil. Such garden spots are maintenance-free islands of natural beauty and they are worthy of protection. The plants that grow there are true pioneers, eking out a living in the thinnest and poorest of soils. In August, they know the meaning of drought. Yet they survive.
The early pioneers on granitic outcrops are herbaceous plants. Mosses and wildflowers, such as the white-flowered three-toothed cinquefoil (Potentilla tridentata) that blooms in early June, flourish in long narrow rock fissures holding an inch or less of course, sandy soil formed from lichen-weathered rock.
As the cracks widen and soil layers deepen and become enriched with decaying organic matter, woody plants take hold. In June, look for shallow-rooted lowbush blueberries bearing their white bells just above the gray crustose lichens. And in pockets of deeper soil, the swales between boulders, you’ll find colonies of native rhodora (Rhododendron canadense), their spindly stems festooned with lovely rosy purple blooms.
In even deeper pockets, larger shrubs such as the wild raisin (Viburnum nudum var. cassinoides) and small trees struggle for survival, their roots penetrating every subterranean crack and fissure for purchase and moisture. In August, if you walk quietly, you may run across a wood thrush perched within the heart of a viburnum, feasting on wild raisins, too heavy to fly away.
In coastal landscapes, dominant deciduous trees of granitic outcrops include, in order of bloom, serviceberries (Amelanchier sp.), choke cherry (Prunus virginiana) and American mountain ash (Sorbus americana). These three species provide a succession of white flowers on rocky escarpments from early May to mid-June. And while the birds quickly harvest the serviceberries and cherries, mountain ash’s broad white clusters of flowers give way to clusters of orange berries that ripen to full color in late summer and may persist through winter until the robins finally take them.
Trees grow slowly on granitic outcrops; size belies age. While the largest known American mountain ash in the United States, growing in West Virginia, is more than 60 feet tall, those growing on rocky crags in Maine seldom exceed 20 feet and often take the form of large multistemmed shrubs. Bigger is not always better. The shorter stature of these trees brings their flower and fruit displays to eye level, often with the dark gray granite as background.
The stressful environment of a granitic outcrop also develops character, each tree a unique individual. A few are remarkably straight and symmetrical, but most have contorted trunks and branches that developed over years of drought, cold and wind.
In the past I have advocated more frequent use of our native mountain ash species, both Sorbus americana and a similar inland species, Sorbus decora, the showy mountain ash, as landscape trees. When teaching woody plants in Orono, I challenged my students to explain why we favor the European species (Sorbus aucuparia) as a garden tree when both native species are equally lovely, as easy to propagate, equally stress-tolerant and more disease-resistant. It seems that we favor the trees that our forebears brought with them to this country and that early on became established in commerce.
I have changed my mind about bringing these native trees into managed landscapes. After spending a decade of springs with American mountain ash in its native habitat, watching it flower across the rocky landscape of coastal Maine, I think that it grows where it belongs; a native mountain ash growing in a lawn, mulched with a ring of bark, strikes a discordant note.
Lucky are those gardeners who in spring can look beyond their cultivated garden beds to clusters of bright white flowers against gray rock, or into the corner of a distant swale where rhodoras raise purple clouds toward the sky.
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