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Spiraea flowers in proportion to the light it gets. Six hours of direct sun is the working minimum and eight or more gives the solid sheets of flower the shrub is grown for. In part shade it stays alive and leafy, throws long weak stems reaching for light, and flowers thinly along the top edge only. The colored-leaf summer types lose their color in shade as well, greening off to a dull mid-green that is not what anyone planted them for.
Judge the site in full leaf, not in early spring. A bed under deciduous trees can look bright while the canopy is bare and be deep shade by early summer, which is precisely when the summer-flowering types are making the growth that will carry their flowers. For spring types the critical light is the previous summer's light, because that is when next spring's buds are formed, so the same test applies.
In very hot climates, afternoon shade is acceptable and sometimes better than full exposure, particularly for the golden-leaved summer forms, whose foliage scorches at the edges in relentless sun and dry wind. Morning sun with shade after the middle of the afternoon is a reasonable compromise there. Everywhere else, give it the open position.
Soil, drainage and spacing
Spiraea is genuinely undemanding about soil. It grows in clay, in sand, in slightly acidic and slightly alkaline ground, and it copes with poor fertility. The one thing it will not tolerate is standing water. Ground that stays saturated for days after rain causes root rot and slow dieback, and no amount of feeding disguises it. If in doubt, dig a hole a foot (30 cm) deep, fill it with water and see how long it takes to drain. Several hours is fine; overnight is a problem, and that site needs a raised bed or a different plant.
Spacing is the decision people regret. Spiraea does not grow as a single trunk, it suckers and thickens from the crown, so its footprint widens every year until it fills the space available. Read the mature width for the type you have and plant at that distance from walls, paths and neighboring shrubs rather than at the width it is on planting day. A young plant in a pot looks absurdly small against a spacing of four feet (1.2 m) and it will close that gap in two or three seasons.
For a low informal hedge or a mass planting, space at roughly two-thirds of the mature width so the plants knit together but each still has a crown you can reach into. Packing them tighter than that makes renewal pruning impossible, because you cannot get to the base of any individual plant, and the whole run degenerates into one shared thicket of old wood.
| Situation | Spacing between plants | Distance from a wall or path | Notes |
|---|---|---|---|
| Compact summer type in a bed | 24-36 inches (60-90 cm) | 24 inches (60 cm) | Kept small by the annual late-winter cut |
| Arching spring type as a specimen | 60-84 inches (1.5-2.1 m) | 48 inches (1.2 m) | Needs room to arch or the shape is lost |
| Low informal hedge or mass planting | 24-30 inches (60-75 cm) | 30 inches (75 cm) | Two-thirds of mature width so crowns stay reachable |
| Container | One plant per pot | Not applicable | Minimum 14 inches (35 cm) wide, drainage holes clear |
Setting the plant
The best planting seasons are autumn and early spring, when the soil is workable and the plant can root before it has to support a full canopy in heat. Container-grown plants can go in at almost any time if you are willing to water, but a shrub planted in the middle of summer will need attention every few days for weeks, and one planted in autumn will barely need you at all.
Dig wide rather than deep. A hole two to three times the width of the rootball and no deeper than the rootball itself is right. Planting too deep is a common cause of a shrub that sulks for two years: the crown, where all those new basal shoots come from, needs to sit at or a fraction above the surrounding soil level, not buried.
Container plants often come out with the roots circling the inside of the pot. Tease those out or slice down the sides of the rootball in three or four places with a knife before planting. Roots left circling will keep circling, and the plant never anchors properly. Backfill with the soil that came out of the hole rather than a rich imported mix, which only encourages the roots to stay inside the hole.
- Soak the rootball first Stand the pot in water until bubbles stop rising, or water it heavily an hour before planting. A dry rootball planted into moist soil stays dry, because water travels around it rather than into it.
- Check the depth with a straight edge Lay a cane or spade handle across the hole. The top of the rootball should sit level with it or a fraction proud, allowing for the soil settling an inch (2-3 cm) after watering.
- Backfill in stages and firm gently Add soil in two or three layers, firming each with your hands rather than stamping. Leave no large air pockets against the roots, but do not compact the ground into a solid pan.
- Water in and mulch, keeping the crown clear Give at least two gallons (8 liters) slowly so it soaks rather than runs off. Mulch two to three inches (5-8 cm) deep over the root zone, pulled back a hand's width from the stems so the crown stays dry and open.
The first year
Water is the only serious task in year one. A newly planted spiraea has its roots confined to the volume of the old pot until it pushes out, so it dries out far faster than the surrounding soil suggests. Water deeply once or twice a week through the first growing season rather than sprinkling daily, and check by pushing a finger into the soil at the edge of the rootball rather than judging by the surface.
Do not feed at planting and do not feed in the first season. Spiraea grows quickly in ordinary soil and fertilizer on a plant that has not yet rooted out produces soft growth the roots cannot support. If the soil is genuinely poor, an inch (2-3 cm) of compost spread on the surface in spring does everything that is needed.
Expect very little flower in the first year and do not read anything into it. The plant is spending on roots. Structural pruning should also wait: remove damaged or dead shoots at any time, but leave the shape alone until the shrub has been in the ground a full year and you have seen when it flowers.
Record the planting date, the position in the bed, and the flowering weeks that first season. The flowering dates settle which pruning group the shrub belongs to, which is the single most useful fact about it, and the planting date lets you judge whether slow establishment is a problem or simply the normal first two years.
If the shrub arrives bare-root in the dormant season, get it into the ground within a day or two and keep the roots covered and damp until you do. Bare-root spiraea establishes very well, but the roots have no tolerance for drying out in the wind while you decide where to put it.