Growing Hydrangeas in a Cold Climate: Types, Zones and Care

Contents

• The five hydrangea groups that matter
• Which hydrangea survives your zone?
• How much sun do hydrangeas need?
• Soil and pH
• How to plant a hydrangea step by step
• Watering: why it is called hydrangea
• Feeding
• Why is my blue hydrangea pink?
• Pruning in brief: old wood or new wood
• Winter protection
• Why is my hydrangea not blooming?
• Hydrangeas in pots
• Dried flower heads and winter interest
• Wilting leaves, pests and diseases
• Zone and timing chart
• Common mistakes
• Frequently asked questions

The five hydrangea groups that matter

Match the group to your zone and the shrub looks after itself. Get it wrong and no amount of feeding will help. The groups differ on the one question that governs pruning and hardiness alike: does the shrub flower on this year's shoots, or on wood it built last summer?

Panicle hydrangea (Hydrangea paniculata)

The most reliable hydrangea for a cold garden. The heads are cone-shaped panicles, 15–30 cm (6–12 in) long, opening creamy white in July and ageing through pink to rust by September. The shrub reaches 1.5–3 m (5–10 ft) and takes far more sun than the others.

The key point: it blooms on new wood. If it dies back that hardly matters, because the buds form on shoots that appear in spring. That is why it flowers year after year in zone 3 while a bigleaf stands green in zone 5.

Widely sold cultivars: 'Limelight', its compact form 'Little Lime', 'Bobo' for small gardens, the early 'Quick Fire', 'Fire Light', 'Pinky Winky', and the old PeeGee type 'Grandiflora'.

Smooth hydrangea (Hydrangea arborescens)

The one with the big round white balls. 'Annabelle' is best known and its heads reach 25 cm (10 in) across. The shrub grows to 1–1.5 m (3–5 ft) and flowers July into September. Native to eastern North America, it blooms on new wood and is dependable even where winter takes the whole top.

The weakness is the stems: a wet head is heavy and after rain the shrub lies flat. Use a grow-through ring set early in spring, or a stiffer selection such as 'Incrediball'. 'Invincibelle Spirit II' gives pink, and 'Haas' Halo' is a lacecap pollinators use.

Bigleaf hydrangea (Hydrangea macrophylla)

What most people picture: fat round heads in blue, pink or purple, with thick glossy leaves. It is also the most tender, flowering almost entirely on old wood, so the buds sit exposed in the stems all winter. One winter at minus 25 °C (-13 °F) without snow cover removes the whole display.

Mopheads carry dense globes of sterile florets; lacecaps carry a flat plate of fertile flowers ringed by showy sterile ones, and lacecaps are usually a little tougher.

Reblooming cultivars make the group workable in zone 5. 'Endless Summer', 'Twist-n-Shout' and the Let's Dance series flower on both old and new wood, so even when winter takes the tops you get flowers in August. Older cultivars such as 'Nikko Blue' belong in zone 6 and warmer, or in a pot. Mountain hydrangea, Hydrangea serrata, is a smaller relative with hardier buds: try 'Tuff Stuff' or 'Bluebird' where a bigleaf gives up.

Oakleaf hydrangea (Hydrangea quercifolia)

The most underrated of the five. The leaves are deeply lobed like an oak's and turn burgundy in autumn, so the shrub earns its place without flowers at all. The blooms are elongated white cones in June and July that fade to rosy brown.

Hardiness is the limit: USDA zone 5 in shelter, comfortable from zone 6. It blooms on old wood, so the same risk applies as with bigleaf. 'Snow Queen' and 'Alice' are the large forms, 'Ruby Slippers' the compact one.

Climbing hydrangea (Hydrangea anomala subsp. petiolaris)

The answer to the hardest spot in the garden: a tall north wall in shade. It attaches with aerial roots straight onto wood, brick and stone without a trellis, and reaches 6–8 m (20–26 ft). The heads are flat white lacecaps about 20 cm (8 in) across in June.

