One of the most calming and beautiful sights in nature can only be seen in the winter landscapes of temperate regions. After leaf-fall, when winter has truly settled in, our eyes are sensitized again to the true differences in the array of browns and grays of deciduous trees. Then the little woods and thickets of wild places become meditative works of art—lattices of beautifully interwoven trunks and stems, subtly arrayed in earth tones blending from sand to rose, and striped and fringed with the ivories and charcoals of sunlight and shadow. There is no vision more completely peaceful than the sight of a blue-gray pond or marsh, shining in the low late sun, through the warm browns of this winter woodland lattice.
There are many woody plants that combine to create this magnificent effect, and a group that is sure to be found near woodland ponds and marshes are the alders, Alnus spp. The genus Alnus is a close relative of the more familiar birch (Betula). Alnus contains about 35 species native to various parts of the world, and includes many exceptional garden and landscape plants, as well as the lovely wild alders of wet places in many parts of the United States.
The alders range in size and habit from large trees to shrubby multi-stemmed plants. They all have warm, light brown bark, and handsome branching patterns, providing special winter interest. They are often planted for the yellow-brown male flowers that are borne on long graceful catkins in late winter which, though not especially showy, add valuable color to the landscape at that time of year. Female flowers are borne on purple-brown, cone-like, stubby strobili which persist through the following winter. Both male and female flowering structures make wonderful ornaments to the branches of alder. They add great charm and character to the tree's winter silhouette as they dangle from the limbs, dancing to the slightest breeze like silent wind chimes.
Because almost all of the alders have the unusual attribute (among woody plants) of being able to fix nitrogen with their roots, in the manner of legumes, they are excellent plants for poor soils and waste places. They are also among the most tolerant of woody species for wet and flooded soils so that certain species can be found naturalized along watercourses and wetlands from Florida to Maine (and in various other areas of the United States). In spite of their beauty and utility, alders are not commonly grown in nurseries and gardens in the southeastern United States. This is a situation that needs to be changed, for alders have a great deal to offer modern gardens and landscapes.
Among the many alders that you may encounter in the eastern United States, several stand out as particularly garden worthy. All are hardy to Zone 5 (with A. rugosa, A. glutinosa, and A. incana being significantly more cold hardy, to Zone 3). Alders can be propagated from fresh seed sown directly or from dried seed sown following a cold pretreatment (three months at 40°F). Germination is often at low rates with alders. Vegetative propagation from cuttings can be equally chancy with reports of some success with softwood cuttings taken in summer, treated with rooting promoter and rooted under mist. Cultivars are generally propagated by grafting onto species rootstock. Alders are relatively tough plants with few insect or disease problems. They prefer moist sites and will tolerate flooding and submergence, but will often thrive in dry, droughty soils as well. Full sun or part shade gives good growth but full sun will give the best flowering and fruiting.
Alnus cordata, Italian alder, is one of the least common and most attractive alders. It grows to be a large tree to 50' with upright branching. It is native to Italy and has a particularly handsome, flame shaped silhouette with rich green, glossy foliage.
Alnus glutinosa, common alder, is native to parts of Asia, Africa, and Europe, but has naturalized extensively along watercourses in the United States. It can grow to be large tree, to 80', but is more often seen as a multi-stemmed small tree in cultivation and even as a shrub in the wild. The foliage of common alder is especially handsome with leaves dark-green, shiny, rounded and as large as 4" across. Foliage remains uniformly good all summer but there is little fall color development. Common alder is among the least tolerant of the hot summers of the Southeast. There a few named cultivars available, to be found only with a great deal of hunting. 'Aurea' has new gold foliage and is relatively slow growing, 'Charles Howlett' has variegated and non-uniformly shaped foliage, 'Imperialis' and 'Laciniata' have cut-leafed foliage and 'Pyramidalis' grows in a strikingly upright, columnar habit.
Alnus hirsuta, Manchurian alder, is native to Asia, and is perhaps the most beautiful alder. It is a large tree to 60' with a softly oval crown and intricate branching hung with prolific, ornamental catkins and strobili. The foliage is a lovely olive green all summer and turns burnished yellow-tan in fall.
Alnus incana, white alder, is a European species, very common in its native Europe but less so in the United States. It is a large tree, to 60', with a broadly oval habit, and somewhat dull, medium green, tapered foliage. There are a few cultivars of this species. 'Aurea' has new yellow foliage and bright yellow-orange male catkins. 'Laciniata' is a cut leaf form with exceptionally deep lobing and handsome character. 'Pendula' is a beautiful weeping form.
Alnus rugosa, speckled alder, is native throughout the eastern half of the US. It is a shrubby, multi-stemmed plant or small tree to 20' with dark, murky green foliage and more coarse habit than the previously described alders.
Alnus serrulata, hazel alder, is also native through much of the eastern seaboard of the US with similar habit and appearance to that of A. rugosa but with slightly different foliage. Both A. rugosa and A. serrulata are likely to remain best used in naturalized settings where the branches and catkins are a wonderful addition to the winter landscape.
Winter is a time for reflection, both in the backwaters of our own minds, and in the still blue pools of woodland ponds made visible by winter's bared limbs. The elegantly branched crowns and delicate, warm-toned ornaments of alder's catkins are especially delightful in the midst of such mental and visual quietude. We have only to plant alder in our own landscapes to bring this calm beauty of the winter woodland home to rest in the garden.