Hydrangeas – masters of chemistry

Lacecaps and mopheads, cultivars of Hydrangea macrophylla, also known as Big Leaf Hydrangeas, are popular shrubs that have been around for decades and over the years become stalwarts of British summer gardens. Even bright summer bedding plants can’t compete with the eye-catching neon blue or pink flowers of the Mopheads pom-pom like flower heads.

These giant blooms, which are often as big as your hat, are in fact, a mass of petal-like bracts and well known for their ability to curiously change colour in different soils.The colour change is due to the soil pH or acidity, which affects the availability of aluminium, so using their in-built ‘chemistry set’ they determine their own flower colour!

Hydrangeas with blue or pink flowers tend to be blue in acid soil conditions, mauve in acid to neutral soil conditions, and pink in alkaline or limy conditions

To get the best flower colour, you must therefore choose varieties that are known to be reliable for your soil or be prepared to use Vitax Hydrangea Colourant to boost the blue shades. This chemical is added to the soil when planting or to the compost in a pot. It can also be diluted in water and watered on to established plants. This should maintain the colour, but be patient. It does not always work instantly.

Only the white flowers are colour fast regardless of soil type. The variety Madame Emile Moulliere is the best ‘white’ but needs shade for the brightest blooms, which unfortunately tints to pink as they age. Another noteworthy white is Zebra, which has near black stems and rich green leaves that are a great contrast to the large white flowers

The variety Amethyst is one of the best-loved varieties and produces large frilled double flowers of pink or mauve on a plant up to 75cm tall over several weeks. One of the best ‘blues’ is Europa, which is slightly tall at 90cm and has the knack of turning pink when there’s a hint of lime at its roots. The variety Parsifal can be either blue or pink, or in neutral soils, a curious mix on all colours in between.

You also might like the new variety Rembrandt Vibrant Verde, which has green mop head flowers that change colour throughout the season. And the two unusual Green Lips varieties, which are a bi-coloured hydrangea that has pink and green or purple green blooms

Magical hydrangeas are marketed as Four Season Plants as they have four different colour changes throughout the growing season and will reliably flower for up 150 days. For example, the flowers on the variety Ruby Tuesday open green then develop a tinge of red before becoming pale pink to mid to deep intense red in autumn.And by the start of autumn the clear leaves are gloriously tinted.

If you prefer blue flowers, then Coral Blue is the variety for you. The flowers start off the season pale green then turn a deeper shade of green-purple before morphing to a beautiful blue-lilac shade prior to making its final change to dark autumnal green. All this ‘activity’ makes them useful for pots to decorate your doorstep where they’ll turn the heads of passers-by!

The lacecaps varieties have just as much going for them, but the flowers are subtle, with flatter flower heads that resemble pinwheels. They are made up of showy sterile flowers in a round a centre of fertile, bead-like flowers. These are more graceful shrubs than the mop heads and fit quite naturally into woodland setting.

The pH of the soil affects their flower colour too. You’ll make an impressive display using the popular variety Mariesii Perfecta, which in light shade can be rich blue, mauve or lilac blue, or pale pink if the soil is limy. It’s a strong growing plant, so best left to make a large shrub.

The flowers of Mariessa Perfecta are a dark pink but in acidic soil they will change to a beautiful blue colour

Another interesting variety is ‘Quadricolor’, which has variegated leaves in pale and dark green with white and lemon yellow markings. It makes an elegant specimen up to 1.5m tall when covered with its pale mauve-pink flowers, which are a deepershade on acid soil, from June to November.

Another impressive chameleon varieties is Glam Rock ‘Horwack’ whose flowers are constantly changing colour

New last year was the variety Lady Mata Hari, which produces dark coloured and lighter flowers in one head. New this year is the spectacular variety Runaway Bride called Snow White, which produces six times more blooms than the average hydrangea. It was named the Plant of the Year at this year’s Chelsea Flower Show. The lacecap flowers, which are flushed pink, appear from late spring/early summer until well into autumn.

Hydrangea Runaway Bride ‘Snow White’ is available from www.thompson-morgan.com, from £13.99

Hydrangeas essential requirement is water, which they need in copious amounts for them to be able to maintain those fleshy stems and big leaves. As heavy drinkers they will only thrive in rich, moist soil that has been enriched with well-rotted garden compost or manure rather than fertilizer when planting and then mulched annually.

You can preserve the flowers for life, by simply arranging them in a vase, with or without water, and leaving them to dry. It is not necessary to hang hydrangeas up side down to dry unless the stems are very thin and weak. If however, you prefer to retain the natural hydrangea colour, you must use Silica Gel to dry the fresh blooms

There is one draw back of both types of hydrangea and that is, they produce flowers from the topmost bud of each stem, and if that is damaged in winter or if pruned at the wrong time, they will not flower.

The golden rule of “if in doubt – don’t” should therefore be applied when it comes to pruning. You can cut out deadwood at any time of the year, but if light pruning is necessary to reduce the size or shape the shrub, stems must be cut back in June or July, after the shrub blooms but before flower buds form during late summer for the following year. Bear in mind too that the new stems will not necessarily produce flowers for a couple of years!

In cold regions it’s best to leave the flower heads on Macrophylla hydrangeas to protect the buds over winter, then cut back each stem to the first pair of fat buds behind the flower in spring

Paniculata hydrangeas are best pruned hard in late winter, cutting back by half to two-thirds to retain a strong branch framework at the base of the plant. Doing this produces vigorous shoots with flowers at the tips.

After pruning, or at any time in the growing season, feed by sprinkling Vitax Hydrangea Feed around the plants, work it into the soil surface with a fork or hoe and water thoroughly.

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