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 includes around fifty species, the most well-known of which are Alnus glutinosa or the Common Alder, Alnus incana or the Grey Alder, and Alnus cordata or the Italian Alder. Alders are commonly found in damp woodlands, even swampy areas, and along riverbanks. A group of alders is called an alder grove.

Alnus glutinosa, botanical plate from 1862

Did you know? You may sometimes come across the species name “alnifolia” in the Latin names of certain plants. For example, in the case of Clethra alnifolia. “Alnifolia” simply means “with alder-like leaves“.

Alders are generally deciduous trees, of medium size (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. Additionally, 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 resistant to rot. However, outside of water, alder wood degrades very quickly. Alder wood is still used today for making pontoon pilings. In fact, the city of Venice was built on alder pilings. Alder wood features hypertrophied lenticels that aid in tissue oxygenation.

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

Once pollinated, the female catkins transform into small woody cones called strobiles, housing the seeds, which are winged achenes. These seeds are particularly favoured by granivorous birds such as the European Greenfinch, the 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 a particularly important species in the ecology of natural habitats surrounding certain waterways. This area is called the riparian zone. The riparian zone encompasses the wooded and/or herbaceous area along small waterways and serves as an ecotone, a transitional zone between two different ecosystems. These transitional zones are always interesting in terms of biodiversity, as they bring together representatives of the 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 (soil enrichment), pollutant remover, and, of course, a shelter and food source for wildlife (notably for the caterpillar of the Purple Emperor – Aparura ilia).

Our most beautiful species and cultivars

Alnus glutinosa

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 mainly used for revegetation and to prevent bank erosion.

• Flowering time February to May
• Height at maturity 25 m

See the product sheet

Alnus glutinosa Imperialis

Alnus glutinosa 'Imperialis' stands out from our common European alder with its very beautiful foliage, divided into narrow, pointed lobes, reminiscent of ferns.

• Flowering time April, May
• Height at maturity 9 m

See the product sheet

Alnus incana

The white alder or Alnus incana is a medium-sized deciduous tree named for its decorative smooth grey-white bark. Its dark green ovate and pointed leaves, with grey-white undersides, are characteristic.

• Flowering time March to May
• Height at maturity 20 m

See the product sheet

Alnus cordata - Italian Alder

The Corsican alder is a medium-sized deciduous tree with very aesthetic leaves, shaped like dark green hearts, tough and shiny, which remain on the tree for a long time before falling in winter. Native to Corsica and southern Italy, it is found in humid woodlands at altitude and along watercourses.

• Flowering time April, May
• Height at maturity 25 m

See the product sheet

Planting the alder

Where to Plant?

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

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

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

When to Plant?

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

Alders in flooded terrain 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 soil) extracted from the top 25-30 cm. Then, make a second pile with the soil 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. Determine the direction of the 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;
• Refill the planting hole first with the soil from the second pile, then with the fertile soil;
• Level the soil and firm it with your foot to ensure the soil adheres to the roots and eliminates 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 generously until the soil can no longer absorb water;
• Apply a layer of mulch 7 to 10 cm thick. This acts as winter protection, maintains consistent moisture, and prevents weed growth in early spring.

In Containers

• Soak the root ball in a bucket of water to rehydrate it. Then remove the plant from its pot;
• Dig a planting hole two to three times the size of the root ball;
• Add one to three spadefuls of well-rotted compost;
• Place the plant in the planting hole;
• Backfill with soil and firm lightly;
• Water generously until the soil can no longer absorb water;
• Spread a layer of mulch around the base to maintain good moisture around the roots. This will also limit weed growth.

Maintenance of the Alder Tree

Maintenance simply involves monitoring watering during the first year if rainfall is insufficient. After that, 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 sprout 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 rates—no more than one year old!

As alder is a pioneer species, spontaneous seedlings are very common. In this case, you can simply dig up young plants to relocate them elsewhere in your garden.

By Propagation by Cuttings

Two options are available to you.

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

→ Also read How to Propagate Alder?

Alnus sieboldiana (©Harum Koh)

Pests and Diseases

Phytophtora alni or Alder Decline 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 an overly high concentration of alders in a confined area. Alder decline disease first appeared around 1993. The alder develops yellowed and 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 our 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 the white alder. The eaten leaves take on a lacy appearance. They turn brown and fall quickly. However, there’s no need to worry—the alder tolerates this damage well and will quickly compensate by increasing its photosynthesis rate.

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

Lesser Purple Emperor (Aparura ilia)

Nothing to worry about—quite the opposite! The alder is the host plant for a magnificent 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 doesn’t suffer unduly from them.

→ Learn more about alder diseases and pests in our advice sheet.

Combining Alder in the Garden

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

To do it justice, it should be grown as a solitary specimen. However, this doesn’t mean its base can’t be adorned. The decorative-wood dogwoods are perfect companions for alders. They thrive in heavy, moist soils. A few plants of Cornus sanguinea ‘Magic Flame’, whose green-yellow foliage turns orange in autumn, would be an excellent choice. These vibrant colours will enhance the soft green foliage of the ‘Imperialis’ alder.

For ground cover, opt for understated yet effective perennial plants. A few Carex morrowii ‘Irish Green’, with their evergreen, glossy green foliage, and perhaps a lesser-known plant: a carpet of Veronica repens ‘Sunshine’, a charming little creeping perennial with golden foliage. This luminous carpet of creeping speedwell will persist more or less depending on the winter’s severity and will 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 the south of 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 turns green 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 pioneer species 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 poles;
• This rot-proof 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 sawdust is still used today for smoking meat and fish;
• The bark of the alder was used for tanning and dyeing leather.