Great willowherb

How to recognise it

Habit

Tall, upright perennial of 80–150 cm, sometimes up to 2 m, forming dense patches by means of underground runners.

Leaf

Lance-shaped, 6–12 cm, finely toothed, softly hairy and opposite. Stalkless, with a base that half clasps the stem.

Flower

Purplish pink, 15–25 mm across, with four petals that each have a notch. In the centre a white stigma with four lobes in a cross.

Fruit/Seed

A long, narrow capsule 5–8 cm long that splits into four strips. The seeds have a white tuft of hairs and fly off on the wind.

Stem

Round, branched and densely covered in soft, partly glandular hairs: it feels woolly.

Look-alikes

Rosebay willowherb has a long terminal spike, alternate leaves and petals without a notch. Hoary willowherb has pale flowers 6–9 mm across and leaves that do not clasp the stem. Purple loosestrife has upright spikes of flowers with six petals.

Flowering: June–September · Pruning: March and September

Why grow it?

• Long flowering season from June to September, with plenty of nectar and pollen for bumblebees, bees and hoverflies.
• Food plant for the caterpillars of the elephant hawk-moth, a pink and olive-green moth whose caterpillar looks like a little snake.
• Ideal for quickly bringing greenery and colour to a pond edge, a damp corner or a ditch.
• The sturdy stems stand through winter and then give shelter to insects and spiders.
• Its rhizomes help to hold together banks and ditch sides.

Great willowherb: where does it grow best?

• In the wild: ditch banks, stream and river banks, reedbeds, marshes, damp rough ground and moist verges.
• Sun to half shade.
• Moist to wet, fertile soil; it tolerates temporary flooding.
• In the garden: a pond edge, a rain garden or wildlife pond, a damp corner or a ditch. Too vigorous for an ordinary border.

Goes well with

• Purple loosestrife: Purple spikes on the same wet bank, also excellent for bees.
• Meadowsweet: Creamy, scented plumes in July.
• Hemp-agrimony: Pink flower heads in late summer, covered in butterflies.
• Yellow flag: Flowers earlier, in May–June, with sword-shaped leaves at the water’s edge.
• Water mint: A low, fragrant groundcover on the bank.

Great willowherb: when and how to plant it?

1. Buy a plant from a native or aquatic plant nursery, or sow. Never dig plants up from the wild.
2. Plant in March–April or September–October in moist soil or on a pond edge, with the root ball in the wet zone but not under water.
3. Allow 60–100 cm of space, and bear in mind that it spreads by underground runners.
4. To keep it in check, plant it in a large sunken tub or behind a root barrier dug in to a depth of 30 cm.
5. Sowing: scatter the seed on moist soil in spring without covering it; it germinates readily in the light.
6. Keep the soil moist during the first season.

Care

Water

Only needed if the soil dries out; in a wet spot, never.

Feeding

Not needed: it finds enough food in moist soil.

Spreading

Spreads by runners and seed. Each spring, cut around the edges with a spade and pull out runners beyond the area you want.

Self-seeding

If you don’t want seedlings, cut the flowering stems in September, before the capsules split open.

Mowing

Along a ditch or in damp rough ground, mow once a year, in September or March, and remove the cuttings.

Tip: Give it its own damp corner where it is allowed to spread: there it feeds more insects than almost any other waterside plant. In a small garden, a sunken tub is the answer.

Pruning and mowing

Pruning months: March and September.

When: In September if you want to prevent seeding, or in March if you leave the stems standing over winter.

How:

• Cut the flowering stems just before the capsules split if you don’t want fluff all over the garden.
• Preferably leave some stems standing over winter as shelter for insects.
• In March, cut the old stems just above the ground, before the new shoots appear.

Caution: Before mowing or cutting in summer, first check the plants for elephant hawk-moth caterpillars.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In March–April or October, slice off a piece of rhizome with shoots and replant it elsewhere.

Seed

Sow in spring on moist soil, without covering. It also self-seeds very readily.

Great willowherb: how does it get through the winter?

Hardy to −25 °C.

• The top growth dies back; the brown stems with remnants of fluffy seed stay standing for a long time.
• The plant overwinters by means of its rhizomes in the soil.
• Fully hardy, to about −25 °C.

Diseases and pests

• Spreading: Quickly takes up too much space in a small garden. Plant it in a sunken tub or behind a root barrier and cut around the edges every spring.
• Seedlings: The fluff blows into other corners of the garden. Cut the stems in September or weed out seedlings in spring.
• Flopping: On rich soil or in shade the stems grow tall and fall over. A sunnier spot or support from twiggy sticks helps.
• Powdery mildew: White powder on the leaves in dry summers. Harmless; water during drought.

Through the year

• March: Cut back old stems. Cut around the edges with a spade.
• April: New shoots. Plant or divide.
• June: Flowering begins.
• July: Full flower. Look for elephant hawk-moth caterpillars.
• August: Flowering. Bumblebees and hoverflies on the flowers.
• September: Last flowers. Cut the stems if you don’t want seed.
• October: The fluff blows away. You can still plant or divide.
• December: Dry stems as winter shelter.

Did you know?

• Epilobium means ‘on the pod’: the flower sits on top of the long, narrow ovary, which is visible even before the flower opens.
• The elephant hawk-moth caterpillar has false eyespots; when threatened, it pulls in its head, swells its front end and looks like a small snake.
• The light seeds with their tuft of hairs drift far on the wind: that is how the plant quickly turns up on new banks and freshly disturbed wet ground.