OUTDOOR BONSAI CARE GUIDE
Alder Bonsai Care
A practical guide to growing, watering, wiring, pruning and repotting hardy alder bonsai trees.
Botanical nameAlnus
Native rangeEurope, western Asia and Northern Africa
Bonsai stylesFormal upright, informal upright and group
HardinessUSDA zones 4–9
About Alder Bonsai
Alder is a hardy deciduous tree with petiolate, alternate and rounded leaves that are particularly sticky when young. Its bark is dark grey and fissured.
Unlike many trees, alders are well adapted to wet soils near rivers and lakes. Alder wood hardens when it contacts water and is notably resistant to it.
The flowers are unisexual. Male flowers form drooping catkins that appear before the leaves, while female flowers form oval structures resembling small pine cones.
Can You Bonsai an Alder Tree?
Yes. Alder trees make adaptable outdoor bonsai and are easy to train, prune and shape. Their naturally large leaves can sometimes appear out of proportion to the tree. Alder bonsai are especially popular in Europe and are gradually becoming better known among North American growers.
The Alnus genus contains almost 35 species. Two species are used particularly often for bonsai:
Alnus glutinosa
Common alder or black alder has dark green, ovate leaves and produces yellowish-brown catkins.
Alnus cordata
Italian alder has shiny, dark green, heart-shaped leaves and also produces yellowish-brown catkins.
01
Best Location
Alder bonsai trees can be placed in full sun during summer. Move them into partial shade when the weather becomes very hot. These trees thrive in humid conditions.
Alders are winter-hardy, including when kept on benches. During prolonged cold weather, move the bonsai container into a protected shed or greenhouse.
Wild trees can tolerate extremely low temperatures, but a container-grown bonsai should not remain below freezing for prolonged periods.
02
Propagation
Common alder can be propagated from seed. Soak the seeds in water for one to two days, mix them with horticultural sand in a plastic bag, and keep the bag refrigerated for one month.
Sow the stratified seeds in spring and keep the seed tray consistently moist, as moisture helps speed germination.
Alternatively, take hardwood cuttings during dormancy after the leaves have fallen or before the buds begin to form.
03
Watering
Water both commonly grown alder species freely throughout the growing season.
Alder tolerates wet soil and naturally grows near bodies of water. However, the soil should not remain waterlogged continuously because this can cause root rot.
During winter, keep the compost just moist.
04
Wiring
Wire an alder bonsai in spring or early summer. New growth is usually too tender, so work with growth from the previous year after it has hardened.
Remove the wire as soon as the branches hold their position, which may take only a couple of months.
Alder bark marks easily. Check the wire frequently; light scars do not necessarily harm the tree and may add character, but deep wire damage should be avoided.
05
Pruning
Alders are vigorous growers. Throughout the growing season, prune new shoots back to approximately one-third of their length, leaving two leaves, to maintain the bonsai’s shape.
To maintain structure, prune back to old wood in spring. Heavy pruning can be performed at repotting time just before the new buds burst.
Pinch the tips of new shoots weekly during summer to keep the tree compact and encourage short internodes. Complete defoliation may be performed every three years to reduce leaf size.
06
Repotting
Alder is vigorous and generally requires annual repotting in early spring.
Up to approximately half of the root ball may be pruned during repotting. Alder is not especially particular about compost, but it dislikes lime.
Use a deep pot and a basic free-draining bonsai soil mix.
07
Feeding
From early spring through late summer, apply a general fertilizer once a month. Choose a fertilizer that is low in nitrogen.
08
Pests and Diseases
Alder bonsai may occasionally be infested by aphids or scale insects. Treat an active infestation appropriately and maintain good growing conditions.
Fungal problems include bracket rot and leaf spot. Cut affected areas back to healthy wood, seal pruning wounds and dispose of infected material safely.
Although alder prefers moist soil, prolonged waterlogging can cause potentially fatal root rot. Branches may also suffer occasional dieback; prune affected growth back to healthy tissue.