OrnamentalbulbousSome care needed

Dahlia

Tender tubers that flower from midsummer until the first frost blackens them, and then present a real annual decision: lift and store, or mulch heavily and gamble. On free-draining soil in a mild district leaving them in works most years; on cold wet clay it does not, and the tubers rot rather than freeze.

Position: Full sun
Water: high demand
Drainage: Free-draining, Moisture-retentive
Grows to: 45–150 cm × 45–90 cm
Hardiness: H3
Harvest: July to October
Difficulty: Some care needed

August

In flower · Harvest

Dahlia’s year

Resolved for United Kingdom (general). Every window is stored against a climate event rather than a date, so a late frost pocket runs later than the band shows. What else August calls for.

Activity	A	M	J	A	S	O	N
Plant	April.	May.
In flower			July.	August.	September.	October.
Harvest			July.	August.	September.	October.
Protect						October.	November.

Timing that travels

The band above resolves this plant's windows into months for one region. These are the windows themselves, stored against a climate event rather than a date — which is the form that survives being carried to a colder garden, a frost pocket, or next year.

Conditions it needs

Light: Full sun
Frost: Half hardy
Hardiness rating: H3 (RHS)
Grows best at: 15–25 °C
Soil: Loam, Sandy
Soil pH: 6–7.5
Drainage: Free-draining, Moisture-retentive
Water needs: high
Drought: poor tolerance
Waterlogging: poor tolerance
Heat: good tolerance
Wind and exposure: poor tolerance

Where these figures come from

LightSourced guidancehigh confidence

Royal Horticultural Society: Dahlia — position

Checked 2026-08-13

FrostSourced guidancehigh confidence

Royal Horticultural Society: Dahlia — hardiness

Checked 2026-08-13

Hardiness ratingSourced guidancemoderate confidence

Royal Horticultural Society: RHS hardiness ratings

Checked 2026-08-13

Grows best atSourced guidancemoderate confidence

Royal Horticultural Society: Dahlia — temperature

Checked 2026-08-13

SoilSourced guidancemoderate confidence

Royal Horticultural Society: Dahlia — soil

Checked 2026-08-13

Soil pHSourced guidancemoderate confidence

Plant Finder — Missouri Botanical Garden: Dahlia — culture

Checked 2026-08-13

DrainageSourced guidancemoderate confidence

Royal Horticultural Society: Dahlia — soil

Checked 2026-08-13

Water needsHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

DroughtHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

WaterloggingHorticultural consensusmoderate confidence

Tubers rot in cold wet ground, which is what kills those left in over winter.

No external citation — see the evidence type above.

Checked 2026-08-13

HeatHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Wind and exposureHorticultural consensusmoderate confidence

Tall cultivars need staking or they snap.

No external citation — see the evidence type above.

Checked 2026-08-13

On your soil

Computed by comparing this plant's recorded requirements against each soil's recorded characteristics. A recorded horticultural fact overrides the comparison where GrowingHQ holds one.

Soil	Verdict	Why
Heavy clay	Works, with one condition	Dahlia will crop on heavy clay, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. For a crop lifted before winter the risk is not rot but the season: waterlogged ground is cold and unworkable in spring, so sowing is late and germination patchy.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.
Sandy	Works, with one condition	Dahlia will crop on sandy, but one thing has to be dealt with first. It has little drought tolerance and this soil dries out fast — it will check and bolt in the first dry spell without watering.Water and mulch will carry it through. On this soil the watering is not optional in a dry spell, which is the difference between a crop and a failure.
Chalky and alkaline	Works, with one condition	Dahlia will crop on chalky and alkaline, but one thing has to be dealt with first. It has little drought tolerance and this soil dries out fast — it will check and bolt in the first dry spell without watering.Water and mulch will carry it through. On this soil the watering is not optional in a dry spell, which is the difference between a crop and a failure.
Loam	Prefers this soil	Loam suits Dahlia as it stands: The pH suits it: it wants 6 to 7.5, and this soil is in that range. The free drainage is exactly what it wants. Loam is among the soils it is recorded as suiting. The soil holds moisture, which saves a great deal of watering.
Silty	Prefers this soil	Silty suits Dahlia as it stands: The pH suits it: it wants 6 to 7.5, and this soil is in that range. The free drainage is exactly what it wants. The soil holds moisture, which saves a great deal of watering.
Peaty and acid	Works, with one condition	Dahlia will crop on peaty and acid, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. For a crop lifted before winter the risk is not rot but the season: waterlogged ground is cold and unworkable in spring, so sowing is late and germination patchy.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.
Shallow over rock	Works, with one condition	Dahlia will crop on shallow over rock, but one thing has to be dealt with first. It has little drought tolerance and this soil dries out fast — it will check and bolt in the first dry spell without watering.Water and mulch will carry it through. On this soil the watering is not optional in a dry spell, which is the difference between a crop and a failure.
Made ground and urban soil	Works, with one condition	Dahlia will crop on made ground and urban soil, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. For a crop lifted before winter the risk is not rot but the season: waterlogged ground is cold and unworkable in spring, so sowing is late and germination patchy.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.

Not sure which you have? Four hand tests will settle it.

Space and spacing

Mature height: 45–150 cm
Mature spread: 45–90 cm
Rooting depth: 40 cm
Spacing: 45–75 cm apart

Where these figures come from

Mature heightHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Mature spreadHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Rooting depthHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
SpacingSourced guidancemoderate confidence: Royal Horticultural Society: Dahlia — spacing
Checked 2026-08-13

Measured centre to centre. Plants set closer than this compete for light and water long before they look crowded, and the crowding cannot be undone once they are in.

