Planting Thunberg Spirea: Sun, Spacing and Room for the Arch

Sun decides how much white you get

Thunberg spirea flowers on wood that ripened the previous summer, so the light it receives in summer is what buys the display the following spring. Six hours of direct sun is the working minimum and eight or more is where the shrub earns its reputation, with every wiry stem sheathed in small white clusters before a single leaf opens. In part shade the plant stays alive, keeps its graceful shape and flowers thinly along the top third only, and no amount of feeding or watering will make up the difference.

Judge the site at the height of summer, not while the shrub is in bloom. A corner that is bright and open in late winter, then shaded once the deciduous trees overhead leaf out, is a shaded site as far as bud set is concerned. This trips people up constantly, because the shrub looks perfectly happy in leaf and simply produces less flower each year as the canopy above it closes in.

Wind is not a problem. The stems are thin, tough and whippy, and the fine foliage passes air rather than catching it, so this is one of the better shrubs for an exposed boundary or a coastal garden. Late frost is not a problem either in the sense that matters here: the flowers open early, and a hard frost can brown the open blossom for that season, but it does not damage the plant or its wood.

Poor soil is fine, wet soil is not

This is a shrub for thin, hungry, stony ground. It grows on gravelly banks, on the raw subsoil left behind by building work, on chalky ground and on acid sand, and it tolerates a soil pH anywhere from about 5.5 to 7.5 without complaint. Ordinary garden soil suits it perfectly. Rich, heavily manured ground actually works against you, pushing long soft stems that flop and set fewer buds.

The one condition it will not accept is ground that stays saturated. Roots sitting in airless wet soil through a mild winter rot, and the first sign is usually a section of the shrub that fails to leaf out in spring while the rest looks normal. If water stands in the planting hole for more than a few hours after you fill it, plant on a raised mound six to eight inches (15 to 20 cm) high rather than trying to improve the hole itself.

Established plants are genuinely drought tolerant. The fine foliage loses little water and the root system runs wide and shallow, so once the shrub is through its second summer it will carry a normal dry spell without wilting. That toughness is the main reason it ends up on banks, in parking strips and along drives where nothing gets watered.

Allow for the spread, and for the suckers

A mature Thunberg spirea stands three to five feet (0.9 to 1.5 m) tall and spreads four to six feet (1.2 to 1.8 m) across, wider than it is high. The outline is a fountain: dozens of slender stems rising from the base and bending outward until the tips almost touch the ground. That shape is the plant. Boxed between a path and a wall, the same shrub becomes a sheared green mound that flowers on its outer skin, and you have spent money on a plant to then remove the reason for buying it.

It also suckers, slowly. New shoots push up from the root system a few inches outside the existing crown each year, and over a decade a single plant thickens into a low thicket a foot or two wider than it started. This is slow, shallow spread from the crown outward, not a runner that appears across the lawn, and it is easy to hold in check with an edging iron once a year. Allow for it rather than fighting it: the thicket habit is exactly what makes this a good informal hedge or bank cover.

For a continuous informal hedge, set plants three feet (0.9 m) apart and expect them to close into an unbroken band in three or four seasons. As a single specimen, give the shrub its full mature width plus two feet (0.6 m) of clear ground all round so you never have to cut into it for access.

Setting the plant

The best planting window is autumn, once the leaves have colored and dropped and the soil is still warm, because the roots move while the top is dormant and the shrub is established before it meets its first summer. Late winter to very early spring, before the buds swell, is the second choice. Container-grown plants can go in during the growing season if you commit to watering them through the first dry spell, but you will lose a season of growth to the check.

Dig a hole two to three times the width of the root ball and no deeper than the root ball is tall. Width matters because the roots run out sideways in the top foot of soil; depth is where people go wrong, since backfill settles and a shrub set two inches low ends with its crown buried, which rots the base and pushes odd suckers from below ground.

Backfill with the soil you dug out, not with a bag of rich compost. A pocket of luxury soil inside ordinary ground teaches the roots to circle in the pocket instead of running out into the surrounding ground, and this shrub has no need of the extra fertility in any case.

1. Break the sides of the hole In clay, run a fork down the walls to score the smeared surface a spade leaves behind. A glazed hole in clay holds water like a bucket and will drown the roots over a wet winter.
2. Tease out the root ball If roots are circling the pot, cut down through them in three or four places with a knife and pull the outer inch (2.5 cm) loose. Circling roots stay circling and strangle the shrub years later.
3. Set the depth with a straight edge Lay a cane across the hole and set the top of the root ball level with it, or a shade proud. Never below. Buried crowns are the single most common cause of a young spirea failing.
4. Water in heavily, then mulch Give five to ten gallons (20 to 40 liters) slowly so it soaks in rather than running off, then mulch two inches (5 cm) deep over the root area, pulled back a hand's width from the stems.
5. Cut nothing but damage Do not tidy a newly planted shrub into shape. Remove broken or dead wood only. Every stem you shorten is flower buds you will not see, and the plant needs its full leaf area to root out.

The first two seasons, and what to write down

Water deeply once a week through the first growing season whenever a week passes without an inch (25 mm) of rain, and keep going into autumn if the season stays dry. One long soak beats three light sprinklings, because it wets the full depth of the root ball and pulls the roots downward. From the second summer onward you can largely stop; by the third year the shrub is on its own except in a genuine drought.

Do not feed at planting and do not feed through the first summer. Thunberg spirea grows readily on poor ground, and nitrogen simply lengthens the stems at the expense of the flower buds that are forming in those same weeks.

Write down the planting date, the exact position measured from something permanent such as a fence post or a corner of the house, and the mature width you have allowed for. Note the size of the plant as it went in, and mark the outer edge of the crown so that in three years you can see how far it has crept outward. Add one line each spring recording when the first flowers opened and roughly how far down the stems the bloom reached. Those two notes together tell you, over a few years, whether the site is giving the shrub enough summer sun or whether a neighboring tree is quietly shading it out.

If you are planting where an existing shrub failed, check the depth of the old root crown before blaming the soil. A crown buried two or three inches below grade will kill a spirea slowly enough that people assume the site is bad when the planting was the problem.