Diarmuid Gavin: We are a nation obsessed with hydrangeas – here’s everything you need to know about growing them

The main questions involve when to prune and how to change the colour of a flower. This interest underlies the great love we have for this shrub. In fact, if I’m feeling unloved on my Instagram, there is one picture that I post of hydrangeas in a garden in the west coast of Ireland and I’m guaranteed thousands of likes.

Even though there’s not much happening in hydrangea land just now, it is the time to act. Mophead hydrangeas, with their full round heads of large petals, are the most commonly planted hydrangeas, and often the temptation is to tidy them up as soon as they finish flowering in late summer. It’s best to wait until spring to do this as the dead flowerheads can provide protection to the tender buds forming below. In spring, remove the brown flowerheads and cut just above the first pair of buds below.

Lacecaps are the other commonly planted hydrangea — aptly named for the circle of large petals around a flat cap-like flowerhead. These can be deadheaded when the flowers go over in late summer.

With established mopheads and lacecaps, you can also remove one or two stems right back to base to encourage fresh growth and, by doing this every year, you keep the plant producing new stems while also getting lots of flowers from the existing stems. However, if your shrub has become very straggly and you want to knock it back into shape, you can hard-prune the entire bush, but you will forfeit this year’s flowers.

Another commonly planted hydrangea is ‘Annabelle’, which in summer produces large globes of flowers which start off as pale green and mature to white before fading to a limey green and is often featured in cool contemporary front gardens. ‘Annabelle’ is a Hydrangea arborescens and so is pruned in a different manner from the mopheads and lacecaps.

Flowering on fresh growth, she can be pruned quite hard now without losing any flower buds. If you prune hard, you will get a more compact bush with larger flowerheads, and this can avoid the floppiness that sometimes happens with Annabelle.

However, this is best done with well-established shrubs — in the first few years, I’d recommend pruning less severely, cutting stems back by one-third. You can also hedge your bets and cut some stems back hard and some less so — this will give you a mix of flowers at different heights.

Prune just above healthy-looking buds. If you don’t prune at all, the shrub gets bigger and you’ll only have flowers at the top of each stem. The pruned flowers make good dried-flower arrangements.

Paniculata hydrangeas are also pruned the same way as, again, these flower on this season’s growth. Paniculatas are gorgeous shrubs with conical-shaped flower heads, and an interesting feature is the two-toned effect you get as the flowers fade.

‘Limelight’ flowers open as pale green maturing to cream and then flushed with pink as autumn approaches. ‘Vanilla fraise’ is one of my favourites, opening white but then flushing to a rich raspberry pink.

And as to how to change the colour of your hydrangea from pink to blue or vice versa? Well it’s all down to what type of soil you have. If your soil is acidic, the petals will be blue, and if its neutral to alkaline, they will be pink. The best way to ensure ideal conditions is in a container, where you can control the soil condition by purchasing special acidic compost and occasionally topping up with aluminium sulphate.

Plant of the week

Forsythia There’s a moment in March when a very boring shrub in my garden starts to glow and, within a week, it becomes the star attraction, lighting up the borders with its golden yellow flowers. This is forsythia and it earns its spot every year with this cheerful and floriferous display. A good shrub for problem soil such as waterlogged or dry. Prune after flowering if necessary to keep its size under control or in shape.

Reader Q&A

How do you look after an amaryllis which has finished flowering? Can it be reused for next year? Rory

Amaryllis is the indoor bulb which produces gorgeous big, often rich-red flowers around Christmas and New Year, and yes, with a bit of care, it will flower every year. Keep it watered and fed until late September and then let it die back and go into dormancy. Leave it in the garage for a couple of months and then bring it back indoors in December into the light, start watering and feeding again and hopefully it will rebloom.

​Submit your gardening questions to Diarmuid via his Instagram @diarmuidgavin using the hashtag #weekendgarden