How to choose a Hydrangea

Hydrangeas are some of the most recognisable of all garden plants. They are deservedly popular for their showy flowers in summer, but with so many cultivars available just how do you choose the right one?

Firstly a little about Hydrangea flowers. Hydrangea flowers are themselves tiny and insignificant but are surrounded by sterile florets, and it is these that we see as 'flowers' in the flowerhead. Typical Hydrangeas fall into three groups, the mopheads with their dense domed heads of florets, the lacecaps with their tiny flowers surrounded by florets in typically flat heads, and the H. paniculata varieties with upright panicles of flowers. There are thousands of different Hydrangeas available, so choosing a variety can be difficult. I offer the following as advice.

Firstly avoid Hydrangea macrophylla 'Merveille Sanguine'. It's a popular old French variety and is found in many gardening books. The biggest appeal of this variety is its dark red-tinted leaves, but I'll be blunt and say that it's just not that good a variety. I've never seen a plant that is growing spectacularly well; typically this variety gets very sparse and woody over the years, despite careful pruning, and gets rather tatty and untidy. This, coupled with the fact that it doesn't seem to be particularly hardy, makes me happy to turn discerning gardeners away from this variety, despite its popularity.

H. 'Merveille Sanguine'-looks nice, but leave it at the garden centre!

Dusky leaves of H. 'Merveille Sanguine'

Thankfully if you want Hydrangeas with dark leaves there are alternatives, although they are lacecap varieties whereas H. 'Merveille Sanguine' is a mophead. I have two favourites that I am happy to recommend. H. 'Selina' is a fairly new variety and has beautiful deep burgundy leaves (especially in full sun) and large heads of pink/blue flowers (depending on soil type- see below).

Hydrangea 'Selina'

Deep burgundy leaves of H. 'Selina'

H. 'Dolce Kiss' (Dolkiss) has similarly good deep burgundy leaves to 'Selina' but has white flowers edged pink. I particularly like this variety for the two-tone effect of the flowers and rate this very highly. The flowers of this variety are very similar to H. serrata 'Kiyosumi', an old Japanese variety (I think a wild form), and I'd be very surprised if the latter wasn't a parent of H. 'Dolce Kiss'!

Leaves of H. 'Dolce kiss'

H. 'Dolce Kiss'- love these flowers!

My second piece of advice is to be careful what you buy. Garden centres will try to entice you to buy a nice Hydrangea full of flower but beware! Many garden centres sell Hydrangeas that have been treated with a product called Cycocel, which is a growth retardant. Plants are sprayed with this hormone product which shortens the spaces between leaves (internodes), making the plant more compact but not affecting flowering (or even enhancing flowering). This product is used in the mass production of Hydrangeas, Rhododendrons and sundry other plants to make those attractive dome shaped plants covered in flowers that are so appealing to garden centre shoppers. Resist! Although a Cycocel treated plant looks nice when you buy it, the following year it will behave differently, making longer internodes and flowering more typically, and over the next few years will turn into a rather ordinary Hydrangea, losing the appeal it had when you bought it. I strongly recommend that you go to a nursery and buy a Hydrangea based on the merits of the variety rather than buying what looks pretty at the garden centre. Buy a Hydrangea with strong stems (it might even strike you as looking a little leggy in the pot) and big healthy leaves, but don't expect more than a few flowerheads; you're buying a plant to look spectacular in your garden, not on the nursery! Ask about varieties, get advice about growing Hydrangeas in your area, and make an informed choice. By keeping away from hormone treated plants and choosing a variety carefully you will be buying a plant that will perform better in years to come.

While I'm recommending varieties I'd like to show you a few others that I like. H. 'Zorro' is an improvement on the old variety H. macrophylla 'Nigra', a variety grown principally for its black stems. H. 'Zorro' is a stronger variety, and produces large flower heads of pink or blue, depending on soil type (again, see below).

Hydrangea 'Zorro'- note the black stems!

Another favourite is H. 'Rotschwanz', a variety with rich deep red flowers on alkaline soils. This is a fairly compact variety and well worth growing.

Hydrangea 'Rotschwanz'

Possibly the best known fact about Hydrangeas is that their flowers vary in colour according to the pH of the soil they are growing in; pink alkaline or neutral soils, blue in acidic soils. Although this is correct it's not the pH of the soil that truly affects the colours but the amount of aluminium in the soil. If you have an alkaline soil and want your Hydrangea to turn blue you can add as many old iron nails, pine needles, tea bags etc. as you like but they won't have any effect. Add aluminium, for example by using a 'Hydrangea colourant', and your plant will, after a few applications, produce blue florets. It is worth noting though that it is a lot easier to turn a pink Hydrangea blue than to turn a blue Hydrangea pink; if the aluminium content of the soil is sufficient to make the Hydrangea flowers blue then it is impossible to stop the plant taking it up. Your best option if you wanted a pink Hydrangea but have acidic soil would be to grow your plant in a pot, but Hydrangeas are never really happy in pots so your plant would struggle. White flowered Hydrangeas are not affected by pH, and it is only H. macrophylla and H. serrata cultivars that change, so none of the H. paniculata varieties or any of the climbers are affected.

Hydrangea 'Zorro' changing from blue to pink

As important as pH is to the flower colour it is worth talking generally about soil and cultivation. Hydrangeas enjoy a deep, rich soil which retains enough water for them in summer while still draining in winter; Hydrangeas in full growth can get quite thirsty! In hot areas it's worth planting them with some shade from the sun during the hottest part of the day to help stop them drying out and to preserve the colour of their flowers and stop them fading. Most Hydrangeas are hardy, but in some areas H. macrophylla and varieties can suffer from cold winters, and as we've seen some varieties (like H. macrophylla 'Merveille Sanguine') can be more tender than others. If in doubt H. serrata varieties tend to be hardier. I'm not really sure where hybrids between H. serrata and H. macrophylla (such as H. 'Selina') are with hardiness; only time will tell. Hydrangeas tend to resent being in pots for any length of time. 

Hydrangea 'Mathilda Gutges' flowering happily