Moving or dividing perennials in the autumn is a great way to reduce your work next spring. The cool, moist weather is an ideal time for perennial roots to become well established, even in cold-winter regions.Gardeners often ask us when the best season is to move specific perennials, so we have a working "rule of thumb" for timing. John's Rule-of-Thumb for when to move or divide perennials:
- If the plant blooms between early spring and late June, then early fall division/moving is ideal.
- If the plant blooms after late June, then early spring division is ideal.
- fall-blooming ornamental grasses usually remain gorgeous well into the winter. It seems a real shame to cut them back to the ground before late winter or early spring. Some gardeners are now waiting even beyond THAT, and enjoying the effect of wheat-colored grass clumps contrasting with spring-flowering bulbs!
- seed-heads of certain perennials provide food for finches and other birds, and they look great against a blanket of snow. Most late-flowering daisy-type perennials are on this list (like Rudbeckia and Purple Coneflower), but others with nice seed-heads and sturdy stems include: Achillea, Agastache, Aster, Astilbe, Baptisia, Buddleia, Chelone, Cimicifuga, Eryngium, Eupatorium, taller Sedum, and a few others.
- there is a common theory that the dead tops of perennials help to trap the snow, which is the very best insulation against cold temperatures. In regions with erratic snowcover and mid-winter thaws, the tiny bit of extra snow that is actually trapped may in fact be of little benefit.
- many perennials have very little winter interest. Cutting these types back in the fall effectively "clears the clutter" and makes the ones you leave look even better. Consider cutting these down in late fall: Alchemilla, Anemone, Campanula, Centaurea, Coreopsis, Delphinium, Dicentra, Euphorbia, Geranium, Hemerocallis, Hosta, Lychnis, Monarda, Nepeta, Oenothera, Phlox (tall types), Trollius, Veronica.
- Autumn-flowering ornamental grasses, if you live in Zones 4, 5 or 6 and they were planted after the beginning of August.
- Japanese Anemones, if you live in Zones 4, 5, or 6 and they were planted after mid July. Probably only required for the first winter.
- You are trying to grow any perennials rated one or more Zones warmer than your region. e.g. you live in Zone 4 and are trying to grow a Zone 5 or 6 plant.