The Alder: Planting and Growing

Botany and description

Alders are trees belonging to the Betulaceae family, primarily growing in the northern hemisphere on moist soils. The genus Alnus comprises around fifty species, the most well-known in our regions being Alnus glutinosa or Common Alder, Alnus incana or Grey Alder, and Alnus cordata or Italian Alder. Alders are typically found in damp woodlands, even swampy areas, and along riverbanks. A group of alders is called an alder carr.

Alnus glutinosa, botanical plate from 1862

Did you know? You may sometimes come across the species name “alnifolia” in Latin plant names, such as in Clethra alnifolia. “Alnifolia” simply means “with alder-like leaves“.

Alders are generally deciduous trees of medium height (20 to 30 m). Only a few species have evergreen or semi-evergreen foliage and grow in subtropical forests. The foliage is alternate, usually dark green in colour. The leaves turn yellow in autumn before falling late in the season. In the Common Alder, the buds are purple and sticky.

Leaves of Alnus glutinosa, Alnus incana and Alnus cordata

The alder lives in symbiosis with around twenty mycorrhizal fungi. Moreover, the alder develops nodules on its roots that house bacteria of the genus Frankia. These bacteria fix nitrogen from the air, thereby enriching the soil around the roots. Thanks to this, the alder is considered a pioneer species. The tree “prepares the ground” for other plants to establish themselves. The lifespan of an alder is approximately 60 to 80 years.

Alder wood is light, soft, homogeneous and rot-resistant. However, outside of water, alder wood deteriorates very quickly. Alder wood is still used today for making pontoon pillars. In fact, the city of Venice was built on alder wood pilings. Alder wood features enlarged lenticels that aid tissue oxygenation.

The spring flowering, from late February to early April, consists of flexible, pendulous male and female inflorescences: catkins. Alders are monoecious, meaning male and female flowers are found on the same tree. The catkins hang at the tips of the branches: female catkins are purple and located at the base of the male catkins.

Once pollinated, the female catkins transform into small woody cones called strobili, housing the seeds – winged achenes. These seeds are particularly favoured by granivorous birds such as the European Greenfinch, European Goldfinch, and the aptly named Siskin. These birds disperse the seeds, but the seeds are also carried by wind and water.

Good to know: the Common Alder is particularly important in the ecology of natural habitats surrounding certain watercourses. This area is called the riparian woodland. Riparian woodlands encompass the wooded and/or herbaceous zones along small watercourses and serve as ecotones – transition zones between two different ecosystems. These transition zones are always interesting from a biodiversity perspective, as they bring together representatives of flora and fauna from two different types of ecosystems (this is known as the “edge effect“). The alder plays its role as a bank stabiliser, nitrogen fixer (enriching the soil), pollution-filtering tree, and of course, as shelter and food for wildlife (notably for the caterpillar of the Purple Emperor – Aparura ilia).

Our Finest Species and Cultivars

[product sku=”876082″ blog_description=”The glutinous alder is a medium-sized deciduous tree, emblematic of wetlands and watercourses. It is naturally widespread throughout Europe up to Scandinavia, excluding the Mediterranean region. This tree is primarily used for landscaping and preventing riverbank erosion.” template=”listing1″ /]

[product sku=”821871″ blog_description=”The Alnus glutinosa ‘Imperialis’ stands out from our common European alder with its beautifully lobed foliage featuring narrow, pointed leaves reminiscent of ferns.” template=”listing1″ /]

[product sku=”876051″ blog_description=”The white alder or Alnus incana is a medium-sized deciduous tree named for its decorative smooth, grey-white bark. Its characteristic dark green ovate, pointed leaves with grey-white undersides are distinctive.” template=”listing1″ /]

[product sku=”876061″ blog_description=”The Corsican alder is a medium-sized deciduous tree with highly aesthetic, heart-shaped dark green leaves that are leathery and glossy, remaining on the tree for an extended period before falling in winter. Native to Corsica and southern Italy, it thrives in humid upland woods and along watercourses.” template=”listing1″ /]

Planting the alder

Where to Plant?

