How to grow hydrangeas

Hydrangeas are one of the few shrubs that can put on a truly spectacular display at the height of summer and beyond. The range of flower shapes and colours is now more diverse than ever, and, excitingly, new mophead and lace-cap varieties not only look more modern but many now flower on both old and new wood. The older ones only flowered on previous year’s growth. 

This revolution in breeding means they should flower for longer, as they’ll bloom first on last year’s wood and then later on this year’s wood. Also, hard frosts or overzealous spring pruning won’t lose a whole year’s worth of blooms. 

Which? Gardening magazine grew a range of popular varieties to see which would give us the best display.

How to grow hydrangeas: month by month

Best hydrangea varieties

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How we test hydrangeas

After planting, we kept them watered until they were established and then watered them during dry spells. We grew them for two years, assessing the plants regularly for duration and impact of flowering; colour on our slightly acidic soil; plant shape and the impact of the foliage; plants’ susceptibility to common problems

Caring for your plants

Planting

Soil needs to be moisture-retentive, so add plenty of well-rotted compost when planting and mulch with compost annually in spring.

It’s best to plant in partial shade, although hydrangeas will do well in cooler morning sun if they have shade in the hottest part of the day.

Watering 

Plants will soon wilt if the soil is dry, so be prepared to water if they do start to dry out, especially when they droop. Water well occasionally, rather than little and often.

Deadheading

Leave flower heads on in winter to protect the buds, pruning in early spring when leaves appear.

Pruning

It’s important to stand still and study your hydrangeas before diving in: look at what’s flowered, what hasn’t; what’s old, what’s new; what’s strong and what’s weak.

Pruning generally falls into two groups, the first being for those hydrangeas that flower on fresh growth produced in the current year. These include H. paniculata, where last year’s stems can be taken back to one or two sets of buds above older growth. The taller you want your plant, the higher you need to prune. Now and again old stems can be taken right back to regenerate new growth from the framework. H. arborescens also flowers on the current season’s wood. Here the majority of stems can be cut right down to the ground, leaving only a few of the thickest and strongest stems shortened to just 10-15cm.

The second group includes H. macrophylla and H. serrata, which both flower best on terminal buds produced last year. If all the shoots are pruned, you’ll have no flowers. So, take out all the weak, flowered stems first, cutting right down to the ground or to the next strong non-flowered shoot (flowered stems have last year’s old flower heads on them).

If there are plenty of strong, non-flowered stems with terminal buds left on the plant, then take out all the remaining flowered wood. Then thin out the non-flowered stems, removing the weakest and congested, leaving an open and well-balanced plant. If there aren’t enough non-flowered stems then you will need to retain some of the strongest flowered wood, pruning these back to the first set of strong buds.

Blue hydrangeas

If you’re buying a new hydrangea that you want to have blue flowers, choose a mophead type that changes colour such as ‘Endless Summer’. Other types either retain the colour in their blooms completely or will only deepen or fade their existing colour.

If your soil pH is acid (less than pH 5.5 or so), hydrangea flowers will be blue or more likely to turn blue. Most soils in the north and west of the UK are acidic, whereas the Midlands, East Anglia and the South East, excluding London, tend to be slightly alkaline due to the chalkiness of the soil, and that will keep hydrangeas a resolute bright pink.  

It’s easier to control the soil pH and salts by keeping the plant in a pot, but if you want to plant in the ground it will be worth testing the soil to check the pH. In our previous tests, we’ve found that garden soil pH testing kits are generally accurate and are cheap; well worth it to see if the spot you’re planning to plant in will allow you to achieve the colour you want. It might be worth testing a few potential sites as the pH can vary across your garden. If necessary you can prepare the soil by adding a hydrangea colourant.

Common growing problems

Frost damage

Frost can burn the leaves in spring. Don't worry if this happens as the plant will recover. You can tidy up the damaged growth once the danger of frost has passed.

Hydrangea scale

Hydrangea scale is thought to be on the increase. This pest is hard to see until the wax-covered white eggs are laid on the stems and undersides of leaves in early summer. You can try to remove the eggs if there aren’t too many. Spraying with a contact insecticide such as Best Buy Bayer Natria Bug Control can also help. This should be done in July when the nymphs have hatched.

Read more about scale insects.