Blue blossom (California lilac) Ceanothus thyrsiflorus
Blue blossom (California lilac) (Ceanothus thyrsiflorus) is an evergreen perennial shrub in the Rhamnaceae family, native to California and Oregon coast (coastal scrub and forest edges). It grows 1.5–4 m (4.9–13 ft) tall and 2–5 m (6.6–16 ft) wide, wants full sun and low water, and is hardy to about -10 °C (14 °F). Space plants about 2.5 m (8.2 ft) apart.
Plan Blue blossom (California lilac) in your garden →
Free. Draw your beds, then watch it grow through the year in 2D and 3D.
Blue blossom (California lilac) at a glance
| Type | Perennial shrub, evergreen |
|---|---|
| Family | Rhamnaceae |
| Mature height | 1.5–4 m (4.9–13 ft) |
| Mature spread | 2–5 m (6.6–16 ft) |
| Spacing | 2.5 m (8.2 ft) centre to centre |
| Sun | Full sun, at least 6 h of direct sun a day |
| Water | Low; drought tolerance high, waterlogging tolerance none |
| Soil pH | 5.5–7.5 |
| Cold hardiness | About -10 °C (14 °F) once established (roughly USDA zone 8) |
| Heat stress | Likely above about 38 °C (100 °F) |
| Flowering | from about March for 4–7 weeks (northern hemisphere; later in cold climates, earlier in warm ones) |
| Wind tolerance | Moderate |
| Pollinator value | High |
| Edible | Not grown as food |
| Native to | California and Oregon coast (coastal scrub and forest edges) |
| Common in gardens of | 🇺🇸 United States, 🇬🇧 United Kingdom, 🇫🇷 France |
Is Blue blossom (California lilac) toxic?
- People: not known to be toxic
- Cats: not known to be toxic
- Dogs: not known to be toxic
No toxicity reported in ASPCA or standard poison references.
If a child or pet may have eaten part of a plant, call a poison centre or vet straight away.
Companion planting: what to grow with Blue blossom (California lilac)
No well-documented helpful pairing. Group it with plants that like full sun and low water.
See Blue blossom (California lilac) in a botanical garden
It grows in this sample garden, which you can walk through in 3D:
- UC Botanical Garden, Berkeley (6 plants)
Questions about Blue blossom (California lilac)
- How far apart should I plant Blue blossom (California lilac)?
- About 2.5 m (8.2 ft) apart, centre to centre. A mature plant spreads 2–5 m (6.6–16 ft).
- Does Blue blossom (California lilac) need full sun?
- Full sun: at least 6 hours of direct sun a day.
- How much water does Blue blossom (California lilac) need?
- Low water. Drought tolerance is high and waterlogging tolerance none.
- How cold-hardy is Blue blossom (California lilac)?
- It tolerates about -10 °C (14 °F) once established, roughly USDA zone 8.
- When does Blue blossom (California lilac) flower?
- From about March for 4–7 weeks (northern hemisphere; later in cold climates, earlier in warm ones).
- Is Blue blossom (California lilac) poisonous to cats and dogs?
- For cats it is not known to be toxic; for dogs not known to be toxic; for people not known to be toxic. No toxicity reported in ASPCA or standard poison references.
- What grows well with Blue blossom (California lilac)?
- There is no well-documented companion pairing for Blue blossom (California lilac). Group it with plants that like the same sun (full sun) and water (low).
Sources and evidence
Evidence grade B (B: extension service or peer-reviewed).
- Calscape: California native plants, California Native Plant Society
- RHS Plant Finder, Royal Horticultural Society
Notes and assumptions
- Clouds of sky-blue flowers in spring, alive with bees; fixes nitrogen (Frankia); short-lived (10–20 years).
Reference values for planning, not horticultural, veterinary or medical advice. Varieties, soils and local climates differ: check with a local nursery or extension service.
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