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.
Exceptions to the rule are: Peonies (move/divide in fall only), Oriental Poppies (move/divide in August), Bearded Iris (move/divide in July through September) and true Lilies (move/divide in mid to late fall).
Of course, you can always break the rules and see what happens. Just remember that if you move or divide a big, bushy perennial always cut back the foliage by at least half to prevent serious wilting. This helps to keep the leaf mass in proportion to the reduced number of roots!
Once Autumn has truly arrived….
A couple of good, hard frosts makes a big difference in the garden. Some perennials immediately begin to go dormant, while others seem to want to hang on into late fall. To those new gardeners out there, we encourage you to consider leaving most perennials alone in the fall if you are unsure of what winter interest they might provide. It would seem a shame, for instance, to cut back those big, beautiful clumps of ornamental grasses in the fall, ruining any opportunity to hear them rustling in the winter winds, or to enjoy the contrast of their wheat-coloured stems against clean, fresh snow. Winter interest is entirely subjective, and only you can decide what is attractive to your eye, or what looks tired and messy.
Here are a few tips and ideas:
- 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.
Certain perennials naturally carry over a low clump of evergreen leaves near the ground, known as a “rosette“. Although you can trim the upright stems back, these lower leaves need to be left alone in the fall. By spring they often look a little worse for wear, but a quick trim with scissors (only the brown or dead parts) will tidy the plants up again. In this group are: Achillea, Aster, Coreopsis, Digitalis, Erigeron, Fragaria, Gaillardia, Geum, Heuchera, Bearded Iris, Shasta Daisies, Penstemon, Poppies, Polemonium, Potentilla, Salvia, Scabiosa, Stachys, Tiarella, Verbascum, and many of the hardy ferns.
Evergreen perennials and alpines should not be trimmed in the fall. Usually the best time to trim these is immediately after blooming, if at all. Leave these ones alone in the fall: Ajuga, Alyssum, Arabis, Armeria, Artemisia ’Powis Castle’ and ‘Huntingdon’, Aubrieta, Aurinia, Bergenia, Cerastium, Corydalis, Dianthus, Epimedium (trim in late winter, before new buds appear), evergreen Euphorbia, Helianthemum, Helleborus, Heuchera, Iberis, Kniphofia, Lamium, Lavender, Liriope, Origanum, Phlox (creeping types), Primula, Pulmonaria, Sagina, Saxifraga, Sedum (many creeping types), Sempervivum, Teucrium, Thymus, Viola.
Certain woody-stemmed perennials are better left alone in the fall, and pruned back in the spring, leaving about 6 inches of woody stem for the new buds to appear from. These include: Buddleia, Caryopteris, Erysimum ’Bowles’ Mauve’, Fuchsia, Hypericum, Lavatera, Perovskia (Russian Sage), Phygelius, Santolina.
And, finally, certain perennials with associated disease or insect problems should not only be cut back in the fall, but care should be taken to remove and destroy the leaf litter below them, where insects and pathogens may be hiding. Among these: Alcea (Hollyhocks), Aquilegia (Columbine), Crocosmia, Delphinium, Helenium, Heliopsis, Hemerocallis (Daylily), Iris (Bearded types, leave green leaves alone but remove all dead ones), true Lilies, Monarda, Peonies, Summer Phlox, Tricyrtis, and Veronica (tall types).
Late Fall mulching – do you need to bother?
If the perennials in your garden are mostly things that are rated hardy for your region or colder, the expense, time and effort that goes into late fall mulching is hard to justify, in many cases. The idea of a mulch is to add a layer of insulation on top of the soil, preventing sudden changes in soil temperature (from either deep freezing OR thawing), changes that can wreak havoc to the root systems of tender plants. Regions with reliable snow cover already enjoy the advantages of a natural snow mulch — nature’s insulation. However, where snow is unreliable, a late fall mulch can help in certain cases.
Consider trying it in these instances:
- 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.
Mulching materials should be organic matter that remains loose and won’t pack down to suffocate your plants. Good choices might be dried leaves (a mix of different types is best — not too heavy on the maple leaves), clean straw, chopped dead tops from other perennials, evergreen boughs from pruning, marsh hay (lucky you, if you have it available!).
Bad choices: peat moss, garden soil, newspaper, sheets of plastic or garbage bags. All of these have a smothering capability. Some gardeners are also reporting good success using various foam products available for this at garden centers.
Mulch can be simply piled high on top of your plants, but a depth of 6 to 8 inches or more is ideal. Those dandy mulch forms that are used on roses might be handy, or you can easily make your own cages from chicken wire.
A word about Hardy Mums
Hardy Mums — more aptly called Garden Mums these days, are something gardeners often ask us about. We thought it would be worth explaining a thing or two about the modern selection of Chrysanthemums and how they respond, so gardeners better understand them.
Basically, Mums are mostly now being bred to produce exuberant cushions of stunning, glorious colour in the containers at the time you buy them. For the most part, they are being bred as a temporary holiday plant, to be enjoyed while in flower and then discarded afterwards. To be honest, the price of Mums in most regions makes this an affordable thing to do.
The hassle of overwintering them and then pinching several times each season in future years (May through July) may not be worth the effort in the end. Mums need regular fertilizing, a full sun location, regular insect control, and constant watering through droughts in order to ever again achieve that perfect cushion look they had when you bought them. Without all of those things, more often than not they end up looking tall, spindly and bedraggled in their second season. Gardeners in northern regions may also find that their Mums never again flower before the frosts kill the buds.
So… if you choose to give it a try, mulch your mums well after the ground freezes. Check them periodically in late winter and spring to make sure the frost has not heaved them from the ground, and press the rootball back in gently if it has. In spring, cut back the dead foliage and wait until about the end of May to see if they survived or not.
Need help with storing your tender bulbs?
Here is a handy link with lots of details on how to go about storing those tender Summer-blooming bulbs, like Dahlias, Cannas, Calla Lilies, etc. This links to lots of other bulb information as well, so it’s a handy resource for the perennial gardener!