Thanks to its ability to fix nitrogen, the alder can thrive in poor soils. These trees require soil that remains moist, even wet. In fact, they are among the few trees (along with willows) in our regions that can grow in marshy or regularly flooded ground.

Note that Alnus cordata or the Italian Alder tolerates dry and chalky soils much better than white and glutinous alders.

Alders grow faster and more harmoniously in full sun. They tolerate partial shade or, at a push, full shade, but this comes at the expense of their habit and longevity.

When to Plant?

Alders are best planted in November-December or in spring, in April.

Alders in flooded ground near a watercourse

How to Plant?

Bare-Root (Autumn Planting)

• Dig a hole twice the volume of the plant’s roots. To respect the different soil layers, make one pile with the topsoil (rich and fertile) from the first 25-30 cm of depth. Then, make a second pile with the subsoil removed beyond 30 cm depth;
• Loosen the soil at the bottom of the planting hole. Add compost or potting mix and blend;
• A bare-root tree must be staked. Identify the direction of prevailing winds—the stake should face this direction to help the tree withstand the wind. Place it at the edge of the hole, not the centre;
• Position the tree or bush upright;
• Backfill the hole first with the subsoil, then with the fertile topsoil;
• Level the soil and firm it with your foot to ensure good root-to-soil contact and eliminate air pockets;
• Secure the tree to the stake with your choice of ties (coconut twine, rubber strap). Do not tie too tightly, as the trunk will thicken during growth;
• With the remaining soil, create a watering basin around the base of the tree or bush;
• Water thoroughly until the soil can no longer absorb water;
• Apply a mulch layer 7 to 10 cm thick. This acts as winter protection, maintains consistent moisture, and prevents weed growth in early spring.

Container-Grown

• Soak the rootball in a bucket of water to rehydrate it, then remove from the pot;
• Dig a planting hole two to three times the size of the rootball;
• Add one to three spadefuls of well-rotted compost;
• Place the plant in the hole;
• Backfill with soil and firm gently;
• Water thoroughly until the soil can no longer absorb water;
• Spread a layer of mulch around the base to retain moisture and suppress weeds.

Maintenance of the Alder

Maintenance simply involves monitoring watering during the first year if rainfall is insufficient. Thereafter, and if the alder has been planted in suitable soil, the tree will manage on its own.

Pruning can be done in late winter (February-March). Some cut back alders during this period to form coppice shoots. Alders, in fact, readily regrow from the stump.

→ Read more: How to prune an alder?

Propagation of the Alder

By sowing

• sow seeds in April-May in a substrate half garden soil and half compost, in a seed tray or pot. Sowing can also be done directly in open ground in loosened soil;
• lightly cover the seeds;
• keep the substrate or soil moist until germination;
• transplant the young plants (10-15 cm tall) into larger pots. Keep your young plants in pots for a full year;
• plant in autumn the following year.

Note: alder seeds do not age well, they must be fresh for maximum germination rate: no more than one year old!

As alder is a pioneer species, self-sown seedlings are very common. In this case, you can simply lift young plants to relocate them elsewhere in your garden.

By propagation by cuttings

Two options are available.

1. Semi-hardwood cuttings taken in August: simply place them either in open ground or in a pot, keeping the substrate always moist but not waterlogged;
2. Truncheon cuttings* (as for willows and poplars) taken in autumn: cut branches one metre long, soak the base in water all winter then plant them in the soil, inserting at least 30 cm deep. *Truncheons are essentially giant cuttings: 2-3 year old branches, stripped of leaves and twigs, at least 1m long (sometimes 2-3m for willows or poplars) that are planted deep in the soil.

→ Read also How to propagate alder?

Alnus sieboldiana (©Harum Koh)

Pests and Diseases

Phytophtora alni or Alder Dieback Disease

Phytophtora alni is a fungus that penetrates the roots, lenticels, and wounds of the glutinous alder and the white alder. It thrives in warm stagnant water, clay soils, and overly dense concentrations of alders in a confined area. Alder dieback disease first appeared around 1993. The alder develops yellowed, sparse foliage, and black spots appear on the trunk. There is no treatment, but coppicing can sometimes revive the tree. Note that this disease is mainly found in natural riparian forests but is relatively rare in gardens.

