Tuberous Jerusalem sage

Tuberous Jerusalem sage: how do you recognise it?

Habit

Sturdy, upright clump of 1–1.5 m high and about 60 cm wide, with square, often dark-tinted stems.

Leaves

Large, heart-shaped to triangular, wrinkled and toothed, up to about 25 cm at the base; the leaves get smaller higher up the stem.

Flower

Pink-lilac, hooded lipped flowers in whorls (tiers) along the upper part of the stem, from June to August.

Seed

After flowering, sturdy whorls holding the seed stay in place; they are decorative into winter.

Roots

Thick, tuberous roots; the plant roots fairly deeply and so copes well with dry spells.

Flowering: June–August · Plants: March–May and September–October · Pruning: March

Why grow it?

• Flowers from June to August, with tiers of pink-lilac blooms up tall stems.
• Bumblebees and bees love it, so it is a good plant for an insect-friendly border.
• Drought-tolerant: a strong choice for sunny, dry spots and the gravel garden.
• The dark stems and seedheads stay attractive in winter, especially with frost on them.
• Little work: fully hardy (to about −30 °C) and rarely in need of support on lean soil.

Tuberous Jerusalem sage: where does it grow best?

• Full sun. In part shade it grows looser and the stems flop more easily.
• Ordinary garden soil: sand, loam or clay, acid to chalky. The soil must drain well, because it dislikes wet feet in winter.
• Dry to moderately moist; it also does well on lean soil.
• Put it in the middle or at the back of the border and allow 50–60 cm between plants.
• An open spot is fine, as long as it is not too windy.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Not fussy

Moisture

Dry or normal

Size

1.2 m tall, 0.6 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Sage: Purple-blue spikes in the same sun and dry soil, facing the pink tiers.
• Globe thistle: Blue globes and an open, sturdy structure beside the upright stems.
• Yarrow: Flat heads in yellow or white as a calm contrast with the whorls.
• Feather reed grass: A tall, fine grass that loosens up the sturdy clump.
• Stonecrop: A low plant for the edge that flowers late in the season.

Tuberous Jerusalem sage: when and how to plant it?

Plant in March–May and September–October.

1. Plant in March–May or in September–October, when the soil is neither frozen nor waterlogged.
2. Dig a hole twice as wide as the root ball. Lighten heavy clay with coarse sand or grit so the water drains away.
3. Set the plant at the same depth as in its pot; the crown must not end up deeper.
4. Space plants 50–60 cm apart.
5. Water well straight away, then weekly in dry weather until it is established.
6. Spread a thin layer of compost or fine gravel around the plant, but keep the crown clear.

Care

Watering

Rarely needed after the first year. Water only in prolonged drought, then generously and at the base.

Feeding

On poor soil, a spadeful of compost in spring. On over-rich soil the stems turn weak and flop.

Mulching

A thin layer of compost or fine gravel in spring; never a thick blanket against the crown.

Spent flowers

Leave the seed whorls standing for winter. If you do not want seedlings, cut them off before the seed ripens.

Support

Usually not needed in full sun. On rich soil or in part shade a ring or stake holds the clump together.

Tip: Above all give it well-drained soil, because wet winters are its worst enemy. Leave the stems and seed whorls standing until March: they look good in winter and shelter insects.

Tuberous Jerusalem sage: when to prune it?

Pruning months: March.

When: In March, before the new shoots appear: cut away last year’s old stems then.

How:

• Cut the dead stems down in March to a few centimetres above the ground.
• After flowering, cut off the faded whorls if you do not want seedlings.
• Remove withered leaves at the base so air can move through the clump.

Caution: Do not cut back too early in autumn: the seedheads look good in winter and the stems shelter insects.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In March–April or in September: lift the clump and divide it with a sharp spade or knife into pieces with roots and buds. Replant straight away.

Sowing

Sow in autumn or early spring in a seed tray and put it outdoors. Germination can be slow and uneven; prick out the seedlings when they are big enough.

Tuberous Jerusalem sage: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy, to about −30 °C: no protection is needed in Belgium.
• The risk is not the cold but wet soil in winter, so make sure it drains well.
• In a large pot (at least 40 cm) it is possible: place it in shelter, make sure the water can drain away and water a little in dry, frost-free weather.

Diseases and pests

• Rot in wet soil: A browning, soft crown after a wet winter. Provide well-drained soil and do not plant too deep.
• Flopping stems: Happens on over-rich soil or in part shade. Feed less, give more sun and add support if needed.
• Slugs: Young shoots are sometimes nibbled in spring. Collect them in the evening or lay a ring of coarse grit.
• Self-sown seedlings: It can seed itself about. Cut off the seed whorls before the seed ripens and weed out seedlings in spring.

Through the year

• March: Cut back the old stems. Plant or divide. Apply compost on poor soil.
• April: Plant or divide. Watch for slugs on the young shoots.
• May: Plant. Add support if needed.
• June: Flowering begins. Bumblebees and bees visit the flowers.
• July: Full flower. Water young plants in prolonged drought.
• August: Last flowers. Let the seed whorls ripen or cut them off.
• September: Plant or divide.
• October: Plant. Leave the seed whorls for winter.
• December: Rest. Stems and seed whorls decorate the garden, especially with frost.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• The species name tuberosa refers to the thickened, tuberous roots.
• Unlike the shrubby Phlomis fruticosa, which is less frost-hardy, this species dies back in winter and is fully hardy.
• The hooded flowers are built for bumblebees and large bees, which crawl into the flower to reach the nectar.