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21 September 2015

Growing Hydrangeas

My wife has been "encouraging" me to grow some hydrangeas in our back garden for a few years, and I am now on the verge of having a go. I must admit that they always look attractive when covered in flowers in other peoples' gardens. Although there are other species, the ones usually seen in gardens are Hydrangea Macrophylla.

Hydrangeas can vary in size from a small shrub, 80cm by 90cms, to a very large shrub, 7m by 2.5m. Climbing hydrangeas can reach 15m in height.

They are happiest in a moist, but well-drained soil, in a cool, semi-shady part of the garden. Exposed east-facing sites, where cold winds may damage young spring growth, should be avoided, as should dry, sunny spots. Dig in plenty of organic material before planting as they hate to be too dry.

There are two types - mopheads and lacecaps - the difference being in the way that the flower heads are formed. Mopheads have domed flower heads made up almost entirely of large sterile sepals. Lacecap florets, however, are flatter and have a central section of tiny, fertile florets made up of both male and female flowers, which in turn are surrounded by the infertile sepals.

Mophead florets vary in colour according to the pH of the soil in which they are grown. An alkaline, chalky soil, will lead to flowers that are predominately pink, whereas a very acidic soil will result in blue flowers.

When planted in largely neutral soils - a pH of 6.5 - you will get a lovely pastel colouring, ranging from soft pinks to lavenders, mauves and true blue.

Burying old metal with a hydrangea is said to encourage pink hydrangeas to produce blue flowers, but this is not true. However, mulching annually with an ericaceous material, such as composted pine needles or bracken, will encourage a blue one to retain its hue if the soil is otherwise neutral.

Hydrangeas do not need pruning but they will need cutting back every so often. As with all pruning, it is important to understand what to do and when to do it.

Mopheads form flowers in spring from buds that formed in late summer, so if you prune in autumn or spring, you will ruin your chances of having flowers that year. Spent flowers form an important protective layer for emerging buds, so should be left in place until spring, when the chance of frost has passed. A hard frost in May can wipe out a year's flowers on even the healthiest mophead.

When you do prune, cut back the old flower heads to the first pair of healthy buds below them. Remove old, crossing or dead wood, cutting it right back to the base of the shrub.

Lacecaps can be treated the same way as mopheads, though as they are hardier they can be pruned in autumn when flowering has finished.

If you grow hydrangeas in pots, it is a good idea to protect them with horticultural fleece, if a late frost is forecast. This will keep them safe from the frost and from the even more damaging early morning sunshine that often follows.

Work plenty of organic matter into the soil prior to planting, and then after planting and in subsequent years apply organic matter as mulch. Well-rotted leafmould, garden compost, farmyard manure or composted bark are all suitable.

Hydrangeas can be fed annually if necessary with fertiliser in late winter or spring. This applies mainly to plants growing in lighter, sandier soils - it shouldn't be required on richer soils. Too much feeding can encourage excessive soft, leafy growth, with plants less likely to develop flower buds and more at risk from frost damage in colder winters.

If you fancy some propogating, Hydrangeas are easy to grow from softwood, semi-ripe or hardwood cuttings, though the climbing types are best layered. Large clumps of Hydrangea macrophylla cultivars often lend themselves to division or layering.

Species hydrangeas can also be propagated from seed in spring.

There will be more Hydrangea information in my next article.
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