It does nothing for the first two or three years while it builds roots, and rarely flowers before year four or five. Hardiness is good, USDA zone 4 to 8. It blooms on old wood and should barely be pruned.

Which hydrangea survives your zone?

USDA hardiness zones run coldest to warmest, so zone 3 is the tough end and zone 6 the comfortable one. A shrub rated zone 4 should survive lows of about minus 34 °C (-30 °F).

But the zone number alone is not enough, and this is where most guides stop too soon. Two kinds of hardiness have to be kept apart. Plant hardiness decides whether roots and stems survive; bud hardiness decides whether the flower buds survive. For a panicle hydrangea these are the same thing. For a bigleaf they are completely different, and a plant can live ten years without flowering.

How to read a zone rating on a label

A bigleaf labelled zone 5 usually means the shrub survives there, not that it flowers there. A panicle labelled zone 3 flowers in zone 3. Same kind of number, completely different promise. Go by the group rather than the digit.

Microclimate moves you half a zone

A position against a south or west brick wall, sheltered from the wind and with reliable snow cover, behaves like half a zone warmer than the map says. An exposed site where wind strips the snow behaves colder. Snow matters most: under 40 cm (16 in) of snow the soil rarely drops below minus 5 °C (23 °F) whatever the air thermometer reads.

What to choose in practice

In zone 3, grow panicle and smooth hydrangeas and nothing else in the ground. In zone 4, add the climber and treat any bigleaf as a pot plant. In zone 5, panicle and smooth are the backbone, reblooming bigleaf works with protection, and oakleaf succeeds in shelter. In zone 6 all six groups are realistic.

How much sun do hydrangeas need?

The standard answer is part shade, which is true but too blunt to act on. What a hydrangea wants is sun in the morning, shade from late morning to mid-afternoon, and shelter from drying wind.

The reason is water balance. Hydrangea leaves are large and thin and lose a lot of moisture. In strong midday sun the roots cannot supply water as fast as the leaves lose it, and the shrub wilts. Wilt every day and it weakens and flowering shortens.

Differences between the groups

Panicle hydrangea takes the most sun and grows in full sun if the soil stays moist, which is why it suits open northern gardens. Smooth hydrangea prefers part shade but handles full sun in zones 3 and 4. Bigleaf and oakleaf want midday protection. The climber is the only one that flowers decently in real shade.

Under a tree works less well than it sounds

A shady spot under the maple sounds ideal and is usually the opposite. An established tree has a dense root mat in the topsoil and drinks the rain before it reaches the hydrangea. Choose the edge of the canopy, dig a generous hole and expect to water for years. Under a Norway maple or birch it is close to hopeless.

The north side of a house, by contrast, is excellent. There is light but no direct sun, and the soil stays cool and damp. The same corner suits shade perennials as underplanting.

Soil and pH

A hydrangea wants soil that is three things at once: rich in organic matter, moisture-retentive and well drained. Organic matter is the key, because it holds water like a sponge and still lets the surplus pass through.

In practice, mix the backfill with plenty of compost, leaf mould or well-rotted manure. On sand this holds moisture; on heavy clay, add coarse grit alongside it so water does not stand around the roots over winter. Winter wet is the commonest cause of death in a newly planted hydrangea.

What pH actually controls

Hydrangeas grow across a wide range, roughly pH 4.5 to 7.0, and best in the slightly acid zone around 5.5. But pH mainly controls one thing, and it applies to only two groups: flower colour on bigleaf and mountain hydrangeas.

On chalky soil above pH 7 a hydrangea often shows iron chlorosis: yellow leaves with green veins still showing. Treat with chelated iron in the short term and acidifying material in the long term. On genuinely chalky ground, a large pot of ericaceous compost beats rebuilding the whole border.

