Get the Best From Mophead and Lacecap Hydrangeas Every Year

From mid to late summer lovers of Hydrangea macrophylla can marvel at this popular mid to late summer flowering shrub with a visit to The Lakeland Horticultural Societies garden at Holehird, near Windermere. Here the National Collection contains 150 cultivars of this the most popular species of hydrangea in the full colour range of blue, mauve, pink, red and white.

Mophead and Lacecap Hydrangeas

Cultivars of H. macrophylla are divided into two groups. “Lacewings” which have flattened flowers consisting of small central fertile flowers and large sterile outer ones are derived from the wild species H. macrophylla var. normalis. The more popular “mopheads” have spherical flowerheads with large sterile flowers.

Some of the first hydrangeas to reach Europe from Japan were the mopheads with their blue or pink blooms. They became popular as pot plants in the 19th century, but now usually grown as hardy shrubs and are well known for their unique ability to change colour in different soils.

How to Make Pink Hydrangea Flowers Turn Blue

This is one of the most frequently raised queries at gardeners’ question times:-

• Flower colour is determined by soil acidity and the presence of aluminium in the soil.
• Those cultivars with blue or pink flowers tend to be blue in acid soils due to high available aluminium levels and pink where soil conditions are alkaline.
• Between the two extremes they tend to be a shade of mauve.
• White and green-flowered cultivars are unaffected by soil pH.
• To keep cultivars with blue flowers blue the soil needs to have a pH of 4.5 to 5.0.
• Soils with a pH higher than this can be treated with hydrangea blueing compounds containing aluminium sulphate which are obtainable from garden centres. They work quickly, but used to excess can interfere with phosphorus levels in the soil and when used repeatedly can cause aluminium in the soil to rise to toxic levels.
• Don’t bother trying this treatment on very alkaline soils, because it won’t work.
• Where the soil is very alkaline or contains free chalk the the only effective solution is to grow blue flowered cultivars in containers.
• To enhance the colour of pink and red flowered cultivars apply ground limestone at 75-100g per sq metre in winter.
• In areas where the tap water is hard its use will raise the pH of the soil and so hydrangeas should only be watered with rainwater.

Correct Way to Pruning Hydrangea Macrophylla

Although these hydrangeas will perform adequately with the minimum amount of attention regular pruning enhances flower quality. Conversely incorrect pruning will result in shrubs not flowering. Very little formative pruning is required. The following guidelines refer to established shrubs:-

• Flowers are produced on the previous season’s wood and the old flowerheads are best left on overwinter as they protect the new growth beneath them.
• Pruning is best done in early to mid spring.
• Cut back weak, thin shoots to ground level
• Prune out one or two of the oldest shoots to the base of the plant.
• Branches which flowered last year should be pruned back by about a third to fat buds; these will produce flowers.
• Old, neglected bushes may be rejuvenated by cutting the whole plant back to the base in spring, but they will not flower the following summer.

Providing the Ideal Conditions for Hydrangeas

All hydrangeas are woodland plants in their natural habitat. They are most at home in a moist, but well-drained soil preferring a cool semi-shaded position. Cold winter are likely to damage soft, spring growth and dry sunny spots are best avoided. Great benefits will be had by the incorporation of plenty of organic matter into the soil prior to planting with regular additions as a mulch in future years.