Late summer into early fall is an excellent time to divide many perennials. Prime party time is now past, so the holes you make in the border when lifting plants won’t mater aesthetically. The garden appears in its full horrific overgrown glory in late summer, too, rather than wearing that misleadingly circumspect demeanor of spring (when you no doubt said to yourself: “Aah, I can get away without splitting those daisies this year”.) Most important of all, there are still several weeks of nice, cool weather with relatively warm soil temperatures before hard frost – optimal conditions for establishment of your new plants.

Some plants can be divided spring or fall and nothing you do, shorts of using a blowtorch, will bother them much. The scales are tilted in favor of late summer or early fall, though, because such plants will then emerge with a more symmetrical shape in spring. The caveat is that late summer-fall divisions should not be too small; some roots should already be in place to help anchor the new plants.

Here are some tips for dividing perennials:

· If I require more hostas, I quarter the tough-rooted old plants in late summer with an old (very strong) butcher knife, but a sharp spade or small handsaw will also work. This method works well for other plants with woody crowns such as astilbe, goatsbeard, Solomon’s seal, snakeroot, globe thistle, fall sedums, yellow loosestrife, liatris, veronica, meadowsweet, purple coneflower, penstemon, monkshood, physostegia and Siberian iris.

· With daylilies, cut the foliage down to a short fan, then dig up the clump. (Based on my own experience, don’t use a wimpy spade or fork to divide daylilies – the tool is no match for these tough customers!) Once the clump is out, pry apart the roots which can then be replanted.

· The rhizomes of bearded iris will crowd each other out over time, resulting in reduced bloom. Cut the foliage back to a few inches, dig up the clump, then using a sharp clean knife, divide it into several rhizomes, each with its own fan. Check for borer rot, cutting off diseased sections and dipping the rhizome in a 1:10 bleach-and-water solution. Then plant each rhizome on a mound in the center of the new planting hole, letting the roots spread out into the hole. Make sure the rhizome is right at the surface of the soil, since irises like to bake in the sun.

· Peonies don’t need dividing, but if you want additional plants, dig them up, separating the roots into sections containing 2 or 3 “eyes” which must be planted no more than 1-1/2 to 2 inches below the soil surface. Plants will take a few years to bloom again.

· With shallow-rooted plants, pay attention to the blooming season. Spring-flowering primroses, rockcress, aubrieta, Jacob’s ladder, lady’s mantle, hellebore and doronicum are best divided by hand after they bloom in spring so they have a chance to re-establish. Similarly, summer-blooming chrysanthemums, Shasta daisies and heleniums are best divided in spring; metabolically, they seem less capable of recovering from late disturbance. On the other hand, I’ve used my spade to divide Michaelmas daisies (fall asters) in bloom and moved them around with no ill effects. Speed and lots of water, naturally, help the process immeasurably.

· Perennial geraniums (cranesbills) are very tricky and best divided in very early spring.

· Monarda, yarrow, oenothera, and artemesia lend themselves easily to dividing, since the stems arise from little sections of running roots and the plants tend to wander away on their own anyway. These can often be divided by hand but the large yarrows may need a sharp spade.

· Summer phlox should be divided every 3-4 years by replanting the outermost sections and discarding the middle.

· Big ornamental grasses like maiden grass (Miscanthus sinensis) and switch grass (Panicum virgatum) are a challenge to divide when they’ve established their tough root systems. But since old plants sometimes die out in the middle, it’s best to rejuvenate the clump by dividing with a very sturdy, sharp spade that’s up to the job. Warm season grasses should be divided during active growth in spring to give the roots lots of time to establish before winter

· Some plants are best left alone. Columbine, trillium, basket-of-gold, candytuft (Iberis), balloon flower, butterfly weed (Asclepias), delphinium, campanula, gas plant, baby’s breath, dianthus, bleeding heart, lavender, sea holly, red-hot poker (Kniphofia), flax, lupine and Russian sage are either deeply tap-rooted, woody sub-shrubs, or just plain resentful of interference.

Newly divided plants benefit from some manure or compost being worked into the soil around them, but don’t use nitrogen fertilizers at the time of division, which only encourage new foliage growth. If you divide plants in late summer or fall, give them a little mulch (shredded leaves, etc.) when frigid weather arrives, extending the time the division has to acclimatize and establish roots.

Adapted from a column that appeared originally in Toronto Gardens

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