Bigleaf Hydrangeas in a Cold Climate

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In Britanny, you can see endless bigleaf hydrangea hedges.

I get to see spectacular bigleaf hydrangeas when I travel to milder climates. I’ve seen hedges of them 8 feet (2.5 m) high and just as wide in Brittany, France, covered with hundreds of purplish blue globular flower heads. And outstanding specimens in Vancouver, Virginia and the Azores. It’s enough to make any gardener want to try them!

But that’s not so easy in colder climates. You see, although nurseries stick labels with encouraging hardiness zones like zone 5 or even zone 4 in the pots of bigleaf hydrangea, in fact, these are essentially zone 6 plants, and zone 7 is even better. Like many shrubs, you can grow them beyond their normal hardiness zone, but with decreasing results as the climate gets colder. Where I live, in USDA zone 3b (AgCan zone 4b), for example, they die back to ground or nearly so in most winters. And they only bloom irregularly, on much shorter plants.

Yet, you can get them to bloom quite well, even into zone 3a. The  secret is to convince them they are growing in zone 7!

Portrait of a Capricious Plant

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In Japan, bigleaf hydrangea fills forests with its blooms.

The bigleaf hydrangea (Hydrangea macrophylla) has several common names: hortensia, florists hydrangea, French hydrangea, blue hydrangea, etc. It’s native to Asia, especially Japan,where it grows in deep forests at the foot of tall trees, well protected from the wind.

In North America, it’s often sold as gift plant for Mother’s Day or Easter, but, although it can survive for  short time indoors, it really needs cool winters to bloom again. Therefore the bigleaf hydrangea doesn’t really make a good houseplant. Alternatively, it is also widely used as an outdoor shrub in milder climates.

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Lacecap hydrangea.

This hydrangea offers two flower types. The original form is the lacecap hydrangea, usually shortened to simply lacecap, which produces, at the end of each stem, a large flat umbel of small fertile florets surrounded by a ring of much larger sterile florets with 4 very broad petals. The showy sterile florets attract pollinating insects to less attractive fertile flowers in the center. There are many lacecap varieties, but few are commercially available, ‘Twist-n-Shout’ being one exception.

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The popular ‘Endless Summer’ is a mophead type, with rounded balls of sterile flowers.

Varieties with globular inflorescences, called mophead hydrangeas or simply mopheads, which are by far the most popular in culture. They originated as mutations from the lacecap types. They are almost entirely composed of sterile, large-petaled flowers and are therefore very showy, although not too attractive to insects, as they offer no pollen or nectar. There are hundreds of mophead varieties, including the popular garden variety, ‘Endless Summer’.

To really flourish, the bigleaf hydrangea needs a fairly cool summer and a moderately cold winter, but without a prolonged period of freezing. It will happily grow in full sun in areas with cool or foggy summers, but elsewhere does best in partial shade and even in fairly deep shade, because the soil and air are cooler and more humid there. Because of its large leaves, this hydrangea loses a lot of water to evaporation and will require frequent watering in drier spots. That’s why you won’t find it widely grown in hot, dry climates such as southern California, nor does it thrive in windy spots.

The bigleaf hydrangea likes soil rich in humus that remains slightly moist at all times. So add plenty of compost to the soil at planting time. Always use a mulch as well: it will keep the soil cooler and moister. Mulches that decompose into rich compost, like chopped leaves and ramial chipped wood, are ideal. Fertilize using an slow-release all-purpose organic fertilizer, especially in naturally poor or sandy soils. Avoid nitrogen-rich fertilizers (ones where the first of the three numbers is highest): they tend to stimulate foliage growth at the expense of flowers.

The Bluer the Better

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Purplish to pink to blue flowers on the same plant usually mean the soil is neither acid nor alkaline.