Testing your soil

A garden pH kit is inexpensive. Take samples from five points in the bed, mix them in a jar and test the mixture. If growing blueberries or rhododendrons already works for you, your soil is acid.

How to plant a hydrangea step by step

The best window is late spring, once the soil has warmed and the plant has the whole summer to root. Early autumn is a good second option in zones 5 and 6. Avoid midsummer heat, and avoid autumn planting in zones 3 and 4, where roots will not establish before freeze-up.

1. Stand the pot in a bucket of water for twenty minutes, until the bubbling stops. A dry root ball never takes up water once planted.
2. Dig a hole at least twice as wide as the pot but no deeper. Width helps the roots move sideways, which is where they grow.
3. Break up the base and sides with a fork. On clay, smooth walls make a watertight bucket in the ground.
4. Mix the excavated soil with a third compost or leaf mould. For blue flowers, use ericaceous compost.
5. Tease the root ball apart at the edges and cut any roots circling the pot.
6. Set the plant with the top of the root ball level with the soil surface. Deeper means suffocated roots and rot at the base.
7. Backfill in stages and firm gently with your hand, not your boot.
8. Build a low ring of soil around the plant and water in with 15–20 litres (4–5 gal).
9. Mulch 5–8 cm (2–3 in) with bark, leaf mould or compost, kept clear of the stems.
10. Water every second day for three weeks, then weekly through the first season.

Spacing follows final size: 90–120 cm (3–4 ft) for an 'Annabelle', 120–150 cm (4–5 ft) for a 'Limelight'. It looks sparse in year one and right in year three. Fill the gap with ground cover plants meanwhile.

Watering: why it is called hydrangea

The name is not an accident. Hydrangea comes from the Greek hydor, water, and angeion, vessel. Botanically it refers to the cup shape of the seed capsules, but the plant lives up to it anyway: of the common garden shrubs, this is the thirstiest.

The working figure is about 25 mm (1 in) per week, which for a mature shrub means 20–40 litres (5–10 gal) in dry weather. New plants need water every second day for the first few weeks.

Deep and infrequent, never little and often

A couple of litres every evening only wets the top few centimetres. The roots follow the moisture upward into the layer that dries first, and the plant becomes more vulnerable, not less. Give a full bucket at the base instead, three or four days apart.

Wilting leaves are not always thirst

A hydrangea in strong midday sun often wilts even when the soil is damp, and stands back up by evening. Always check the soil with a finger to the second knuckle before you water. If it is moist and the plant is still flat at seven in the evening, something else is wrong: root damage, planting too deep, or standing water.

Mulch does half the work

A 5–8 cm (2–3 in) layer of bark, leaf mould or compost halves evaporation, evens out soil temperature and slows weeds. Top it up every spring. It is the most labour-saving thing you can do for a hydrangea.

Feeding

Hydrangeas need feeding but suffer from too much, especially nitrogen. An overfed hydrangea makes big dark leaves, long soft shoots and few flowers, and those shoots do not ripen before winter.

The basic regime is simple: one feed in early spring as growth starts, using a balanced granular fertiliser or well-rotted manure, a generous handful per square metre worked into the surface and watered in.

A second feed in June

Panicle and smooth hydrangeas build their whole display on summer growth and respond well to a lighter second feed in early June. Bigleaf rarely needs it. Never feed after mid-July. Late feeding produces soft shoots that do not ripen, one of the surest ways to lose winter hardiness. The same rule applies to growing roses.

Ericaceous feeds work well

Fertilisers sold for rhododendrons and azaleas are acidifying and suit hydrangeas, especially if you want blue flowers. Keep lime, wood ash and bone meal away from a blue bigleaf: all three raise pH and work against the colour.

Why is my blue hydrangea pink?

The classic hydrangea disappointment. You buy a blue plant, put it in the border, and next summer it is pink. Nothing is wrong with the plant; the soil is talking.

