Photo: A. Barra, CC BY 3.0 · source
Shrub · Rhamnaceae
Blueblossom
Ceanothus thyrsiflorus
In other languages: Ceanothus (Nederlands) · Céanothe (Français)
Ceanothus thyrsiflorus is an evergreen shrub from the coast of California and Oregon, where it grows wild on sunny slopes. In May–June it is covered in dense clusters of tiny sky-blue flowers that bees visit in huge numbers. The small, glossy dark green leaves make it an attractive shrub outside the flowering season too. It loves sun, warmth and free-draining soil and copes well with drought, but less well with hard frost and wet winters. Well-known forms include ‘Skylark’, with dark blue flowers, and the low, creeping variety repens.
Light
Full sun
warm and bright
Moisture
Dry to normal
no wet feet
Legal basis
Sand or loam
acid to neutral
Hardiness
down to -15 °C
protect young plants
Size
3 m
tall, 3 m wide
Flowering
May–June
| Pruning months | June–July |
|---|---|
| Height | 3 m |
| Native to Belgium | No, planted |
| Good for bees, butterflies and birds | Yes, for bees |
| Poisonous to people and pets | Not known to be poisonous |
| Flowering | May–June |
Identify
Blueblossom: how do you recognise it?
- Habit
- Broad, dense shrub up to 3 m tall and wide; sometimes taller against a wall.
- Leaves
- Small, oval, glossy dark green, with three distinct veins from the base; evergreen.
- Flowers
- Dense, plume-like clusters of tiny flowers, in May–June.
- Colour
- Light to dark blue, white in a few varieties.
- Roots
- With nodules in which bacteria fix nitrogen from the air.
Flowering: May–June · Plants: April–May · Pruning: June–July
In your garden
Why grow it?
- One of the few shrubs with truly blue flowers, in May–June.
- The flower clusters hum with bees.
- Evergreen: a fresh, glossy shrub even in winter.
- Drought-tolerant once well established: ideal for a sunny, dry spot.
- Lovely as a free-standing shrub, trained against a wall or as an informal hedge.
Blueblossom: where does it grow best?
- Full sun, ideally against a south- or west-facing wall.
- Well-drained sandy or loamy soil, acid to neutral; it will not tolerate heavy, wet clay.
- Sheltered from cold east and north winds, which can scorch the evergreen leaves in winter.
- Allow 2–3 m: it grows fast and dislikes being moved.
- Light
- Sun
- Soil type
- Sand or loam
- Acidity
- Acid or neutral
- Moisture
- Dry or normal
- Size
- 3 m tall, 3 m wide
- Hardiness
- Hardy (to -15 °C)
Goes well with
- Lavender: Also sun-loving and drought-tolerant, with a different shade of blue.
- Rosemary: Evergreen, aromatic and happy in the same dry, sunny spot.
- Rock rose: Flowers in pink and white right after the ceanothus, in the same warm spot.
- Sage: Grey-green leaves and an aromatic scent for a Mediterranean corner.
Maintenance
Blueblossom: when and how to plant it?
Plant in April–May.
- Plant in April–May, so it is well rooted before winter.
- Choose a young plant in a pot: large, older plants establish poorly.
- Dig a hole twice as wide as the root ball; on heavy soil, mix in coarse sand or gravel, not manure.
- Set the plant at the same depth as it was in the pot, firm in and water well.
- Water in dry spells during the first year and, against a wall, fix wires to tie the branches to.
Care
- Water
- In dry spells in the first year; after that it is drought-tolerant and needs little.
- Feeding
- Not needed: too much feeding gives soft, frost-tender growth.
- Mulching
- A layer of gravel or fine bark keeps the base dry and weed-free.
- Tying in
- Against a wall, tie the main branches loosely to wires.
Tip: Plant it in full sun in a sheltered spot with well-drained soil. Prune only lightly, straight after flowering: it regrows poorly from old wood.
Blueblossom: when to prune it?
Pruning months: June–July.
When: Straight after flowering, in late June or early July.
How:
- Lightly shorten the flowered shoots, by no more than a third of the new growth.
- Only cut into green, leafy wood.
- Against a wall, cut back shoots that grow outwards.
Caution: Never cut into old, bare wood: it regrows poorly from there, if at all.
Propagation
- Semi-ripe cuttings
- In July–August, 10 cm cuttings in a sandy cutting mix, sheltered and slightly moist. Overwinter the young plants frost-free.
Blueblossom: how does it get through the winter?
Hardy to −15 °C: protect young plants.
- Hardy to about −15 °C: fine in most Belgian winters, but it can be damaged in severe ones.
- In hard frost, drape horticultural fleece over young plants, especially in the first few years.
- Wet soil in winter is often a greater enemy than the frost itself.
- Shake heavy snow off the branches, or they will break.
Diseases and pests
- Root rot: The shrub wilts and suddenly dies, usually on wet, heavy soil. Plant it in a well-drained spot.
- Frost damage: Brown leaves and dead shoot tips after a severe winter. Wait until it starts growing again, then cut out only the dead wood.
- Short life: It often does not live very long and can become bare at the base after a number of years. Take a cutting in good time as a replacement.
- Yellow leaves: On chalky soil the leaves can turn yellow. Plant in acid to neutral soil.
Through the year
- January: Shake heavy snow off the branches.
- April: Plant. Fix wires against a wall.
- May: First flowers, full of bees. Plant.
- June: In flower. Prune lightly straight after flowering.
- July: Finish pruning. Take semi-ripe cuttings.
- August: Take semi-ripe cuttings. Water only in long dry spells.
- November: Have fleece ready for hard frost.
- December: In hard frost, cover young plants.
Use
Good for wildlife
- Bees
- Nectar and pollen for honeybees and wild bees.
You and this species
Did you know?
- Bacteria living on its roots fix nitrogen from the air, just as in the pea family. That is why it also grows well on poor soil.
- Despite its English name ‘Californian lilac’, it is not related to lilac: it belongs to the buckthorn family.