One unusual characteristic of the bigleaf hydrangea is that its flower color changes according to the soil type: its flowers will be blue in acid soil (a pH of less than 5.5) and pink in alkaline soil (a pH of more than 6.5). In between, at a pH of 5.6 to 6.4, many varieties will show various shades of pink to purple to blue, even on the same plant. The blue coloration is due to the presence of aluminum (a widely available element found it most soils). It is more readily available to plants in acid soils which therefore tend to produce blue flowers and becomes fixed and unavailable in alkaline soils, leading to pink flowers.

In most of eastern and northern North America, soils are naturally acidic enough that flowers tend towards the blue side. If they’re not blue enough for your taste, try lowering the soil pH with sulfur treatments. Even just applying peat moss (an acidic soil amendment) at planting and following that with a peat moss mulch will often give just as good results.

Garden centers often recommend adding aluminum sulphate to the soil, but this product can be toxic to plants if not used very carefully and I don’t recommend it. At any rate, most soils contain an abundance of aluminum: why would you want to add more?

Fewer people seem to want their hydrangeas to be pinker, but if so, apply an alkalinizing agent, such as lime, to the soil. Red and purple varieties too tend to be more intensely colored in alkaline soils. Unless your soil is naturally lacking in aluminum, you’ll find it is harder to get a clear pink in bigleaf hydrangeas than a true blue.

Note too that there are biglear hydrangeas with white flowers. They tend to relatively unaffected by the acidity/alkalinity of the soil, simply turning somewhat pinkish or bluish as they age.

My suggestion: first ensure the survival of your hydrangea by offering the best conditions you can. Only later, when you have mastered its culture, should you consider whether you want to meddle with its color.

Pruning

Considerable confusion arises from the fact that various species in the genus Hydrangea have very different pruning needs.

Most hydrangeas bloom on new wood: the new stems that appear in the spring. You can prune these hydrangeas severely in the fall or spring without reducing their bloom. This group includes the popular smooth hydrangea ‘Annabelle’ (H. arborescens ‘Annabelle’), with its big balls of white flowers in mid-summer, as well as the nearly as popular panicle hydrangea (H. paniculata), a later blooming plant with more elongated inflorescences that change from white to pink as they age.

The bigleaf hydrangea is exactly the opposite: it blooms on old wood, that is, the stems produced previous year. So it’s branches already bear the buds of the flowers of the following summer as they go into winter. If you prune this plant heavily in either fall or spring, eliminating all the branches, it will not flower because you’ll have removed the flower buds for the coming summer.

Be careful what you read on the Internet about pruning this hydrangea and especially where the information comes from! Advice given for mild climates will often suggest pruning quite severely after flowering to control the plant’s growth. However, that kind of pruning is not applicable to hydrangeas grown in cold regions! You’ll see: the cold will keep their growth in check!

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This hydrangea has died back nearly to the ground: simply prune back the dead wood.

In cold climates (certainly zones 3, 4, and 5 and quite possibly even 6), it is better to never prune living stems, especially not in the autumn! You can remove dead flower clusters, but nothing more. Instead, wait until late spring before pruning.

In cold climates, there will inevitably be winter damage and in fact, most of the branches will freeze at least in part. If they freeze to the ground, the plant will probably grow back, but will not bloom that year. On the other hand, if even a short section of the branch is still alive in the spring, it will contain buds that will bloom. So pruning is really very simple in cold climates: just remove the dead wood (easy to recognize because it bears no green buds!). And do it in the late spring, cutting back to about 1/2 inch (1 cm) above the living bud.

Why Don’t Reblooming Varieties Rebloom?

Some cultivars, like ‘Endless Summer’ and ‘All Summer Beauty’, are said to flower on old wood in early summer, but also on new wood, later in the summer, providing a second series of flowers and therefore continuous flowering throughout the season. They said to be “remontant”, a horticultural term that essentially means “they bloom again”. Plus, sellers state, “even if the buds are damaged in winter weather, the plant can still flower on wood it produces that season”.

Great in theory, but not so real in practice. In colder climates, this simply doesn’t work… not after a cold winter, at any rate. I have never seen a clear explanation for this (if anyone knows the cause, I’m all ears!), But I have my own theory. I suspect this second flowering occurs not from truly new shoots that emerge from the soil in the spring, but from secondary sprouts arising from those that survived the winter. So even as its branches are flowering, new buds begin to form at their base, ready to take over. However, to get this second flowering, the plant would need to have branches have survived the cold.

