Keep shrubs healthy
Q
Why did blue flowers go pink? CAROL TADMAN, OUNDLE
A
Hydrangea flowers only turn blue if plants absorb aluminium from the soil. Once the blue pigments have formed, they won’t change, so blue flowers stay blue for the season. However, in alkaline soils, the plant will go on to produce pink flowers the following year. To keep plants producing blue flowers, apply aluminium sulphate in the form of hydrangea colourant before the buds start to form. If you only add a small amount, the flowers may turn mauve or purple.
Q
How do I prune huge old plants? EMMA ANDERSON, CORNWALL
A
It’s difficult to reduce the size of the plant significantly without affecting flowering. But you can prune large, old hydrangeas in spring, cutting out a few of the old stems near the base to thin out the growth and allow the new shoots more space. You could also renew the plant by cutting back all shoots to near ground level in March, which will result in a mass of new growth, but you won’t enjoy any flowers for a year.
Q
Is it easy to keep hydrangeas in pots? JULIA TURNER, ALCESTER, WARWICKSHIRE
A
Hydrangeas are good plants for patio pots and will grow and flower well in sun or part shade. It’s best to use a loam-based compost but John Innes No3 contains lime, which will make the flowers pink. For blue flowers, it’s best to use ericaceous compost. Watering is very important – hydrangeas suffer if they’re allowed to dry out – and they need regular feeding, either weekly with a liquid feed from March to October, or with a controlled-release fertiliser applied each spring.
Q
Are the pink spots on my white hydrangea caused by alkaline soil? NICOLA DAVIES, CARDIFF
A
White hydrangeas aren’t affected by soil acidity, though the tiny, true flowers in the centre of the bracts may be tinged pink or blue according to soil pH. But in full sun, white hydrangeas tend to discolour and develop pink tones, so it’s best to grow them in part shade. The petals also turn red as they age – something that’s more obvious on white flowers than pink or blue ones.
Q
What’s caused leaf edges on my healthy plants to turn brown? ANITA WATSON, HENLEY, OXON
A
Although mopheads (Hydrangea macrophylla) are fairly hardy, their new growth is easily damaged by frost. The old stems aren’t affected by a few degrees of frost but if there’s a late, sharp frost, as we had last April, it can cause browning of the leaf tips or even kill new growth and flower buds. This rarely kills shrubs but can reduce growth and flowering for a season.