Obsidian Storm Cloud Hydrangea

Obsidian Storm Cloud Hydrangea (Hydrangea macrophylla 'Obsidian Storm Cloud') is a mophead grown as much for its stems and leaves as its flowers. Near black stems carry dark, deeply toned foliage, and the large flower heads come through blue to purple depending on your soil. It settles at about 1.2m tall and wide, which keeps it useful in a small garden or a generous pot.

Why you'll love it

• Dark stems and dark foliage. The near black framework gives it presence before the flowers open, which most hydrangeas cannot claim.
• Large mophead flowers. Full rounded heads in blue through purple, held clear of the foliage.
• You can steer the colour. Acidic soil pushes it blue, alkaline soil pushes it pink, and you control that from the bed.
• It stays in bounds. At 1.2m by 1.2m it fits a courtyard or a border without the annual wrestle a full sized hydrangea demands.
• It works in shade. Part shade is its preference, not a compromise, so it fills a position most flowering shrubs refuse.

Where it works best

Feature planting in shade. A single plant under a high canopy, or on the south side of a house, gives flower colour where the garden is usually all green.

Pots and courtyards. A 400mm pot suits one plant, and container growing makes soil pH easy to manage, so you can decide on blue or pink and hold it there.

Borders in small gardens. Spaced about 1m apart the plants meet without crowding, giving a massed effect along a shaded path.

Growing hydrangeas in Australia

Position and climate. Morning sun with afternoon shade is the target, because hot afternoon sun scorches the leaf margins and collapses the flower heads. It suits subtropical, warm temperate and cool temperate gardens, handles humidity, and is frost hardy once established.

Soil. Rich, moisture retentive and free draining, with plenty of compost. For blue and purple heads keep the soil acidic, around pH 5.0 to 5.5, using an acidifying fertiliser or aluminium sulphate. Add lime instead if you want pink.

Water and feed. This is a thirsty plant. Water deeply two or three times a week through summer, more in a pot, and mulch 75mm deep. Feed in early spring and again after the first flush with a fertiliser matched to the colour you want.

Pruning. Cut spent heads back to the first pair of fat buds in late winter, and take out one or two of the oldest stems at ground level each year. Growth is slow, so do not cut harder.

What to expect when it arrives

It comes in three sizes: 200mm in packs of six, 250mm in packs of three, and single 300mm pots. All three are currently supplied on backorder against the next crop, so expect a wait rather than immediate despatch. The 300mm plants are the largest and closest to flowering size.

Hydrangeas are deciduous, so a plant sent in winter may arrive as bare stems with visible buds. That is normal, not a fault. Growth is slow, and a plant usually takes two seasons to look established. Delivered Australia wide, with delivery from $29.95 and a minimum order of $300 before delivery. Every plant carries our 30 day healthy plant guarantee.

Plant care guides and related reading

• How hydrangea colour works, and how to change it
• Browse the full hydrangea range
• Lowering soil pH to hold blue flowers
• More plants for partial shade

Common questions

How do I keep the flowers blue instead of pink?

Keep the soil acidic, around pH 5.0 to 5.5. Aluminium sulphate applied a few times through the growing season is the usual method, and a pot makes that far easier than acidifying a whole bed.

Will it grow in full sun?

Not well. The leaves burn at the edges and the flower heads wilt through the middle of the day. Give it morning sun and afternoon shade, or dappled light under a high canopy.

Is it safe around cats and dogs?

No. Hydrangea leaves, buds and flowers are toxic to cats and dogs if eaten, causing vomiting and stomach upset. Plant it out of reach of a pet that chews foliage.