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Most perennials are planted for the size they will reach in three years. Japanese anemone has to be planted for the area it will occupy in ten. A single plant does very little the first season, thickens in the second, and from the third year onward sends horizontal shoots out through the soil and pushes up new crowns a foot or more away from the original one. Left alone in soil it likes, one plant becomes a colony six feet (1.8 m) across, and it keeps going.
That is not a fault. It is the reason people grow it: a late colony of pale flowers held above everything else in autumn is worth a lot, and few plants fill that gap. But it means the planting decision is really a containment decision. Put it where the spread is an advantage, in a wide bed, at the back of a border, in the rough ground under shrubs, along a boundary, and it is one of the easiest plants you will ever grow. Put it in a tidy four-foot bed next to slow, precious neighbors and you will be fighting it every spring for the rest of the plant's life.
The other half of that decision is how hard it is to undo. Every piece of root left behind when you dig it out grows into a new plant, including fragments the width of a pencil lead, so there is no clean removal and the position you choose now is effectively permanent.
| Position | How it behaves | Verdict |
|---|---|---|
| Wide border, back rank, among shrubs | Runs freely, shrub roots slow it, stems lean on the shrubs | Ideal |
| Bed edged by mown lawn or paving | Runners that cross the edge are cut off by mowing | Very good, self-limiting |
| Narrow bed with slow, low neighbors | Swamps them within three or four years | Avoid |
| Full sun with dry soil | Leaf margins scorch, growth stalls, few flowers | Avoid |
| Deep dry shade under a mature tree | Survives, spreads slowly, flowers thinly | Tolerated, not good |
| Large container, 15 in (38 cm) or more | Contained, but needs watering all summer and dividing every third year | Workable compromise |
Soil: the one condition it will not compromise on
Japanese anemone is easy about light and fussy about summer moisture. It will take full sun, half a day of sun, or open shade, but in every one of those positions it wants soil that does not dry out between midsummer and the end of flowering. That is exactly the stretch when it is building flower stems, so drought at that moment costs you the display.
In practice, sun and moisture trade off against each other. In soil that stays reliably damp, it will take a good deal of direct sun and flower heavily. In light, sandy or hungry soil, or over the roots of a large tree, it needs shade from the hottest part of the afternoon or the leaf edges brown and curl and the flower count drops. If you have a choice, morning sun with afternoon shade suits it in almost any garden.
Improve the soil over a wide area rather than in a single hole. Fork two to three inches (5 to 8 cm) of garden compost or well-rotted manure into the top eight inches (20 cm) of a patch at least three feet (90 cm) across. The plant will be rooting through all of it within two seasons, and organic matter worked in now is the cheapest insurance against a dry summer.
The one thing to avoid is ground that sits wet all winter. Cold is rarely what kills this plant; a waterlogged dormant crown is. On heavy clay, plant on a slight rise, or work in grit along with the compost.
If your soil is heavy clay that bakes hard in summer and puddles in winter, plant in spring rather than autumn and mulch heavily. Clay that has been improved with organic matter is one of the best soils for this plant; unimproved clay is one of the worst.
Planting, step by step
Spring planting is safer than autumn planting. A plant set out as growth is starting has the whole season to put roots down before its first winter, and winter losses of young plants in cold wet soil are common. If you plant in autumn, do it early enough that the roots move before the soil goes cold, and keep the crown clear of a thick wet mulch.
- Water the plant in its pot first Stand the pot in water until bubbles stop, half an hour or so. A dry rootball planted into moist soil often stays dry, because water runs around the outside of it rather than into it. This is the single most common reason a new plant fails in its first summer.
- Dig wide and shallow, not deep Make the hole twice the width of the rootball and no deeper than it. Loosen the sides so the roots are not sitting in a smooth-walled pit. Depth matters less than width here, because the plant spreads horizontally in the top six to eight inches (15 to 20 cm) of soil.
- Set the crown level with the surface The point where the shoots meet the roots should sit at soil level or a fraction above, never buried. Buried crowns rot over winter. Backfill with the soil you took out, mixed with a couple of handfuls of compost, and firm it with your hands rather than your heel.
- Space at 18 to 24 inches (45 to 60 cm) That looks absurdly generous in the first season and about right in the third. Do not close the gaps with anything you want to keep long term, because the anemone will occupy the space between the plants within two years. Annuals in the first two summers are the sensible filler.
- Water thoroughly, then mulch Give each plant a full gallon (4 to 5 liters) at planting, slowly, so it soaks in rather than running off. Then mulch two inches (5 cm) deep with compost or bark, keeping the material an inch clear of the crown. Water deeply once a week through the first summer whenever there has been no real rain.
Containment is decided now, not later
If the position has a hard edge already, paving, a mown lawn, a gravel path you rake, you need nothing else. Runners that cross into mown grass are cut off every time you mow, and the colony holds its shape by itself. This is by far the easiest way to grow it.
If it does not, install a barrier at planting, when it costs twenty minutes rather than a weekend of digging. Sink a rigid vertical barrier, heavy pond liner or a bottomless container, into the soil around the planting area so the top inch stands proud and the bottom sits ten to twelve inches (25 to 30 cm) down, below the depth the horizontal shoots travel at. Leaving the rim proud matters, because shoots that reach a flush barrier run over the top of it.
The alternative is an annual chore: once a year in spring, push a spade vertically all the way around the clump at the line you want to hold, and pull out anything that has crossed it. Ten minutes a year, done consistently, holds the colony indefinitely. Skipped twice running, it does not.
The first season, and what to record
Expect a disappointing first year. A newly planted Japanese anemone typically makes a low clump of leaves, throws up one or two short flower stems, or none at all, and gives no sign of the plant it will become. This is normal and is not a sign that anything is wrong. The second year is noticeably better. The third year is the plant you were promised. Digging it up in year one because it sulked is the most common mistake people make with it.
Write down the planting date, the exact position, and how far the crown sits from a fixed reference such as a fence post or a path edge. That last measurement is the one you will want. Three years on, when you are deciding whether the colony has moved, a note saying the crown was 30 inches (75 cm) from the post settles the question in seconds, and guesswork does not.
Note the date the first flower opened and the date the last one faded as well. After two seasons of records you will know exactly which weeks of your own autumn this plant covers, which is what you need when deciding what to plant alongside it.