Dividing and moving perennials in late summer — when, which, and how to do it right

Late summer and early autumn are the most intense moment for dividing and moving perennials. German gardeners say it plainly: „August ist die optimale Zeit, Stauden zu teilen“ — August is the best time to divide perennials. In the US the same movement runs under „garden refresh“: moving, dividing, planning the border for the new season. If this year a clump has bloomed poorly, is dying in the middle or has overwhelmed its neighbours — now is the window to fix it without risk.

Why this moment

Perennials are divided twice a year: in spring (March–April, as shoots just emerge) and in late summer — from mid-August to mid-September. This second window has two strong points. First, the plant is past its main flowering and all the energy is going back into the roots. Second — and this matters most — there are still 6–8 weeks of warm, moist soil to grow new roots before dormancy. A perennial moved in October overwinters poorly rooted and either dies in the frost or gets off to a weak start in spring.

Watering in September is critical here: a freshly divided perennial has no full root system yet. The rules for how much water are gathered in the watering guide, and the timing of jobs across the whole season in the garden calendar.

Perennials that welcome division now

Rule of thumb: clump-forming species that flower in spring or early summer (after flowering), tolerate division very well.

• Daylily — the classic division candidate. Divide with garden forks (not a spade), split into 3–5 „fans“ with roots; it flowers the following summer.
• Siebold’s plantain lily (hosta) — divide in September, just as the leaves start to fade. Large clumps often have a „dead“ crown in the middle — cut it out and replant the outer sections.
• Heart-leaved bergenia — the rhizome creeps outward; cut off the outer, strong pieces with leaves and throw the middle on the compost.
• Bearded iris — divide every 3–4 years, right after flowering (late July – August). Plant the rhizomes shallowly, half above the ground — otherwise they will not flower.
• Garden phlox and purple coneflower — divide every 3–4 years at the turn of August and September. This rejuvenates the clump and reduces mildew on phlox (better air flow).

Perennials better divided in SPRING

Rule: late-autumn-flowering species and those sensitive to root disturbance in the cold are left for March–April.

• New York aster — divide in spring, as soon as new shoots emerge. Divided in autumn it goes into frost in mid-bloom — that usually means lost flowers and weak rooting.
• Garden chrysanthemums — same, wait until spring.
• Ornamental grasses (e.g. Chinese silver grass) — divide only in spring, when new shoots start. Divided in autumn they often fail in winter.

Peony, oriental poppy and the „perennial you shouldn’t touch“

Three categories need special care:

• Chinese peony — divide only at the turn of August and September and only if you MUST. A peony planted too deep (eyes >5 cm below the soil) will not flower for years — plant it shallowly. After division it often waits 2–3 seasons before returning to form.
• Oriental poppy — its long tap root does not tolerate disturbance; if you must move it, do so in August, right after the foliage dies down, and dig as deep as you can.
• Perennials freshly planted this year — simply do not divide. Wait a year or two until they settle in.

How to divide — step by step

1. Water the plant a day before. A dry root ball crumbles, roots tear, the plant loses more than it needs to.
2. Dig up the whole clump, cutting the roots vertically with a spade in a 20–30 cm radius around the shoots.
3. Split with garden forks, not a spade. Push two forks back-to-back into the middle of the clump and lever them apart — roots separate along natural joints instead of shredding.
4. Discard the middle of the old, woody clump. Healthy young pieces from the outside are your new material.
5. Cut the foliage back by 1/3–1/2, to reduce evaporation until new roots take hold.
6. Plant at the same depth as before (exceptions: iris — shallow, peony — shallow), feed the planting hole with compost (see fertilising guide), water thoroughly.
7. Mulch with 5–8 cm of bark or wood chips — it holds moisture during rooting and shelters young roots from the first frost.

Dividing is a practically free way to fill up a border, rejuvenate old clumps and get rid of „doughnut“ perennials with dead middles. Use the August–September window, close the winter jobs following the guide to preparing the garden for winter — and in spring the border starts up with the vigour of freshly planted stock, only for free.