The mechanism is aluminium. Blue develops when the plant takes up aluminium ions that bind to the pigment in the flower. Aluminium is in most soils but only available when pH is low. Below pH 5.5 the flower turns blue, above pH 6.5 pink, and in between you get muddy purples on the same shrub.

Nurseries grow their stock in acidic peat with added aluminium sulfate. Move that plant into ordinary garden soil at pH 6.5 and it loses access to aluminium and flowers pink. The plant has not changed; the soil has.

Keeping the flowers blue

Lower the pH and hold it there. Plant in ericaceous compost, mulch with bark and pine needles, feed with an ericaceous fertiliser, and water with rainwater rather than hard tap water. For a faster result, apply aluminium sulfate at about 15–25 g per 10 litres (1/2–1 oz per 2.5 gal) two or three times a season. Test as you go, because overdoing it damages roots.

Getting pink

Raise the pH with garden lime in autumn, roughly a handful per square metre, and avoid acidifying mulches. Pink is easier than blue, because most gardens start near neutral.

It only works on two groups

Only bigleaf and mountain hydrangeas change colour with pH. Panicle hydrangeas cannot be recoloured no matter how much aluminium sulfate you apply. The white-to-pink shift in 'Vanilla Strawberry' and 'Limelight' is age and cool nights, not chemistry. The same goes for smooth and oakleaf hydrangeas, and white bigleaf cultivars have no pigment to work with either.

Pruning in brief: old wood or new wood

The whole of hydrangea pruning rests on one question: are next year's flowers already on the plant, or will they form next spring?

Panicle and smooth hydrangeas flower on new wood. They can be cut back hard in late winter or early spring and are better for it: a hard-cut stem answers with a stronger panicle.

Bigleaf, oakleaf, climbing and mountain hydrangeas flower on old wood. Their buds formed last August and sit at the tops of the stems. Cut them down in spring and you remove the whole summer's display. Here you only take out dead wood once the buds have swelled, and thin one old stem at the base now and then.

The details, including where to place the cut, how to handle a reblooming cultivar, how to renovate an old shrub and what to do with an overgrown climber, are covered group by group in the guide to pruning hydrangeas. The principle above is enough to keep you from doing anything irreversible meanwhile.

Winter protection

The need follows the same split as pruning. A panicle hydrangea in zone 4 needs none. A bigleaf in zone 5 lives or dies by it.

Protect everything in year one

A newly planted hydrangea is more vulnerable than an established one. Pile 15–20 cm (6–8 in) of shredded leaves or bark over the root zone once the ground has cooled but before it freezes hard. The point is not warmth but steadiness: freeze-thaw cycles hurt roots more than steady cold.

Bigleaf and oakleaf in zones 5 and 6

Here the buds need protecting, not just the roots. Tie the stems loosely together in late autumn. Set a cylinder of chicken wire around the shrub, fill it with dry oak or beech leaves, which do not pack down, and cap it with evergreen boughs. A loose wrap of burlap works too, but never plastic, which traps condensation and causes rot.

Remove the protection in stages in spring, on an overcast day. Shoots that come out of a dark leaf pile into April sun will scorch.

Spring frost is more dangerous than winter cold

A hydrangea that shrugged off minus 25 °C (-13 °F) in January can lose its whole display to one frost in late May, when the new shoots are soft. If your bigleaf has just broken bud and frost is forecast, throw frost cloth over it in the evening. Five minutes of work saves a summer of flowers.

Why is my hydrangea not blooming?

The most common hydrangea question by a distance, and in nine cases out of ten the answer is one of five. Work through them in order.

1. Winter-killed flower buds

The commonest cause in a cold climate, and it applies only to the old-wood groups. Check the stems in May. If they are brown and dry from the tip down while the base pushes healthy leaves, the buds were killed. Protect next winter, or switch to a reblooming cultivar.

