Growing Dahlias: Planting, Care and Overwintering in Cold Climates

Contents

• What is a dahlia tuber, and how is it different from a bulb?
• Dahlia types: decorative, cactus, ball, pompon, single and waterlily
• Which dahlia varieties do best in a short season?
• Site, soil and bed preparation
• Starting dahlia tubers indoors
• When can you plant dahlias out, and how deep?
• Staking, support and pinching
• Watering and feeding: potassium, not nitrogen
• Deadheading keeps dahlias flowering until frost
• Slugs and other pests
• Growing dahlias in containers
• Lifting and storing tubers over winter
• Dividing dahlia tubers in spring
• Why is my dahlia not blooming?
• Zone and timing chart for USDA zones 3–6
• How to grow dahlias step by step
• Common mistakes
• Frequently asked questions

What is a dahlia tuber, and how is it different from a bulb?

They sit on the same shelf in the garden centre, and catalogues cheerfully call both of them bulbs. But they are built differently, and the difference decides how you handle them.

A bulb is a plant folded up: cut a tulip bulb in half and you see a tiny flower bud surrounded by fleshy scales, with the growing point inside. A dahlia tuber is a thickened root. It stores starch and water but holds no flower bud and no scales, so it cannot sprout on its own. The eyes sit on the crown, the collar of tissue where the tuber joins last year's stem.

Why the difference matters

A dahlia tuber without a piece of crown is lost, even though it is perfectly healthy. It has no eye to grow from and can sit in the box for a year doing nothing. That is the usual reason a division fails: you cut the clump into neat pieces and only half come up. Every division needs a piece of crown with at least one visible eye, plus an intact tuber with an unbroken neck.

Check the tuber when you buy

A good tuber is firm like a carrot and still carries an obvious piece of last year's stem. A slim tuber with a sound neck makes a fine plant, while a fat tuber with no neck is worthless. Light and papery means it dried out; soft patches are rot.

Dahlia types: decorative, cactus, ball, pompon, single and waterlily

Dahlias are classified by flower form, not by colour or height. The class is usually on the label and says more about how the plant reads in a border than the variety name does. There are over twenty thousand cultivars but only a handful of forms.

Decorative dahlias

Fully double blooms with no visible disc and broad, flat petals, from a few centimetres across to 25 cm (10 in). The largest are sold as dinnerplate dahlias and need staking and room.

Cactus and semi-cactus

The petals are narrow and rolled into pointed quills, so the bloom looks like a starburst; semi-cactus petals are broader at the base. Rain runs off a cactus dahlia more easily than off a tightly packed decorative, which matters in a wet August.

Ball and pompon dahlias

A ball dahlia is fully double and globe-shaped or slightly flattened, with petals rolled along their edges. A pompon is the same in miniature, under 5 cm (2 in). Neither is easily wrecked by rain.

Single and waterlily dahlias

Singles carry one ring of petals around an open yellow centre that bees and hoverflies can use, while a fully double dahlia is effectively closed to them. If you want colour that also does some work, this is the group to pick, ideally alongside other bee friendly plants. A waterlily dahlia is double but noticeably flat, with a depth of no more than a third of its diameter.

Other forms

Collarettes carry a ring of short petals inside the outer ring, and anemone types a cushion of tubular florets. Mignon is a height class rather than a form: plants under 50 cm (20 in) for pots and border edges.

Which dahlia varieties do best in a short season?

Any dahlia will grow through a northern summer, because the tuber comes up in autumn anyway. What separates them is the time to first bloom, how well they shrug off rain, and how much staking they demand.

Varieties worth starting with

'Café au Lait' is the most requested dahlia on the continent, a dinnerplate decorative with blooms up to 25 cm (10 in) in a muted shade between cream and dusty apricot. It is also demanding: the bloom is heavy and a downpour can ruin an open flower.

'Bishop of Llandaff' goes the other way, with single to semi-double scarlet flowers against near-black bronze foliage, about a metre (3 ft) tall and sturdier than its reputation suggests. The open centre pulls in bumblebees all August, and seed-grown 'Bishop's Children' gives the same dark foliage in mixed colours.

Among ball dahlias, 'Cornel Bronze' in warm apricot and 'Jowey Winnie' in dusty pink turn up in almost every catalogue and are close to weatherproof. 'Franz Kafka' is the classic lavender pompon, 'Gerrie Hoek' a soft pink waterlily sold for eighty years, and 'Karma Choc' a dark wine-red bred for cutting. For pots the 'Gallery' and 'Happy Single' series work at around 40 cm (16 in).

