The best-known varieties are the mophead hydrangeas (Hydrangea macrophylla), which are deciduous shrubs growing to around 1-2 metres in height and bearing large rounded flower heads. Lacecap hydrangeas are a smaller group (belonging to the same species), which have more flattened flower heads, where the showy coloured petals form a ring around a cluster of tiny flowers in the centre.
Hydrangea paniculata, which is native to China and Japan, produces large panicles of flowers on a robust shrub, growing to around 4m in height and is more tolerant of sun. New forms include ‘Candlelight’ and Diamond Rouge’, both of which will grow in full sun or part shade.
The oak-leaf hydrangea (Hydrangea quercifolia) is another lovely form, which is native to the USA. It features deeply-lobed leaves, which turn fabulous red shades in autumn, and produces creamy white, cone-shaped flowers in summer.
For sunny areas of the garden, choose forms of Hydrangea paniculata which are more sun-tolerant. ‘Candlelight’ bears creamy white flowers which turn pink as they age; ‘Diamond Rouge’ produces white flowers in summer, which gradually morph through pink to rich red.
For hydrangea blooms with a twist, seek out some of the double-flowered forms. The You & Me Series, from PMA, features a range of gorgeous double-flowered cultivars, including ‘Desire’, ‘Forever’ and ‘Romance’.
Long flowering hydrangeas
One of the newest hydrangeas to appear on the market is ‘Magical Revolution’, which produces classic mophead-style flowers with a unique quality - they last for up to 150 days! Opening in spring, the blooms go through a colour transformation from pastel blue or pink, through pinky-red shades and onto deep burgundy by autumn. They’re also compact growing plants, so are perfect for pots and small gardens too.
The Endless Summer range of hydrangeas also offers a long flowering season in the garden, since the plants flower on both old and new wood. The plants can be pruned back all over and will bloom again in about 8 – 9 weeks.
Apart from the white flowering varieties (which always stay white), hydrangeas – such as Hydrangea macrophylla varieties – are known for their ability to change colour, producing blue flowers in acidic soils (below pH 7) and pink in alkaline soils (above pH 7). You can manipulate the look of your hydrangeas to suit, but you'll need to add lime (to make the soil alkaline) or aluminium sulphate (to make the soil acidic) to the soil before buds form.
If you’re desperate to change the colour of your plant, there are bluing or pinking tonics available for the purpose. However, the best advice is to just enjoy the flower colour your soil produces, rather than trying to radically alter its pH level – which might also have a negative effect on the other plants growing nearby.
Alternatively, consider the option of growing your hydrangea in a pot. That way, you’ll have total control of the soil conditions. The flowers of white hydrangeas are not affected by the pH of the soil.
How to grow hydrangeas
Hydrangeas can be planted at almost any time of year, except when the ground is frozen in winter. Plant your hydrangeas at the depth they're at in their pot. Be sure to look at the guidelines on the plant tag to be sure that you space the plants properly.
Some hydrangea varieties, like oakleaf hydrangea, get quite large, while others are bred to be dwarves. When it comes to hydrangea soil, be sure that you're planting your shrub in a pourous, moist soil.
Mophead varieties of hydrangea grow best in filtered shade, or else a combination of sun in the morning and shade in the afternoon. They won’t thrive in heavy all-day shade, especially where there is no air movement around their leaves, as this tends to lead to problems like powdery mildew on the foliage. Hydrangea quercifolia and H. paniculata both tolerate more sun – just avoid positions which are blasted by hot westerly afternoon sun in summer.
Hydrangeas thrive in well-draining, organically rich soil, so dig in additional compost and cow manure at planting. They also love moisture, so add some water-retaining crystals to the soil as you backfill. After planting, mulch the surface of the soil to a depth of about 3- 5cm.
Feed hydrangeas in early spring, with a dose of controlled-release fertiliser or manure pellets. This will give them plenty of nutrients to draw on while forming the flowers. Choose a fertiliser low in nitrogen, as too much of it encourages leaf growth to happen at the expense of your blooms. Phosphorous stimulates flowering and potassium improves the quality of your flowers. Top up the mulch layer in late spring or early summer, and once the plants are in bloom, feed them with occasional doses of a soluble fertiliser, using a flower and fruit formulation.
Hydrangeas are thirsty plants so properly watering them is crucial, the prefix of the plant's name, hydra, even indicates so! Even a day or two without water can affect your hydrangeas, so be sure to water them often. If you notice that your hydrangea shrub is wilting, give it a good dose of water and it should perk back up.
Nasties are usually minimal for this plant. White-coated hydrangea scale may appear and can be picked off, but if the infestation is serious, treat with a product such as Eco-oil.
How to grow from cuttings
Hydrangeas are easy to grow from cuttings taken during the spring or summer months. Take tip cuttings about 10 – 15cm long, making the cut just below a pair of leaves. Remove this bottom pair of leaves, then cut the remaining leaves in half, to reduce water loss. Plant the cuttings into pots filled with propagating mix and place in a shady but bright spot. Don’t let the soil dry out, keeping it just damp but not waterlogged.
How to prune hydrangeas
Many gardeners are hesitant to prune hydrangeas because they think it is counterproductive to get rid of the blooms, but that couldn't be further from the truth. Pruning your hydrangea blooms will only help your shrub's growth for next year.
Pruning can be done anytime after flowering finishes, although you can leave it until mid-winter if this suits. Only trim back the stems that flowered, making the cuts just above a plump pair of buds.
When pruning a hydrangea, it's best to take off no more than one-third of the plant at any one time. Your objective also determines how you prune it. If you are just doing some shaping of plants that are too tall, you want to take the top growth down a little. If you need to do a severe pruning or rejuvenate the plant, you may want to take the branches all the way down to near the ground.
Pruning Pee Gee hydrangea
Pee Gee hydrangea (Hydrangea paniculata, which includes varieties such as 'Limelight') as well as smooth hydrangea (Hydrangea arborescens, also called seven bark and best known for the variety 'Annabelle') bloom on new wood. Timing for the pruning of these types of hydrangea is not as critical—you can prune in winter or early spring and they will grow and flower in the same season.
Pruning big leaf hydrangea
Big leaf or mophead hydrangea (Hydrangea macrophylla) and oakleaf hydrangea (Hydrangea quercifolia) both bloom on the previous season's growth. If you prune these at the wrong time of the year, they either won't flower or will flower sporadically. They should be pruned soon after flowering in summer, and never too severely—less than one-third of the plant at any one time.
Some newer varieties of big leaf hydrangea, including the popular 'Endless Summer', have been bred to bloom on new wood as well as old wood. This makes them better for cold climates because even if the old stems suffer cold damage, new growth will still bloom. This also provides more flexibility in pruning, since you can prune it at any time of the year and it should still bloom. Even so, pruning right after bloom will maximise flowering.
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