Indigofera heterantha: growing guide, care tips

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Indigofera heterantha is a graceful, wall-shrub-like member of the pea family, clothed in fine pinnate foliage that gives it an almost fern-like softness. From midsummer until the first frosts it carries upright spikes of small rose-purple pea flowers, making it one of the longest-flowering shrubs you can grow against a sunny wall. Its greatest quirk is that it flowers on the current season’s growth, so even if winter cuts it to the ground it will bounce back and bloom the same year.

Growing conditions

Give Himalayan indigo the sunniest, warmest spot you have. It needs a full day of sun to ripen its wood and produce a steady succession of flower spikes; in shade the growth becomes lax and flowering is sparse. A south- or west-facing wall or fence is ideal, particularly in cooler regions.

Drainage matters more than fertility. This is a plant of dry Himalayan hillsides, so it thrives in light, gritty, free-draining soils and copes happily with chalk, sand and thin stony ground. Neutral to slightly alkaline soil suits it best, but it is unfussy about pH provided the roots never sit in winter wet.

In mild gardens it forms a permanent woody framework; in colder or wetter areas it behaves more like a herbaceous perennial, dying back hard and resprouting from the base. Shelter from cold, drying winds helps preserve the top growth, and being a legume, it fixes its own nitrogen and rarely needs rich soil.

How to plant Indigofera

Plant in mid to late spring, once the soil has begun to warm and hard frosts have passed. Autumn planting is possible in mild coastal areas, but spring gives the roots a full season to establish before their first winter.

• Spacing: Allow 1.5–2 m between plants, or 45–60 cm from a wall, so air can circulate around the arching stems and reduce mildew.
• Depth: Set the top of the rootball level with the surrounding soil — never bury the base of the stems, which encourages rot.
• Soil preparation: Fork in coarse grit or gravel on heavy ground and avoid heavy manuring; lush, soft growth is more frost-tender.
• Watering in: Soak thoroughly after planting, then water weekly through the first summer only, tapering off as growth matures.
• Propagation: Take semi-ripe cuttings in July, or sow scarified seed in spring at 18–20 °C; rooted suckers can also be detached in spring.

Care through the seasons

Spring: Wait until you see buds swelling — usually April or even May, as this shrub breaks late. Cut back frost-damaged stems to healthy wood, or prune the whole plant to a low framework 15–30 cm above the ground for the best flowering shoots. A light mulch of garden compost and a handful of potash-rich feed is all the nourishment it needs.

Summer: Flowering begins in June and continues on the new wood right through the season. Established plants are notably drought-tolerant and rarely need watering; container-grown specimens will, however, need a soak every few days in hot spells. There is no need to deadhead, though snipping off spent spikes tidies the plant and encourages more.

Autumn: Resist the urge to prune — the old stems help protect the crown from winter cold and wet. Collect ripe seed pods in October if you want to raise your own plants. Apply a dry mulch of bark, bracken or leaf mould over the root zone in colder gardens.

Winter: Expect the plant to look dead; it is simply deciduous and very late to stir. In areas colder than −15 °C, or on exposed sites, pile extra mulch over the crown and accept that it may regrow from ground level.

Best varieties to grow

Named cultivars are scarce, so most gardeners grow the species alongside a handful of closely related indigos.

• Indigofera heterantha — the straight species is the one to seek out, holding an RHS Award of Garden Merit for its reliability and long flowering season.
• Indigofera amblyantha — taller and more upright, with softer shell-pink flower spikes over a longer, more open framework.
• Indigofera pseudotinctoria ‘Rose Carpet’ — a low, spreading form reaching only 60 cm, excellent for banks, gravel and the front of a sunny border.
• Indigofera kirilowii — a suckering, thicket-forming species around 1 m tall with bright rose-pink flowers and greater tolerance of cold.

Pests, diseases and common problems

• Winter wet and root rot: Yellowing, failure to shoot in spring and blackened crowns point to waterlogging. Improve drainage with grit, plant on a slight mound and never mulch right up against the stems.
• Powdery mildew: A white bloom on leaves in dry, still conditions, worst on wall-trained plants. Water the roots during prolonged drought, thin congested stems for airflow and remove affected leaves rather than reaching for sprays.
• Failure to flower: Usually caused by too much shade, over-feeding with nitrogen, or pruning too late in spring after the flowering shoots have formed. Move to full sun, feed sparingly and prune only as buds break.