Alnus

This hardy genus is a quick grower and ideal for adding interest to nutrient-poor ground.

Alnus glutinosa - photo: Adrian Thomas
Alnus glutinosa - photo: Adrian Thomas

Poor, wet soil is usually a thing to despair at, and is not at all promising for tree-planting — unless that tree is the alder. It will grow in almost any soil, thrives in damp, even very wet conditions and can survive periodic flooding. Being fast-growing but not too tall adds to its landscaping credentials.

The genus Alnus is from the same family as birch: Betulaceae. Like birch, most alders are pioneer species. They specialise in invading gaps and clearings in forests, and producing numerous small seeds which germinate quickly to colonise bare, often nutrient-poor ground. They are able to do this because, like peas and beans, they possess special nodules on their roots containing nitrogen-fixing bacteria that improve the soil. For this reason, alders are often used as soil improvers on reclamation land. The large shrub A. viridis, the green alder, has been successfully used on derelict sites, where it quickly colonises the land, stabilising the soil and adding fertility.

Alders put on a lot of fast initial growth and have a relatively short life span. They seldom grow very tall — the largest species, A. glutinosa and A. cordata Award of Garden Merit (AGM) will reach 25m — and need good light and open ground.

In their natural habitat they are often overtaken by larger trees that outshade them. A. cordata AGM, A. rubra and A.incana are fast growing, making them good windbreaks. A. cordata AGM, the Italian alder, is also noted for its tolerance to pollution, which, with its upright habit, makes it an ideal street tree.

Alders are perhaps best known for their attractive catkins that often appear in late winter before the leaves. The male catkins are long, pendulous and come in shades of yellow and brown. The female catkins are less conspicuous in spring, but ripen in summer into woody conifer-like cones. These fruit open in autumn to release the seed, but often remain on the tree for several years.

A. cordata AGM, A.glutinosa and A. viridis are the most lime-tolerant species. A. cordata tolerates drier, more calcareous soil than its relatives. All alders are fully hardy.

Unfortunately, all alders are prone to Phytophthora root rot, particularly in the wettest conditions. Over the past decade, many riverside alder have succumbed to the fungus. If alders are going to be planted in very wet conditions, it is very important that the site is tested for Phytophthora before planting. Any new plant material or soil imported to the sites should also be tested.

Alders require little aftercare besides that given to any newly planted tree: regular watering, staking and rabbit- or deer-proofing, if required.

What the specialists say

Hossein Arshadi, director, Hiller Nursery, Hampshire

“We sell four species of alders: A. cordata Award of Garden Merit (AGM), A. glutinosa, A. incana and A. x spaethii. At the moment A. cordata is the preferred choice because of its attractive glossy foliage. It holds its leaves well into December, even January. Its narrow, upright shape makes it a good choice for streets and car parks.

“A. glutinosa, the common alder, thrives in wet conditions. This makes it very popular with landscape architects for plantings where a natural look is required.

“A. incana is very similar to A. glutinosa, but is a more vigorous grower. It has a lovely leaf and makes a reasonable-sized tree. Again, like A. glutinosa it tends to be used in more native-style plantings.

“A. x spaethii is not widely used because it is not very widely known. It has a much longer leaf and is a very attractive tree on account of its long catkins. It’s the kind of tree that makes landscape architects visiting the nursery change their order as soon as they see it. I think once people know it better, you’ll see more of it.

“In terms of growing alders, in the past eight or nine years we’ve seen an increase of Phytophthora infection. This tends to be a problem where trees are planted near river banks or in wet ground, because this increases the potential for the Phytophthora to get into the roots. It is very important to make sure that the site is not contaminated if you are going to plant in wet conditions.”

Jana Nemcova, head grower, Majestic Trees, Hertfordshire

“A. incana and A. glutinosa are very common and get used mainly in natural looking landscapes, such as riverside plantings. They are not formal-looking trees, but both do very well in moist conditions. A. cordata AGM is a more formal plant that you see used a lot in garden plantings. It’s not a big tree, so it’s suited to smaller places.

“In general, alders prefer moist soil and don’t like too much shade. They are very valuable for wet conditions and will grow in any soil types, which makes them a useful tree for difficult situations.”

In Practice

Geoffrey Carr, landscape designer, Gloucestershire

“I believe the alder is a victim of its own strengths. They are renowned for their ability to grow in poor, damp soils and often get planted as a soil-improving pioneer crop, but they tend to be overlooked as a domestic tree and seldom form part of the street scene. The Italian alder’s reputation as a rapid grower contributes to their functional use, but it also has a magnificent conical shape and is ideal as a quick-fix focal point.

“I use alders in domestic gardens instead of willow if the situation calls for a less invasive tree. There are regulations for using them for reclaiming brownfield sites: they must not be planted within 6m of buried utilities.”

Species and Cultivars

•    A. cordata Award of Garden Merit (AGM), Italian alder, is a conical tree that grows to a height of 25m. It has heart-shaped, glossy, dark green leaves. The male catkins open long before the leaves in late winter or early spring, the female catkins by summer. It tolerates a wide range of soil types, but prefers moist conditions.
•    A. glutinosa, common alder, is a broadly conical tree that reaches a height of 25m. It is considerably wider than A. cordata AGM, and has ovate, dark green leaves that are sticky when young. In spring it is covered in groups of pendent yellow catkins.
•    A. glutinosa ‘Aurea’ has pale yellow leaves when young that mature to light green. It grows to 15m tall.
•    A. glutinosa ‘Imperialis’ AGM is an attractive form with deeply cut, mid-green leaves.
•    A. glutinosa ‘Laciniata’ is very similar to ‘Imperialis’ but has a stiffer, stronger growing habit.
•    A. glutinosa ‘Pyramidalis’ has a very narrow, conical habit with upright branches. It is very good for narrow sites.
•    A. incana, grey alder, is a very hardy tree with dark green leaves that are grey underneath. Its yellow catkins appear in late winter, before the leaves. It can reach 20m in height, and is ideal for cold, wet sites.
•    A. incana ‘Aurea’ has yellow leaves that turn pale green in summer. It has orange shoots and catkins in winter. It grows to 10m tall.
•    A. incana ‘Lacinata’ has narrow-lobed leaves.
•    A. incana ‘Pendula’ is an attractive, small, weeping tree, growing to 10m tall and 6m wide.
•    A. japonica, Japanese alder, is a strong medium-sized tree with narrow pointed leaves. It has yellow catkins that open before the leaves in spring. It grows to 20m.
•    A. x spaethii is a fast-growing tree with large leaves that are purplish when young. Catkins appear in late winter to early spring.


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