Deciduous shrub with large rounded flower heads. Pink in alkaline soil, blue in acidic.
Hardiness ratings
| System | Rating | Temperature range | How to read it |
|---|---|---|---|
| USDA hardiness zone | Zone 5–9 | −28.9 °C to −1.1 °C | Plant tolerates down to this zone |
| RHS hardiness rating | H5 | −15 °C to −10 °C | Plant needs at least this level of cold tolerance |
| Canadian plant hardiness zone | Zone 5–9 | −23 °C and warmer | Plant tolerates down to this zone |
| Australian (ANBG) zone | Zone 2–5 | −10 °C to 10 °C | Plant tolerates down to this zone |
About Hydrangea (mophead)
Mophead hydrangeas are the plants behind the familiar claim that flower colour reflects soil pH, and the mechanism is more specific than that suggests: acidity governs how available aluminium is in the soil, and it is aluminium uptake that turns the flowers blue. Without aluminium present, acid soil alone will not do it, and white varieties will not change colour at all. The showy parts are sterile bracts rather than true petals. Most mopheads flower on wood formed the previous year, which is why an over-enthusiastic tidy-up removes the coming season's display.
Growing notes
- Wilts quickly in afternoon sun and dry conditions
- Cut back in spring, not autumn — stems carry next year's buds
Pet caution: Hydrangea (mophead) is listed as potentially harmful to cats and/or dogs. Keep pets from grazing on it, and contact a vet if you suspect your animal has eaten some.
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