Himalayan indigo

Himalayan indigo: how do you recognise it?

Habit

Loose, open shrub, 1.5–2 m tall and wide, with arching branches.

Leaves

Pinnate, 5–10 cm, with 13–21 small oval leaflets; fresh green to grey-green. They appear late in spring.

Flowers

Upright clusters of 5–15 cm with small pinkish-purple pea flowers, from the leaf axils of the new shoots, June to September.

Fruit

Narrow brown pods a few centimetres long.

Roots

Deep and branching, with root nodules in which bacteria fix nitrogen.

Flowering: June–September · Plants: April–May · Pruning: April

Why grow it?

• A very long flowering season, from June to September, when many shrubs have finished.
• Light, graceful foliage with an airy, almost Mediterranean feel.
• Bees and bumblebees readily visit the pea flowers.
• Copes with dry spells once well established: ideal for a warm, sunny spot.
• As a legume it works with soil bacteria that fix nitrogen, so it also grows on poor soil.

Himalayan indigo: where does it grow best?

• Full sun: the warmer and sunnier, the better it flowers.
• Free-draining soil: sand, loam or clay, acid to chalky; poor soil is no problem.
• Dry to normally moist; it dislikes waterlogged roots in winter.
• A sheltered spot, for example in front of a south-facing wall, limits frost damage; you can also train it against the wall there.
• Allow 1.5–2 m per plant.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Not fussy

Moisture

Dry or normal

Size

2 m tall, 2 m wide

Hardiness

Hardy (to -20 °C)

Goes well with

• Lavender: The same sun and dry soil; purple beside pink.
• Russian sage: Violet-blue spires in late summer, just as fond of dry conditions.
• Ornamental grasses: Airy plumes that sway nicely beside the fine foliage.
• Catmint: Blue flowers at its feet from June to September.

Himalayan indigo: when and how to plant it?

Plant in April–May.

1. Plant in April–May, once the soil has warmed up: this gives it time to root well before winter.
2. Dig a hole twice as wide as the root ball; on heavy soil mix in coarse grit or fine gravel.
3. Set the plant at the same depth as it was in the pot and firm the soil well.
4. Leave 1.5–2 m between plants.
5. Water generously straight away and regularly in dry spells during the first summer.
6. Mulch around the base, but keep it away from the stems.

Care

Watering

Regularly in the first summer; after that only during long dry spells.

Feeding

Usually unnecessary, as it fixes its own nitrogen. On very poor soil, a spadeful of compost in April.

Patience

It leafs out late, often in May: bare branches in April are normal.

Support

Against a wall you can tie the branches loosely to wires.

Spent flowers

No need to remove them: new clusters keep coming all summer.

Tip: Do not worry too soon if it is still bare in April: it only comes into leaf in May. Wait until you can see which branches are sprouting before pruning, then remove only the frosted and old wood.

Himalayan indigo: when to prune it?

Pruning months: April.

When: In April, when you can see which branches are sprouting. It flowers on new wood, so pruning costs no flowers.

How:

• Cut frosted and dead branches back to the first healthy bud or to the base.
• Each year, remove a few of the oldest, weakest branches at the base.
• If it was killed to the ground in winter, cut everything short; it will regrow from the base.
• For a more compact shrub, cut all branches back to 30–50 cm in April.

Caution: Do not prune too early: it leafs out late, and only in April or May can you see what has been frosted.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Softwood cuttings

In June, take 8–10 cm cuttings from young shoots and root them in moist cutting compost under plastic.

Semi-ripe cuttings

In July–August, take 10–15 cm heeled cuttings and root them in a mix of sand and compost in a cold frame; overwinter them frost-free.

Himalayan indigo: how does it get through the winter?

Hardy to −20 °C.

• Hardy to about −20 °C; in severe winters the branches may be killed back, but the base usually survives.
• In the first year, cover the base with a layer of leaves or straw to protect the roots.
• A spot in front of a south-facing wall or in a sheltered corner limits frost damage.
• In a pot: keep it sheltered and dry in winter and wrap the pot in hessian.

Diseases and pests

• Frosted branches: Bare, brown branches after a severe winter. Wait until May to see what sprouts, then cut out the dead wood.
• Late leafing: Still no leaves in April: that is normal. Do not throw it out, it only gets going in May.
• Root rot: Yellow leaves and dying branches on wet, heavy soil. Plant it in free-draining soil and avoid standing water.
• Few flowers: Too much shade or too much fertiliser. Give it full sun and do not feed it.

Through the year

• March: No leaves yet: be patient. Remove the winter protection from the base.
• April: Prune: frosted and old branches out. Plant.
• May: The leaves appear. Plant. Water in dry spells.
• June: Flowering begins. Take softwood cuttings.
• July: Flowering. Take semi-ripe cuttings.
• August: Flowering. Water during long dry spells.
• September: Last flowers. The pods ripen.
• November: Young plants: a layer of leaves over the base.
• December: Dormant.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• The dye indigo does not come from this species but mainly from its tropical relative Indigofera tinctoria; the genus name means ‘indigo-bearing’.
• As a legume it fixes nitrogen from the air with the help of bacteria in its root nodules.