Choose for your site

If the bed is windy, stay under a metre (3 ft) and skip the dinnerplates. If your season is short, choose ball, pompon and single types, which flower two to three weeks earlier than the big doubles. Among long flowering perennials, waterlily dahlias sit best, because they do not take over the whole picture.

Site, soil and bed preparation

Dahlias want sun without qualification. Under six hours of direct light the plant goes lanky, the stems thin and the flowers scarce. Morning sun is worth more than afternoon sun, because the leaves dry early and mildew gets less of a foothold. Wind is underrated: a 120 cm (4 ft) plant works like a sail, and a sheltered spot saves half the staking.

The soil should be rich in organic matter, well drained and evenly moist. The tuber rots in standing water but drinks heavily once in flower, which means good structure rather than light or heavy soil. Work a spadeful of compost into each hole and dig a spade's depth. On heavy clay, raise the bed or use raised bed gardening, since meltwater otherwise sits exactly where the tuber goes. Aim for pH 6.0 to 7.0.

Put the stake in before you plant: a cane driven in beside an established plant in July goes straight through the tuber clump.

Starting dahlia tubers indoors

You can put the tuber straight in the ground and it will work. But starting indoors buys three to four weeks of flowering, and where frost arrives in September that is the difference between a dahlia that shows what it can do and one that has barely started.

The second reason carries as much weight. A newly planted tuber sends a soft shoot straight up into slug season, and that shoot often disappears overnight. A started plant with a firm 30 cm (12 in) stem fares far better.

Timing and method

Count four to six weeks back from your planting-out date: early April in zone 6, mid-April in zone 5, late April in zone 4 and early May in zone 3. Start too early and you get a long, pale plant that is hard to harden off.

Use a roomy pot, at least 10 litres (2.5 gallons), or a bucket with drainage holes. Lay the tuber on its side with the crown up and cover with 5–10 cm (2–4 in) of moist potting mix. Water once thoroughly, then very sparingly until the first shoot shows, because wet soil around a leafless tuber is the surest route to rot. Keep the pot bright at 10–15 °C (50–59 °F); warmer gives fast, weak growth. The principles match starting seeds indoors.

Hardening off and free plants

Set the plant outside in part shade for a few hours a day during the last week before planting out; taken straight from a windowsill to full sun it gets scorched patches and loses a couple of weeks. A tuber with several shoots also gives free plants: cut a 10 cm (4 in) shoot off at the crown and push it into moist mix under a cover, and it roots in two to three weeks.

When can you plant dahlias out, and how deep?

A dahlia is as frost-tender as a tomato. One night at −2 °C (28 °F) blackens the foliage, and a plant knocked back in May starts over. Plant out only once frost is past and the soil has warmed to about 15 °C (60 °F): mid to late May in zone 6, late May to early June in zone 5, early June in zone 4 and mid-June in zone 3. The rule that works everywhere is to plant dahlias a few days after your tomatoes.

Depth and spacing

Dig a hole 10–15 cm (4–6 in) deep and twice as wide as the clump. Lay the tuber on its side with the crown and eyes facing up, never standing on end, and cover with 5–10 cm (2–4 in) of soil. Shallower and the tuber dries out; deeper and the shoot takes longer to reach daylight, which raises the risk of rot in cold spring soil. A started plant goes in at the depth it sat in its pot.

Space dwarf types 30–40 cm (12–16 in) apart, medium types 50–60 cm (20–24 in) and tall types 60–90 cm (24–36 in). Crowding costs airflow and invites mildew in August.

Water the hole, not the plant

Water the hole before planting. A leafless tuber uses almost no water, and soaked soil over a newly planted tuber is the most common reason it rots before it starts.

Staking, support and pinching

A 120 cm (4 ft) dahlia carrying five heavy blooms falls over. Not perhaps, but certainly, some time in August when rain and wind arrive together, and the stem snaps at the base.

Stake at planting time

Drive the stake in as you set the tuber, 10–15 cm (4–6 in) away from it. A stout 150 cm (5 ft) bamboo cane handles most varieties; dinnerplate types want a proper post. Tie with soft twine in a figure of eight between stem and cane so the stem cannot chafe, first at 30 cm (12 in) and then every 30 cm as the plant climbs. Check in July that nothing has cut in, because the stem thickens fast.

