Box elder

Box elder: how do you recognise it?

Habit

Tree of 10–20 m with a broad, somewhat irregular crown, often multi-stemmed; the variegated forms stay smaller.

Leaf

Opposite, pinnate with 3–5 (sometimes 7) coarsely toothed or slightly lobed leaflets.

Bark

Young twigs green, often with a blue-white bloom; older bark grey-brown, shallowly fissured.

Flower

Male and female trees are separate: small, greenish, hanging flowers in spring, just before or with the leaves.

Fruit

Paired winged fruits in long hanging clusters, on female trees only.

Difference from ash

Ash has more leaflets (usually 9–13) and thick black buds.

Flowering: March–April · Pruning: July–October

Identify by feature: Pinnate leaves · Opposite leaves · Winged seeds

Looks like it, but isn’t

• Ash Fraxinus excelsior: more leaflets (7–13) and thick black buds. Difference from ash

Why grow it?

• Fast-growing: a shade-giving tree within a few years.
• The variegated form with white leaf margins brightens up a dark corner.
• Undemanding: grows on almost any soil, in sun or partial shade.
• Green, white-bloomed twigs that stand out in winter too.

Box elder: where does it grow best?

• Sun to partial shade.
• Almost any soil: it isn’t fussy.
• Not near a river, stream or nature reserve: it self-seeds there.
• The species grows 10 m wide; the variegated form stays smaller and suits a medium-sized garden.

Light

Sun or partial shade

Legal basis

Any garden soil

Moisture

Normal

Size

15 m tall, 10 m wide

Hardiness

Throughout Belgium, no protection needed

Goes well with

• Holly: Dark evergreen leaves next to the light, variegated foliage.
• Lesser periwinkle: An evergreen ground cover under the crown.
• Common dogwood: A native shrub for the edge, with red twigs in winter next to the green twigs of the box elder.

Good and bad companions

Goes well with

• Tolerate the shade under this tree: oregon grape, rhododendron, oleaster, japanese skimmia, garden privet, camellia, laurustinus, box-leaved honeysuckle

Box elder: when and how to plant it?

1. Plant from November to March; a pot-grown tree can also go in later.
2. Choose a spot well away from water and nature reserves.
3. Dig a large planting hole, plant at the same depth as before and firm the soil; put a stake on the windward side.
4. Water generously after planting and during dry spells in the first year.

Care

Watering

In dry spells during the first year; rarely needed after that.

Feeding

Not needed: it grows fast enough already.

Seedlings

Pull up seedlings in the garden while they are young, roots and all.

Green shoots

On the variegated form, look out every year for all-green shoots and cut them out.

Tip: Want a maple that helps wildlife? Choose the native field maple: about the same size, easy to prune and valuable for insects and birds.

Box elder: when to prune it?

Pruning months: July–October.

When: In summer or early autumn (July–October). In spring it bleeds heavily.

How:

• On the variegated form, cut out green shoots that ‘revert’ as soon as possible, or they will take over the tree.
• Remove dead, crossing and overcrowded branches; keep the crown open.
• Prune young trees in good time: small wounds heal better than large ones.

Caution: Don’t prune from late winter into spring: it then bleeds heavily from the wounds.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Sowing

It already self-seeds more than enough. Seedlings of the variegated form come up green.

Grafting

Variegated forms are grafted or grown from cuttings by the nursery.

Box elder: how does it get through the winter?

Hardy throughout Belgium: no protection needed.

• Fully hardy.
• The green, white-bloomed twigs stand out on the bare tree in winter.

Diseases and pests

• Reversion: On the variegated form, all-green shoots appear that grow faster. Cut them back to variegated wood.
• Self-seeding: Female trees self-seed, sometimes massively along rivers. Pull up seedlings and remove young trees in nature reserves, roots and all.
• Branch breakage: The wood is brittle: branches sometimes snap in storms. Keep the crown open and remove weak forks on young trees.
• Verticillium wilt: Branches suddenly wilting in summer (Verticillium). Cut them back into healthy wood; there is no cure.

Through the year

• March: Last month for planting; don’t prune, it bleeds.
• April: Flowering, just before or with the leaves.
• May: Cut green shoots out of the variegated form.
• June: Pull up seedlings.
• July: Pruning season starts (July–October).
• August: Prune; water young trees in dry spells.
• September: Pruning still possible; the clusters of fruit ripen.
• October: Last month for pruning; weed out seedlings.
• November: Planting season starts.

What can you use it for?

Garden and landscape

Fast-growing garden tree, often in a variegated form with white leaf margins.

Caution: Can self-seed heavily along rivers.

Did you know?

• In gardens often grown as a variegated form with white leaf margins. Native to North America.
• Its pinnate leaves are unusual for a maple and look like those of an ash; the paired winged fruits give it away as a true maple.
• Male and female flowers grow on separate trees: only female trees bear fruit.
• Unlike most maples it is dioecious: there are separate male and female trees.
• Its English name comes from its whitish wood, which recalled box, and its pinnate leaves, which recalled elder.