How to Grow Smooth Hydrangea
Hydrangea arborescens
Smooth hydrangea is a very hardy deciduous shrub with big rounded heads of white flowers from early summer. It flowers on the current season's shoots, so you can cut it back hard every spring without losing that year's flowers. Grow it in moist but well-drained soil in partial shade, or in sun where the soil stays moist. Soil pH does not change the flower colour. The heavy heads can bend the stems after rain, so a support ring helps.
In your Leaftide garden 
- Height
- 150 cm
- Spread
- 150 cm
- Flowers
- June - September
- Light
- Full sun to part shade
- Water
- Moderate
- Feed
- Rarely needed
- Prune
- Hard prune in early spring
Yearly Lifecycle
Care Essentials
Established plants in reasonable soil rarely need feeding. On light, sandy soil or if growth is weak, scatter a general fertiliser around the plant in spring after pruning.
Watch For
- Aphids on soft new growth
- Capsid bug damage to young leaves
- Hydrangea scale on stems
- Powdery mildew in dry spells
- Grey mould on flower heads in wet weather
Companions
Hostas, Ferns, Astilbe, Japanese anemones
Pet Safety
Care Requirements
Light
Partial shade, or sun if the soil stays moist
Grows best in light or partial shade. It takes full sun where the soil stays reliably moist, but in hot, dry spots the leaves scorch and the plant wilts.
Watering
Keep the soil evenly moist; water in dry spells
In the first growing season, water whenever there has been no significant rain for 7-10 days. Established plants still need water in hot, dry weather. Mulch helps the soil hold moisture.
Fertilizing
Rarely needed; a general feed in spring on poor soil
Established plants in reasonable soil need no regular feeding, and too much fertiliser gives soft, leafy growth. On light, sandy soils or if growth is weak, apply a general fertiliser in spring after pruning.
Pruning
Hard prune in early spring; flowers on new growth
In early spring, cut last year's stems back to the lowest pair of healthy buds, leaving a low woody framework about 25 cm high (up to 60 cm for a taller shrub). Remove dead, thin and weak stems at the base. Neglected plants respond well to the same hard pruning.
Spacing
100-150 cm apart
Space plants 100-150 cm apart, or about 1 m for a massed planting or informal hedge. The clump widens slowly with new stems from the base.
Support
A support ring for the heavy flower heads
Large heads can pull the stems over after rain. Put a support ring, or canes and twine, around the clump in spring so the new stems grow up through it. Pruning to a low framework also gives sturdier stems.
Growing Tips
Prune hard in early spring
Smooth hydrangea flowers on shoots that grow this year. In early spring, before growth starts, cut last year's stems back to the lowest pair of healthy buds, leaving a low framework about 25 cm high (or up to 60 cm for a taller shrub). Hard pruning gives bigger heads on stronger stems.
Support the flower heads
Annabelle's heads reach 25 cm across and get heavy after rain, which can bend the stems to the ground. Put a support ring, or a few canes and twine, around the clump in spring before the heads form. Incrediball was bred for stiffer stems and flops less.
Keep the soil moist
Smooth hydrangea wilts quickly when the soil dries out, especially in sun. Water in dry spells and keep a layer of mulch over the roots.
Soil pH does not change the colour
White smooth hydrangeas stay white on any soil, so you need no ericaceous feed or blueing compound. The heads open lime green, turn white and fade back to green as they age.
Popular Varieties
Annabelle
The classic smooth hydrangea and an RHS Award of Garden Merit plant. Rounded heads of white flowers up to 25 cm across open lime green in early summer. Grows to about 1.5 m; the heavy heads can bend the stems after rain, so give it support.
Incrediball
An arborescens hydrangea producing enormous white snowball flower heads up to 30cm across on strong stems that resist flopping. Extremely cold hardy.