Purple gromwell

Purple gromwell: how do you recognise it?

Habit

Upright flowering stems of 20–30 cm and long, trailing shoots that root at the tips; one plant soon covers 60 cm.

Leaves

Lance-shaped, dark green and rough to the touch.

Flower

Funnel-shaped flowers about 1.5 cm across, first reddish purple and then deep gentian blue, in April–June.

Fruit

Hard, glossy white nutlets that look like tiny porcelain beads.

Roots

A short rootstock; new plants grow from the rooting shoots.

Flowering: April–June · Plants: March–April and September–October · Pruning: June–July

Why grow it?

• Deep blue flowers in April–June, a rare colour under shrubs and along woodland edges.
• A fast, dense ground cover for chalky soil, in sun or partial shade.
• Bees and bumblebees readily visit the flowers.
• Once established, it also tolerates the drier soil under shrubs.
• The hard, glossy white seeds are a lovely detail in summer.

Purple gromwell: where does it grow best?

• Sun to partial shade; it is happy under shrubs and along a woodland edge.
• Loam or clay, neutral to chalky.
• Fairly dry to moderately moist; no waterlogged soil.
• Ideal under shrubs, along a woodland edge, on a bank or in a semi-wild garden, where it has room to spread.
• Plant 40–50 cm apart, about 3–4 plants per m².

Light

Sun and partial shade

Soil type

Loam or clay

Acidity

Neutral or chalky

Moisture

Dry or normal

Size

0.3 m tall, 0.6 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Lilac: Flowers at the same time and likes the same chalky soil.
• Wood spurge: Lime-green flowers next to the deep blue, also in partial shade.
• Stinking hellebore: Sturdy evergreen foliage that gives structure to the carpet.
• Guelder rose: A native shrub with white flowers in May, under which the blue carpet shows up beautifully.

Purple gromwell: when and how to plant it?

Plant in March–April and September–October.

1. Plant in March–April or in September–October, when the soil is neither frozen nor waterlogged.
2. Remove perennial weeds and mix some compost into the soil.
3. Dig a hole slightly larger than the pot.
4. Set the plant at the same depth as it was in the pot, 40–50 cm apart.
5. Water well and keep the soil moist for the first few weeks.
6. Mulch with leaf mould; the trailing shoots root into it easily.

Care

Watering

In the first year in dry weather. After that, only during long dry spells.

Feeding

Not needed; a layer of leaf mould in spring is enough.

Keeping it in check

Cut or dig out shoots that go too far straight away.

Weeding

Weed regularly in the first year, until the carpet has closed.

Tidying

In March, remove dead stems and untidy leaves.

Tip: Plant it under shrubs or on a bank where it can spread freely, and shorten the long shoots after flowering: the carpet stays dense and its neighbours are spared.

Purple gromwell: when to prune it?

Pruning months: June–July.

When: After flowering, in June–July, cut back the faded stems and any overlong shoots. In March, clear away dead stems.

How:

• Cut faded flower stems back to just above the foliage.
• Shorten shoots that go too far or grow over other plants; rooted pieces can be potted up or given away.
• In March, remove dead stems and old leaves, so the new foliage has room.

Caution: It spreads quickly by rooting shoots and can smother smaller plants: give it a spot where it is allowed to spread.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Rooted shoots

The trailing shoots root by themselves. In March–April or in September, detach a rooted young plant and replant it straight away.

Seed

In autumn, sow the hard seeds in a pot and leave it outdoors; germination is often slow and erratic.

Purple gromwell: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: no protection is needed in Belgium.
• Some of the leaves may turn brown in winter; the plant shoots again in spring.
• In a pot: stand it against a wall and do not let the compost dry out or become waterlogged.

Diseases and pests

• Spreading too far: The trailing shoots grow over neighbouring plants. Cut them back regularly and consider a buried edging.
• Powdery mildew: White powder on the leaves in dry summers. Water in dry weather and cut off affected stems.
• Bare patches: In deep, dry shade the carpet becomes thin. Add compost and water, and peg shoots down over the bare patches.
• Slugs: They eat the young leaves in spring. Pick them off in the evening.

Through the year

• March: Clear away dead stems. Planting. Add compost.
• April: Flowering begins. Planting. Move rooted shoots.
• May: Full flower, visited by bees.
• June: End of flowering. Cut back faded stems and overlong shoots.
• July: Keep the shoots in check. Water in dry weather.
• August: Water during long dry spells.
• September: Planting. Detach and replant rooted shoots.
• October: Planting. Sow seed in a pot outdoors.
• November: Leaf mould around the plants.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• As in many members of the borage family, including lungwort, the flowers change from reddish purple to blue as they age.
• Its old genus name, Lithospermum, means ‘stone seed’, after its very hard white seeds.
• In Britain it is rare in the wild and grows in only a few places.