The end result is that, in zones 3, 4 and 5 at least, remontant hydrangeas like ‘Endless Summer’ could better be called “Flowerless Summer” many years. If you can’t create the conditions necessary for the first blooming to occur, the plant is unlikely to flower at all.

Winter Protection: the Secret to Successful Bloom

Curiously, the bigleaf hydrangea often blooms better in cold, snowy climates than somewhat warmer ones. Why? Because snow acts as a very good insulator. Where snow falls early, in abundance and stays late, it protects overwintering branches, the ones bearing the flower buds. In climates where snow is only sporadic, melts away repeatedly in winter, or disappears too early in spring, branches will tend to freeze and therefore, bye-bye bloom.

In the colder regions, snowy or not, it is always wise to properly protect bigleaf hydrangeas. Not with a rose cone or geotextiles, as the temperature inside those shelters starts rising too quickly in the spring, pushing the hydrangea to produce new shoots too early… and these are inevitably killed by a late frost. Instead, you need winter protection that slows spring growth or, looking at it from a different point of view, that prolongs winter. And for that, nothing beats a good layer of dead leaves.

Dead leaves: the Best Winter Protection!

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Dead leaves are the very best winter protection!

In the fall, after the first frost, cover your hydrangea with 1 foot or even 2 feet (30 to 60 cm) of fall leaves (you can surround the plant with a “cage” of chicken wire to better hold them in place). Leaves tend to remain cold and moist for a long time, even when the spring sun behinds to warm things up elsewhere, and that’s what you want. Wait until very late spring, when there is no longer any risk of frost, before removing the leaves, usually about the end of tulip season. The mulch will keep the soil cold longer, delaying the start of vegetation… and protecting your future flowers.

When you finally do remove the leaf mulch protection, be ready to cover your hydrangea in case of a late freeze. A large pot or bucket placed upside down over the plants should be sufficient to keep the frost off.

Be careful when removing dead leaves: the plant will already starting to grow under leaves, with pale swelling buds or new shoots and you don’t want to damage them. This is the time to prune, removing dead wood only.

With this treatment, you have a good chance of seeing your bigleaf hydrangea flower almost every year, even into zone 3!

Cold-hardy Cultivars

Even so, some cultivars are better adapted to cold climates than others. Varieties sold as Mother’s Day plants were chosen to be easy to force and therefore bloom early, exactly what you don’t want in a cold climate. They rarely make good choices for outdoor planting further north than zone 7.

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‘Penny Mac’ is said to be the hardiest bigleaf hydrangea.

Among the cultivars that are more cold-resistant, slower to sprout in spring or both, and are therefore the best choices for northern gardeners, there are: ‘All Summer Beauty’, ‘Blushing Bride’, ‘Rose Bouquet’, ‘Early Sensation’, ‘Endless Summer’, ‘Let’s Dance Moonlight’,’ Let’s Dance Starlight’, ‘Let’s Dance Big Easy’, ‘Nikko Blue’, ‘Pink Beauty’ and ‘Twist-n-Shout’, plus such series as ‘Forever and Ever’ and ‘Cityline’. ‘Penny Mac’ has the reputation of being the hardiest and the most floriferous of all the big leaf hydrangeas. It is readily remontant when given decent conditions.

Developing a Zen Attitude

There you go! I have now shared all the secrets for successfully blooming a bigleaf hydrangea in a cold climate. But I find the above too much work for a laidback gardener.

Personally, I give my big leaf hydrangeas no special care other than planting them in a suitable location: in a protected spot in part shade with moist, rich but well-drained soil where autumn leaves naturally accumulate. I’ll admit my results are variable: there are good years with lots of bloom and bad years with little to none. I just figure I appreciate the flowers all that much more when they’re a pleasant surprise than a forgone conclusion!

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