In a container

Minimum volume: 20 litres or more
Minimum depth: 30 cm

Where the container figures come from

Minimum volumeHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Minimum depthHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13

Grows well in a large pot and the tubers can be lifted and stored dry over winter, or the whole pot moved somewhere frost-free. Feed with a high-potassium feed once flowering starts.

Habit and ultimate size

What the plant becomes, rather than what it is when you buy it. The gap between the two is where most planting mistakes live.

In a container: Good in a container long-term

Where these figures come from

In a containerHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13

All four are sold as 'bulbs'. The differences are internal, which is why they are drawn rather than photographed — and each one changes what you physically do with the thing in your hand.

Storage organ: Tuberous root
Survives winter in the ground: No — lift and store, or treat as annual
Planting depth: 10–15 cm

Where these figures come from

Storage organSourced guidancehigh confidence: Royal Horticultural Society: Dahlia — propagation
Checked 2026-08-13
Survives winter in the groundSourced guidancehigh confidence: Royal Horticultural Society: Dahlia — hardiness
Checked 2026-08-13
Planting depthSourced guidancemoderate confidence: Royal Horticultural Society: Dahlia — planting depth
Checked 2026-08-13

A swollen root rather than a stem, so the growth buds are only where it joins the old stem. It cannot be divided into arbitrary pieces the way a tuber can.

Two to three times the bulb's own height is the working rule, and the four plants that break it are the four most often planted wrong. A bearded iris buried by mulch or a tidy neighbour will make leaves for years and never flower again.

Lifting and storing. Lift after the first frost blackens the foliage, cut the stems back, dry the clump upside down for a week or two and store it somewhere frost-free and barely moist. In a mild garden on free-draining soil a deep mulch will often carry it through instead, and increasingly does — but a wet winter kills more stored dahlias than a cold one, at either end of the decision.

source

Horticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

After flowering. Deadhead relentlessly through the season. A dahlia that is allowed to set seed slows down and stops, and the difference between a deadheaded plant and a neglected one is several weeks of flowering.

source

Horticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Where and how you can grow it

Each way of growing is assessed separately, and “not assessed” is a real answer rather than a gap we have filled in with a guess.

Method	Suitability	What it needs
Open ground	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Traditional dug bed	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Raised bed	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Container	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13	20 litres; Staking for tall cultivars
Greenhouse	Workable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Hydroponics	Not suitable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13

Growing it

ProtectionDecide by soil rather than by district. On free-draining soil, cut down after the first frost and mulch thickly; the tubers usually come through. On cold heavy clay, lift them, dry them off and store somewhere frost-free — the risk there is rot rather than freezing, and a mulch makes rot more likely rather than less.
PruningDeadhead continuously and cut flowers freely. Dahlias flower harder the more they are picked, and a plant left to set seed slows down noticeably.
WateringWater sparingly after planting the tubers until growth appears — a tuber sitting in cold wet soil rots rather than sprouting. Once the plants are growing, water generously and regularly through summer, because dahlias carry a great deal of soft growth and flower continuously.
FeedingFeed with a high-potassium liquid feed every fortnight from midsummer until the first frosts. A high-nitrogen feed produces very large plants with soft stems that snap and comparatively few flowers, which is the usual result of using a general-purpose feed on them.

Where this guidance comes from

ProtectionSourced guidancehigh confidence: Royal Horticultural Society: Dahlia — overwintering
Checked 2026-08-13
PruningSourced guidancehigh confidence: Royal Horticultural Society: Dahlia — care
Checked 2026-08-13
WateringSourced guidancehigh confidence: Royal Horticultural Society: Dahlias — watering
Checked 2026-08-13
FeedingSourced guidancehigh confidence: Royal Horticultural Society: Dahlias — feeding
Checked 2026-08-13

The bud you cut to becomes the new shoot, and it grows in the direction it points — which is why an outward-facing bud opens the plant up and an inward-facing one crosses the middle.

Harvesting

When it is readyCut blooms from July until the frosts, choosing flowers that have opened fully — a dahlia cut in tight bud will not open in water. Cut early in the morning. The other harvest is the tubers: lift those a week or two after the first frost has blackened the foliage, which is the signal that the plant has finished moving reserves down into them.
How to pick itCut flower stems long, just above a leaf joint, and stand them in deep water straight away. For tubers, cut the stems down to about 15 cm, fork the clump up from well outside it, wash the soil off and stand the clump upside down for a fortnight so the hollow stems drain — a clump stored with water in the stem rots from the crown outwards.
Keeping itStore the dried tubers in trays of just-damp compost, sand or vermiculite in a frost-free shed or garage, with the crowns exposed. Check them once a month and cut out any soft patches. Tubers stored bone dry shrivel and tubers stored damp rot, so the compost should feel barely moist and no more.

Where the harvest guidance comes from

When it is readySourced guidancehigh confidence: Royal Horticultural Society: Dahlias — cutting and lifting
Checked 2026-08-13
How to pick itSourced guidancehigh confidence: Royal Horticultural Society: Dahlias — lifting and storing
Checked 2026-08-13
Keeping itSourced guidancehigh confidence: Royal Horticultural Society: Dahlias — overwintering
Checked 2026-08-13

What goes wrong

Problem	How often	Notes
Capsid bug	common source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13
Slugs and snails	common source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13	The emerging shoots in spring are among the most slug-vulnerable things in a garden.
Aphids	occasional source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13
Powdery mildew	occasional source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13

Sources for this page

GrowingHQ editorial synthesis · Plant Finder · Royal Horticultural Society

Each figure on this page carries its own citation, under the “where these figures come from” note in its section. Record last reviewed 2026-08-14.

If something is wrong with it

Holes eaten in leaves — 2 causes GrowingHQ holds for dahlia

Something else? Work through it with the diagnostic tool.