Alder has made the leap from nature to the garden. Here, it elevates ponds and moist places as an imposing tree, swinging shrub or pretty bush.
Alder (Alnus) belongs to the birch (Betulaceae) family and can be found across all of Europe, except in northern Scandinavia and Iceland. With a maximum age of 120 years and a high sunlight requirement characterizing young trees, it cannot compete for long with other trees in most locations. The pioneer shrub finds its niche in markedly wet locations or places which are temporarily flooded. Its preference for moist, swampy regions gave it a dubious reputation in Medieval folklore, when it was associated with witchcraft.
Appearance and Growth
Three species of Alder are native to central Europe: the Black Alder (Alnus glutinosa), the Gray Alder (Alnus incana), also known as the White Alder, and the Green Alder (Alnus viridis, syn. Alnus alnobetula). Depending on the species, the Alder appears as a tall deciduous tree, a multi-trunked shrub or a bush. The Black Alder generally grows an impressive 7.87 to 16 inches per year until it develops into an imposing, 98 foot tall tree. It’s trunk is continuously straight right to the top and the crown looks like a relaxed pyramid. The White Alder is even quicker. At a growth rate of 12 to 24 inches per year, it forms a multi-trunked, conical-shaped shrub with branches slightly angled at height, 13 to 26 feet wide and around 3 feet tall. It is rarely seen as a classic, trunked tree shape. The green alder remains smallest. As a wide shrub, it doesn’t grow taller than 10 feet.
The Alder is a deciduous shrub with alternating leaves. The female infructescences are succinct, they remain on the tree in the winter as woody cones and decorate the bare young shoots of the Black and Gray Alder.
Location and Soil
Alders love sunny spots, but they also manage in semi-shade. They are not particularly demanding, but they prefer a nutrient-rich, fresh to wet and slightly acidic soils.
Nurseries offer Alders as bare-rooted or solitary plants with balls or in a container. Fall is the best time to plant bare-rooted specimens. The planting hole should always be wide enough that the roots around the edge to not touch the sides of the hole. Solitary trees with a well rooted ball can be planted all year round, provided there is no frost. However, the best time to plant these is also in the fall, as deciduous shrubs lose a lot of energy to sprouting leaves in the spring. An Alder grows well if the plant hole is about twice the size of the root ball and it is fully refilled with the dug up soil after placement. Filling up the hole and gently treading down the soil helps here.
The alder does not require winter protection. It is well suited to European climates. Alders love moist to wet places. In drier garden soil with low rain fall and heat they must be regularly and generously watered.
Young Black and Gray Alders need a planting cut before being placed in the garden. A nursery will usually do this for you when you purchase your plant. If the nursery does not, you can shorten the crown yourself by about a third. Afterwards, shears should only be used in order to remove dead or diseased wood and branches which are impeding one another or hugely troublesome. This corrective cut is best done in late summer. As the Green Alder is a bush which copes well with pruning, it can be readily thinned in the spring.
In Venice, the tree trunks of the Alder are used as stable supports for the famous Venetian stilt houses. This is because the wood tolerates permanent wet conditions. This characteristic is what makes Black Alders a particularly popular choice for planting around ponds or small lakes. Black and Gray Alders can also frequently be seen in large gardens and parks as striking solitary figures with their sweeping branches. As the Black Alder hardly forms any roots, it can be readily under-planted with shrubs and grasses which tolerate shade. The Green Alder behaves differently. It forms a thick network of roots and secures embankments in an attractive fashion.
Important Species and Varieties
The Black Alder can be easily recognized during budding in the spring by its sticky leaves. In the summer, it has up to 3.94 inch long, round shaped leaves with a serrated margin. Their topside is shiny, dark green, the underside is a light green. The hairless foliage remains on the tree for a long period and does not display fall foliage. Its flowers bud from March as reddish-brown catkins. From the fall, its inflorescences hang as 0.79 inch long, hairless cones on long stems in the branches. The bark of younger trees is a greenish-brown color, shiny, smooth and stands out with numerous angled cork pores. In older trees, it develops into a dark green scaly bark due to tears dividing it into small, angular pieces. The cordate root system of the Black Alder lacks strong side roots. However, it also has a few fine roots underneath the soil surface and on the ends of the deep vertical roots. It exchanges air through large cork pores on the trunk base and the fine roots near the surface. The Black Alder also forms root nodules, pinhead to apple sized swellings from short, thick, fork branched roots. These are home to a bacteria which binds nitrogen in the air and lives in symbiosis with the Alder.
One variety of the Black Alder we recommend is the Cut-leaved Alder (Alnus glutinosa ‘Imperialis’), a 20 to 26 feet, small, slow growing tree or large shrub with narrow, funnel-shaped emerging base trunks and relaxed, elegant overhanging branches which are reminiscent of a bamboo. The leaves are very fine and delicate. Its lobes form on both sides, there are three to four and they are irregular, narrow and cut in up to over half the leaf blade. They often hangover in an arch shape and are curved outwards.
The Gray or White Alder (Alnus incana) gets its name from its pointed leaves. These are overlaid on the underside with a white-gray felt. Its yellow catkins often bloom very early and are already visible from February. In contrast to the Black Alder, it has hairy cones. These sit directly on the branch or on short stems. The bark is gray, shiny and smooth, the roots are flat and wide reaching. If the roots are injured, the Gray Alder forms offshoots.
The Golden Alder (Alnus incana ‘Aurea’) grows either as a small tree with a narrow crown, or as a large shrub with several trunks. Both growth forms of this variety grow to around 39 feet tall. The young shoots sprout light yellow in the spring and also have red markings; in the winter the branches have and apricot-yellow to brownish-orange color. When budding, the leaves are a radiant yellow, later they turn a yellowish-green color. Male catkins are a vibrant orange to copper red. Other characteristics relate to those of the species.
The Green Alder grows as a small shrub with sweeping branches. It generally doesn’t grow taller than 10 feet and has dark green, ovate slightly pointed leaves on elastic, pruning-tolerant branches. While the male catkins have white hair and hang on the branches, the female ones stand on the branches in red clusters. The root system reaches flatly woven over a large area. The bark is grayish-brown at the base, turning to black on the trunks.
The Black Alder propagates through seeds in nature. In contrast, the Green Alder propagates through layers, that is low-hanging branches close to the soil which take root upon contact with the soil. The Gray Alder has a choice: It can propagate generatively through seeds and also vegetatively via root offshoots.
Diseases and Pests
As a native tree, the Alder is well armed against diseases and pests. It generally does not need care in this regard.