Pinching gives you more flowers

It feels wrong to cut the top off a plant that is growing well, but pinching is the most profitable thing you do to a dahlia. An unpinched plant runs up on one stem with a few blooms at the top; a pinched plant is wider, steadier and carries three to four times as many flowers.

When the plant is 20–30 cm (8–12 in) tall with three or four pairs of leaves, snap out the top shoot just above a leaf pair. Two side shoots break from the axils below the cut and branch again in turn. The price is ten to fourteen days of delayed flowering, so in zone 3 you may skip pinching on late varieties.

Disbudding

Each stem produces three buds: a large central one and two smaller ones beside it. Remove the side buds while they are pea-sized and all the energy goes to the central bud, on a longer stem. Leave all three for the maximum number of blooms.

Watering and feeding: potassium, not nitrogen

Dahlias are big plants that flower for months, so they get through a lot of water and feed. But they want it in the right proportions, and that is where most people go wrong.

Watering

Before the shoots are up, almost nothing. After that, deep and infrequent beats little and often: two or three thorough soakings a week in dry spells. Shallow watering makes shallow roots and a plant that wilts on the first hot day. Water at the base. Mulch from midsummer holds moisture evenly, but not before then, because a damp mulch around young shoots is exactly the habitat slugs want.

Feeding

Nitrogen makes leaves. Potassium makes flowers and ripens growth. A dahlia given too much nitrogen turns into an impressive green bush with soft stems, few blooms and tubers that store badly. The mistake is almost always well meant, in the shape of an extra scoop of general fertiliser in June.

Work compost into the planting hole instead, then switch to a potassium-rich feed when the first buds appear, usually late June. Tomato feed does the job, and an analysis like 5-10-10 points the right way. Feed every three weeks, fortnightly in a container, until early September and then stop. Late feeding keeps the plant growing when it should be ripening, and an unripe tuber rarely survives storage.

Deadheading keeps dahlias flowering until frost

A dahlia flowers in order to set seed. Leave the spent blooms on and the plant reads the job as done and slows on new buds. Deadhead weekly and flowering runs until frost, often six extra weeks.

Telling a bud from a spent bloom

This trips up nearly everyone, because a dahlia that has just dropped its petals looks like a bud about to open. An unopened bud is round, hard and even, like a small marble. A spent bloom is pointed and conical, softer to the touch, often with brown petal remnants at the tip.

Cut lower than you think

Do not just nip off the flower head. Follow the stem down to the first or second pair of leaves and cut just above them. No bare stub is left to rot, and the new shoot from the leaf axil is stronger. It is the same logic as cutting for the vase, which is why a dahlia you pick heavily flowers better than one you leave alone.

Slugs and other pests

Ask anyone who has grown dahlias in a cool, damp climate what went wrong the first year and the answer is the same. A newly emerged shoot is soft, juicy and alone in an empty bed. Slugs find it in one night.

The whole risk sits in spring

A dahlia past 30 cm (12 in), with firm leaves and a woody stem base, is essentially safe. The problem lives in the three or four weeks between the shoot breaking ground and the plant getting sturdy, which is why starting tubers indoors is slug protection as much as a way to flower earlier.

Plant out started plants rather than bare tubers, and hold the mulch back until midsummer. Go out with a torch on a few evenings after rain and pick the slugs off by hand, which sounds primitive and is the most effective single measure. Ring the plant with iron phosphate pellets for the first few weeks.

Other pests and diseases

Aphids gather at shoot tips and wash off with water. Earwigs and Japanese beetles chew the petals; earwigs are trapped in an upturned pot of straw on a cane. Powdery mildew shows as a white film in late August and is prevented by airy spacing and watering at the base. Plants with mottled or crumpled leaves that will not grow may carry virus; bin them with the tuber.

Growing dahlias in containers

Dahlias do well in pots, with two provisos: the pot must be much bigger than most people expect, and the watering has to be reliable. A dahlia in a small pot turns into a stunted plant with tubers not worth saving.

Size, soil and feed

Dwarf varieties under 50 cm (20 in) manage in 30 litres (8 US gallons), roughly a 35 cm (14 in) pot. Medium and tall varieties want 40 litres (10 gallons) or more, one plant per pot. Use a heavy, rich potting mix rather than a light seed compost that dries out in half a day in July. Water daily while the plant is in flower, and feed with a potassium-rich liquid fortnightly until early September.