Alder Leaf Beetle (Ageslastica alni)

Also known as the Alder Flea Beetle, the alder leaf beetle is a small beetle that “attacks” the leaves of the glutinous alder and white alder. The eaten leaves take on a lacy appearance, turn brown, and quickly fall. However, there’s no need to worry—the alder tolerates this damage well and quickly compensates by increasing its photosynthesis rate.

Moreover, it has been proven that alders release ethene when attacked by these beetles. Nearby trees are then “warned” of the pest attack and boost their defences by increasing tannic acid concentration in their leaves. As a result, if one alder is heavily infested, neighbouring trees remain largely unaffected.

Lesser Purple Emperor (Aparura ilia)

Nothing to worry about—quite the opposite! The alder is the host plant for a stunning butterfly: the Lesser Purple Emperor. The caterpillar also feeds on willows and poplars. Due to climate changes and the decline of its natural habitat, this butterfly is becoming rarer across Europe. So if you spot these caterpillars on your alders: leave them be! In any case, the alder isn’t significantly harmed.

→ Learn more about alder diseases and pests in our guide.

Pairing alder in the garden

The Alnus glutinosa ‘Imperialis’ is likely one of the most beautiful alders to adopt in the garden. Its foliage, deeply lobed into narrow, pointed segments, almost resembles fern fronds. Its brown branches are adorned as early as March with numerous pendulous yellow-green to brown aments, highly decorative. Suited to medium-sized gardens, this small tree (no more than 10m tall and 3m wide) boasts a very elegant pyramidal habit.

To do it justice, it should be grown as a solitary specimen. But that doesn’t mean its base can’t be dressed. Ornamental-wood dogwoods are perfect companions for alders. They thrive in heavy, moist soils. A few young plants of Cornus sanguinea ‘Magic Flame’ could be chosen, with their green-yellow foliage turning orange in autumn. These vibrant hues will beautifully offset the soft green foliage of the ‘Imperialis’ alder.

For ground cover, opt for understated yet effective creeping perennials. Some Carex morrowii ‘Irish Green’ with their evergreen, glossy green foliage and, perhaps, a lesser-known plant: a carpet of Veronica repens ‘Sunshine’, a charming small creeping perennial with golden foliage. This luminous carpet of creeping speedwell will persist more or less depending on winter severity and come alive in late spring, albeit briefly, with delicate little white flowers.

Alnus glutinosa ‘Imperialis’, Carex morrowii ‘Irish Green’, Cornus sanguinea ‘Magic Flame’, and Veronica repens ‘Sunshine’

Did you know?

• The alder is sometimes called “Vergne” or “Verne” in southern France. This name is a common surname in certain regions of France. A notable example is the French racing driver Jean-Eric Vergne, nicknamed “Jev“;
• The alder is the name of the 9th day of the month of Germinal in the Republican calendar;
• There is a saying that includes the alder: “If the alder greens before the birch, then the summer will be wet; if the birch overtakes the alder, then the season will be dry“;
• The glutinous alder is one of the trees that make up the sacred grove of the druids in Celtic tradition;
• The last ice age, which ended in our region around 12,000 years ago, drastically reduced the European flora. Alders were among the pioneering essential oils that helped repopulate the north-western part of Europe;
• Alder wood becomes almost rot-proof once soaked in water. This is why it was widely used in the construction of stilts in the Netherlands and Venice, where the city still stands thanks to thousands of alder posts;
• This rot-resistant wood is also used for creating breakwaters, formwork, clogs, barrels, or even for making canoes among Native Americans (Alnus rubra or Red Alder);
• Alder wood can also be carved and, thanks to its acoustic characteristics, used in lutherie;
• Alder wood sawdust is still used today for smoking meat and fish;
• The bark of the alder was used for tanning and dyeing leather.