Ornamentalshrub ornamentalStraightforward

Spiraea

Spiraea japonica

The exception in this group, and the reason to read the label. Japanese spiraea flowers in mid-summer on growth made the same year, so it is cut hard back every spring — the exact treatment that would ruin a forsythia or a weigela. The gold-leaved forms are grown as much for foliage as flower.

Position: Full sun or Partial shade
Water: moderate demand
Drainage: Moisture-retentive, Free-draining
Grows to: 60–150 cm × 60–150 cm
Hardiness: H7
Flowers: June to August
Difficulty: Straightforward

Described view

A summer-flowering Spiraea japonica cut to a low framework in spring beside a spring-flowering Spiraea arguta pruned after its flowers

August

In flower

Spiraea’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	F	M	A	J	J	A	O	N
Plant		March.	April.				October.	November.
In flower				June.	July.	August.
Prune	February.	March.

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, Partial shade
Frost: Very hardy
Hardiness rating: H7 (RHS)
Grows best at: 12–27 °C
Soil: Loam, Clay, Sandy, Chalky
Soil pH: 5.5–7.5
Drainage: Moisture-retentive, Free-draining
Water needs: moderate
Drought: moderate tolerance
Waterlogging: moderate tolerance
Heat: good tolerance
Wind and exposure: good tolerance

Where these figures come from

LightSourced guidancemoderate confidence: Royal Horticultural Society: Spiraea — position
Checked 2026-08-13
FrostSourced guidancehigh confidence: Royal Horticultural Society: Spiraea — 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: Spiraea — temperature
Checked 2026-08-13
SoilSourced guidancemoderate confidence: Royal Horticultural Society: Spiraea — soil
Checked 2026-08-13
Soil pHSourced guidancemoderate confidence: Plant Finder — Missouri Botanical Garden: Spiraea japonica — culture
Checked 2026-08-13
DrainageSourced guidancemoderate confidence: Royal Horticultural Society: Spiraea — 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: 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: 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	Spiraea will crop on heavy clay, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. A plant that stays in the ground through winter with its roots in cold, saturated soil rots — which is what kills most of what fails on heavy ground, rather than the cold.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	Prefers this soil	Sandy suits Spiraea as it stands: The pH suits it: it wants 5.5 to 7.5, and this soil is in that range. The free drainage is exactly what it wants. Sandy soil is among the soils it is recorded as suiting.
Chalky and alkaline	Grows well here	Spiraea grows perfectly well on chalky and alkaline without special measures, though it is not the soil it would choose.
Loam	Prefers this soil	Loam suits Spiraea as it stands: The pH suits it: it wants 5.5 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 Spiraea as it stands: The pH suits it: it wants 5.5 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	Spiraea 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. A plant that stays in the ground through winter with its roots in cold, saturated soil rots — which is what kills most of what fails on heavy ground, rather than the cold.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	Prefers this soil	Shallow over rock suits Spiraea as it stands: The free drainage is exactly what it wants. Loam is among the soils it is recorded as suiting.
Made ground and urban soil	Works, with one condition	Spiraea 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. A plant that stays in the ground through winter with its roots in cold, saturated soil rots — which is what kills most of what fails on heavy ground, rather than the cold.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: 60–150 cm
Mature spread: 60–150 cm
Rooting depth: 45 cm
Spacing: 75–120 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: Spiraea — 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.

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.

Foliage: Deciduous
In a container: Good in a container long-term

Where these figures come from

FoliageSourced guidancehigh confidence: Royal Horticultural Society: Spiraea japonica — foliage
Checked 2026-08-13
In a containerHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13

Growth rate: Fast — over about 60 cm a year
Ultimate height: 60–150 cm
Ultimate spread: 60–150 cm
Time to reach it: 5–8 yearsSourced guidance

Where these figures come from

Growth rateSourced guidancemoderate confidence: Royal Horticultural Society: Spiraea japonica — growth rate
Checked 2026-08-13
Ultimate heightSourced guidancemoderate confidence: Royal Horticultural Society: Spiraea japonica — ultimate height
Checked 2026-08-13
Ultimate spreadSourced guidancemoderate confidence: Royal Horticultural Society: Spiraea japonica — ultimate spread
Checked 2026-08-13
Time to reach itSourced guidancelow confidence: Royal Horticultural Society: Spiraea japonica — time to ultimate size
Checked 2026-08-13

Cutting it back. Expects to be cut to 30 cm every spring and flowers better for it, because the flowers come on the current season's growth. The only shrub in this tranche where the instinctive spring tidy-up is the correct treatment rather than the destructive one.

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

Growing it

PruningCut the whole plant back to about 30 cm every spring. Spiraea japonica flowers on the current season's growth, so hard pruning increases the flowers rather than removing them, and produces the brightest foliage on the gold-leaved forms. This is the one shrub in this tranche where the spring tidy-up is exactly right.
Note that the spring-flowering spiraeas — S. 'Arguta' and similar, which make arching white sprays in April — flower on last year's wood and must be pruned after flowering instead. Two plants sold under one genus name with opposite requirements, and the label rarely says which.

Where this guidance comes from

PruningSourced guidancehigh confidence: Royal Horticultural Society: Spiraea japonica — pruning
Checked 2026-08-13
Item 2Sourced guidancehigh confidence: Royal Horticultural Society: Spiraea — pruning groups
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.

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.