Overwintering potted dahlias

There is a shortcut here. Stop watering after the first frost, cut the stems down and move the whole pot, soil and all, somewhere frost-free at 4–7 °C (40–45 °F). The tubers overwinter in their own soil and you never lift them. Bring the pot out in March and start watering.

Lifting and storing tubers over winter

Dahlias are hardy in the ground only in USDA zone 8 and warmer. In zones 3 to 6 the tubers have to come up, the job that puts most beginners off and that takes about twenty minutes a plant.

Wait for frost, but not too long

Do not lift while the plant is still flowering. The last weeks of the season are the most important for tuber formation, and an unripe tuber rots in the box. Wait until the first real frost blackens the foliage, then cut the stems to 10–15 cm (4–6 in) and leave the tubers in the ground another one to two weeks if the weather allows, which cures the skins. Cover the cut stem ends with foil so water cannot run down the hollow stem and start crown rot.

Dig carefully and clean

Push the fork in at least 30 cm (12 in) from the stem, all the way round, and lift from underneath. The clump is wider than you expect and the necks snap easily; a broken neck means that tuber is lost. Wash the clump off, because soil carries the organisms that start rot. Cut damaged areas back to clean white tissue, then dry the clump upside down for three to seven days.

Storage temperature and medium

Aim for 4–7 °C (40–45 °F), dark and frost-free. Below freezing the tuber is dead. Above 10 °C (50 °F) it starts sprouting in January and burns through its reserves in the dark. An unheated basement, a root cellar or a cold garage all work.

Pack the tubers in a box with vermiculite, dry peat, wood shavings or crumpled newspaper so they do not touch. The medium should be barely moist, and a sealed plastic bag gives condensation and rot. Write the variety name straight onto the tuber, because every tuber looks the same in March, and check the box once a month.

Dividing dahlia tubers in spring

A dahlia that has grown one season comes up as a clump of five to fifteen tubers. Dividing it each spring gives more plants and better flowering, while an old clump throws many weak shoots instead of a few strong ones.

Spring rather than autumn

Both work, but spring is easier the first time. The eyes are barely visible in autumn, while by March they have swollen into small pink or white buds you can actually see. Bring the clump out in March, rinse it and leave it somewhere bright and cool for a week so the eyes show.

1. Stand the clump so you can see the crown, the old stem base with its collar of tissue.
2. Find the eyes: small pale or pink buds on the crown, never on the tuber itself.
3. Split the clump in half through the crown with one cut of a clean, sharp knife.
4. Divide each half further so every piece has one tuber, an intact neck and at least one eye.
5. Cut away tubers with no neck, soft tubers and all the fine feeder roots.
6. Let the cut surfaces dry in the air for a day before planting or storing.

If no eyes show after a week in the light, bed the clump in moist soil and divide once shoots appear. Divisions are genetically identical to the parent, the same as when you divide growing peonies and other perennials with fleshy roots.

Why is my dahlia not blooming?

A dahlia that grows well but will not flower is a common complaint. Work through the four explanations in order.

Too much nitrogen. Lush dark leaves and thick, soft stems with no buds point straight at excess nitrogen. If you have used a general fertiliser with a high first number, that is your answer. Switch to a potassium-rich feed and the effect shows within three to four weeks.

Not enough sun. Under four or five hours of direct sun a dahlia flowers poorly whatever you do, and the plant leans towards the light. A sunny spot that frees up is good ground for perennials for full sun.

Started too late. A tuber put straight into the ground in June in zone 4 rarely flowers before frost, least of all a large-flowered variety.

Drought at bud set. A plant that dries out in July drops its buds before they open. Even, generous watering from bud set through flowering decides it.

Yellowing lower leaves in August are normal. Buds that brown without opening usually mean persistent rain. Plants that get shorter year after year need dividing.

Zone and timing chart for USDA zones 3–6

Your zone decides when you can plant out and how long the dahlias flower before frost ends it. The dates follow average frost dates; a cold year shifts everything a week or two later.

In zones 3 and 4 starting indoors is not an improvement but a requirement. Without a head start, a dahlia planted in mid-June barely opens its first blooms before frost, so choose early varieties too.

How to grow dahlias step by step

The whole year in order, from tuber in a bag to tuber in a box. Adjust the dates to your zone using the chart above.