2. Pruning at the wrong time

Second most common. Someone tidied the shrub in autumn or spring and removed every flower bud with it. The symptom is a vigorous, healthy, flowerless plant. Stop cutting for a year and see what happens.

3. Too much shade

Deep shade gives leaves and few flowers in every group except the climber. If a shrub flowered well for ten years and then faded, the surrounding trees have grown. Raise the canopy or move it.

4. Too much nitrogen

A hydrangea at the edge of a lawn gets a full nitrogen feed every time the lawn is fertilised, and the result is large leaves and no flowers. Switch to a lower-nitrogen feed and keep lawn fertiliser out of the bed.

5. The plant is too young

Climbing hydrangea rarely flowers before year four or five, and oakleaf and large panicle types may take two or three years to settle after planting. A healthy young plant simply needs time.

Hydrangeas in pots

Hydrangeas grow well in containers, and in zone 3 or 4 a pot is often the only way to have a bigleaf that actually blooms. The pot moves somewhere cold but frost-free for winter, and the buds come through intact.

Pot, compost and size

Go bigger than you think: at least 40 litres (10 gal), and 60–80 litres (15–20 gal) for a 'Limelight'. A small pot means drought and daily watering, and it must have drainage holes. Use ericaceous or loam-based compost mixed with bark. Compact cultivars work best: 'Bobo', 'Little Lime', 'Endless Summer' and 'Ruby Slippers'.

Water and feed in a container

A pot dries out many times faster than a border. In July a hydrangea in a sunny position often needs water every day, sometimes twice. Feed with a liquid fertiliser fortnightly from May to mid-July, then stop.

Overwintering the pot

Compost in a container freezes through in a way garden soil does not. Three options work. Move the pot into an unheated garage at 0–5 °C (32–41 °F); darkness is fine because the plant is leafless, and water sparingly once a month. Sink the pot into a border and cover it with leaves. Or stand it against a house wall out of the wind, wrapped in bubble wrap.

Dried flower heads and winter interest

One of the best things about hydrangeas is that the flowers stay handsome after they are finished. Panicle heads hold through winter, turn cinnamon and papery, catch frost and snow and keep the garden alive in February. Leave them: they also give the buds underneath a small but real measure of frost protection.

Drying flowers for indoors

Timing decides everything. Do not cut when the head is fresh and soft, or it will flop and lose colour. Wait until it feels papery and slightly stiff, which happens in late August or September.

Two methods work. Hang drying: cut stems 30–40 cm (12–16 in) long, strip the leaves and hang small bunches upside down somewhere dark, dry and airy for two to three weeks. Water drying gives softer colours: stand the stems in a few centimetres of water out of the light and let it be used up. Dried heads last for years away from direct sun, and mix well with the seedheads left by growing peonies.

Wilting leaves, pests and diseases

Hydrangeas are fairly trouble-free, and the problems that do appear are usually about growing conditions rather than pests.

Brown leaf edges

Drought or salt. Dry, crisp brown margins in July almost always mean too little water or too much sun. Near a salted road, de-icing salt may be the cause, and the only answer is to move the plant.

Yellow leaves with green veins

Iron chlorosis, caused by high pH rather than a shortage of iron. The leaf looks like a green net on a yellow background, clearest on the youngest growth. Apply chelated iron for a quick fix and acidify the soil over time.

Mildew, aphids, vine weevil and deer

Powdery mildew shows as a grey-white film late in the season on crowded plants in dry soil; thin the stems for airflow and water at the base rather than over the foliage. Aphids cluster on soft spring tips and wash off with a jet of water. Vine weevil adults notch the leaf edges, ugly but harmless; the grubs in the compost damage roots, especially in pots, and are treated with nematodes in late summer. Slugs graze soft spring shoots. Deer browse hydrangeas readily, so in rural gardens a fence matters more than any spray.

Zone and timing chart

The chart shows what works where and when the work happens. Treat the dates as a frame and let the season confirm them; a sheltered microclimate often behaves like half a zone warmer.