1. Buy or bring out the tubers in March; check each is firm with intact crown.
2. Divide large clumps once the eyes swell, giving every piece a crown, a tuber and an eye.
3. Start indoors four to six weeks before planting out, bright at 10–15 °C (50–59 °F) and watered sparingly.
4. Prepare the bed ahead: full sun, sheltered, dug deeply with compost.
5. Harden the plants off outdoors in shade in the final week.
6. Put the stake in the planting hole before the plant goes in.
7. Plant out after the last frost, with 5–10 cm (2–4 in) of soil over the tuber.
8. Water sparingly until shoots show, then deeply two or three times a week.
9. Protect the young shoots from slugs for three to four weeks.
10. Pinch out the top at 20–30 cm (8–12 in), just above a pair of leaves.
11. Tie in at 30 cm (12 in) and every 30 cm above that.
12. Feed potassium every three weeks from first bud to early September.
13. Deadhead weekly, cutting back to the first or second pair of leaves.
14. Cut the stems down after the first frost and wait one to two weeks.
15. Lift, wash, trim damage and dry upside down for three to seven days.
16. Store at 4–7 °C (40–45 °F) in barely moist packing, labelled by variety.

Common mistakes

• Dividing without a piece of crown. The tuber is full of food but has no eye to grow from. Every division needs crown, neck and a visible eye.
• Planting out too early. One frost blackens the whole plant. Wait until the soil is around 15 °C (60 °F), a few days after the tomatoes go in.
• Watering a leafless tuber generously. It uses almost no water and rots in wet soil. Water the hole before planting and wait for shoots.
• Feeding nitrogen mid-season. An extra scoop of general fertiliser gives a green bush with no flowers and tubers that will not store. Switch to potassium at bud set.
• Staking after the plant is large. The cane goes straight through the tuber clump. Put it in as you plant, and pinch at 20–30 cm (8–12 in).
• Lifting too early or storing in a sealed bag. Unripe tubers and condensation both mean rot. Wait for frost, dry the clumps and store them airy.

Frequently asked questions

When should you plant dahlia tubers?
Plant out only after the last frost, once the soil has reached about 15 °C (60 °F). That means mid to late May in zone 6, late May to early June in zone 5, early June in zone 4 and mid-June in zone 3. To flower earlier, wake the tubers indoors four to six weeks before that date.

How deep should you plant dahlia tubers?
Dig a hole 10–15 cm (4–6 in) deep, lay the tuber on its side with the crown and eyes facing up, and cover with 5–10 cm (2–4 in) of soil. Shallower and the tuber dries out. Deeper and the shoot takes longer to surface, raising the risk of rot in cold soil.

Do dahlia tubers have to be lifted for winter?
In USDA zones 3 to 7, yes, since dahlias are hardy in the ground only from zone 8 upwards. Cut the stems after the first frost, lift the clump, wash it, dry it upside down for three to seven days and store at 4–7 °C (40–45 °F). Potted plants can overwinter in their pot.

What temperature should dahlia tubers be stored at?
Between 4 and 7 °C (40–45 °F), dark and frost-free. Below freezing the tuber dies. Above 10 °C (50 °F) it sprouts in January and exhausts itself in the dark. An unheated basement, a root cellar or a frost-free garage usually sits in range, with barely moist packing.

Should you pinch dahlias?
Yes, in nearly every case. When the plant is 20–30 cm (8–12 in) tall with three or four pairs of leaves, snap out the top shoot just above a leaf pair. The plant branches and carries three to four times as many blooms. The cost is ten to fourteen days of delayed flowering.

Why is my dahlia not blooming?
The usual cause is too much nitrogen: lush leaves, thick stems and no buds. Switch to a potassium-rich feed. The next most common is too little sun, since under four or five hours of direct light a dahlia flowers poorly. Drought in July also makes the plant drop buds.

How do you protect dahlias from slugs?
The whole risk sits in the three or four weeks after planting out, while the shoots are soft. Plant started plants rather than bare tubers, hold the mulch back until midsummer, pick slugs by torchlight after rain, and ring the plant with iron phosphate pellets.

Can you grow dahlias in pots?
Yes, but the pot has to be large. Dwarf varieties need at least 30 litres (8 US gallons), medium and tall types 40 litres (10 gallons) or more, with drainage holes and a heavy, rich mix. Water daily while in flower and feed potassium fortnightly until early September.

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