Bloom time shifts by roughly a week per zone: a 'Limelight' that opens in mid-July in zone 6 opens in early August in zone 3. In the coldest zones late cultivars can be caught by frost before they colour up, so choose early types such as 'Quick Fire' there.

Common mistakes

• Wrong group for the zone. A bigleaf planted in zone 4 without protection usually survives but never flowers. Check the group, not just the number on the tag.
• Planting too deep. The top of the root ball belongs level with the soil surface, or you get starved roots and rot at the base.
• Little and often watering. A couple of litres each evening only wets the surface and pulls roots into the layer that dries first. Give a full bucket every third day.
• Too much nitrogen. Lawn fertiliser drifting into the bed gives big leaves and no flowers. Feed once in spring and stop by mid-July.
• Hard spring pruning of a bigleaf. The buds are already at the tops of last year's stems. Cut them and the summer goes with them.
• Plastic as winter protection. It traps condensation and swings in temperature. Use leaves, evergreen boughs, burlap or frost cloth that breathes.
• Lime around a blue hydrangea. Lime, wood ash and bone meal raise pH and turn blue flowers pink within a season.
• A pot that is too small. Under about 30 litres (8 gal) the compost dries out daily in July and the plant never thrives.

Frequently asked questions

Which hydrangea is hardiest?
Panicle hydrangea, Hydrangea paniculata, rated USDA zone 3 to 8. It flowers on shoots produced the same spring, so even if the shrub dies back you still get blooms. Smooth hydrangea, including 'Annabelle', is equally hardy. Bigleaf is the tender one and belongs in zone 6, or zone 5 with protection.

Where should I plant a hydrangea?
In morning sun with shade through the middle of the day, sheltered from drying wind, in rich soil that stays evenly moist. The north side of a house suits climbing and bigleaf hydrangeas especially well. Avoid the ground beside the trunk of a large tree, where the tree takes all the water.

How often should I water hydrangeas?
Deeply and infrequently rather than little and often. A mature shrub wants about 25 mm (1 in) of water a week in dry weather, roughly 20–40 litres (5–10 gal) over two soakings. New plants need water every second day for three weeks, and a pot needs it daily in July.

How do I make my hydrangea blue?
Lower the soil pH below 5.5 so aluminium becomes available. Plant in ericaceous compost, mulch with bark and pine needles, feed with an ericaceous fertiliser and water with rainwater. To speed it up, apply aluminium sulfate at 15–25 g per 10 litres (1/2–1 oz per 2.5 gal) two or three times a season. It only works on bigleaf and mountain hydrangeas.

Can you turn a white panicle hydrangea blue?
No. Panicle and smooth hydrangeas lack the pigment that reacts with aluminium, so soil pH has no effect on them. The white-to-pink shift in 'Limelight' or 'Vanilla Strawberry' comes from the flowers ageing and from cool September nights. For blue you need a bigleaf or a mountain hydrangea.

When is the best time to plant hydrangeas?
Late spring, once the soil has warmed and the plant has a full season to root. Early autumn also works in zones 5 and 6. Avoid planting during a midsummer heatwave, and avoid autumn planting in zones 3 and 4, where roots will not establish before the ground freezes.

Should I cut hydrangeas back in the autumn?
No. Leave the flower heads standing. They look good through winter and give the buds beneath a little protection. Panicle and smooth types are pruned in late winter or early spring instead, and on the old-wood groups you only remove dead wood once the buds swell.

Why are my hydrangea leaves turning yellow?
Yellow leaves with green veins showing through are iron chlorosis, caused by soil pH being too high rather than by a shortage of iron in the ground. Apply chelated iron for a quick response and acidify the soil over time. If whole leaves go evenly yellow and drop, suspect waterlogging instead.

If you would like a reminder when it is time to feed, protect and prune in your own zone, our newsletter sends one along with the rest of the season's jobs.