Plants often provide clues when they need more room: They die out in the centre, produce smaller leaves or flowers, and bloom less often, gardener and horticulturalist Heather West says.Jurgute/Getty Images
Fall may be the thriftiest season in the garden.
It’s the time of year when established perennials can be divided and replanted to fill bare spots, create new beds, or shared with friends and neighbours.
Better still, dividing plants isn’t simply a money-saving exercise. Done at the right time, it can also improve the health and strength of the plants already in your garden.
Gardener and horticulturalist Heather West, in Muskoka, Ont., says many perennials benefit from being divided every three to five years. “As plants mature, their roots can become crowded, forcing them to compete for water and nutrients,” she says. “The dense growth can make plants more susceptible to disease.”
Fortunately, West adds, plants often provide clues when they need more room: They die out in the centre, produce smaller leaves or flowers, and bloom less often.
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“Fall’s cooler temperatures are ideal because they put less stress on newly divided plants, while the soil remains warm enough for roots to become established before winter,” she says. “All of this means that, come spring, they are ready to put their energy into new growth.”
Dividing perennials doesn’t require special skills or expensive equipment. A garden spade or fork, a sharp knife, and a little planning are often all that’s needed. Here are West’s tips for dividing, moving and swapping plants so your garden has the best chance of thriving next year.
Timing is key
Canadian gardeners have to calculate the timing for themselves because the ground freezes at very different dates in, say, Whitehorse or Halifax or Vancouver. In Muskoka, for instance, West started to divide plants in early September, because it was warm and wet.
She also recommends transplanting on an overcast day, when exposed roots are less likely to dry out.
Fall has another advantage over spring: it’s easier to see what is working – and what isn’t – in your garden.
“I can take stock of what areas in my garden look overcrowded and messy, versus which beds appear sparse and need more color,” says West, who is the head gardener at Bigwin Island Golf Club. “In the spring, many plants, like hostas and astilbes, don’t emerge until May, even early June. Which makes it hard to identify them.”
Which plants can be moved in the fall
As a general rule, spring- and summer-blooming perennials can be divided in autumn, while late-blooming plants such as asters and chrysanthemums are better left until spring.
Among the perennials that can be divided in fall are daylilies, bee balm, lamium, catmint, coneflowers, black-eyed Susans, phlox, perennial geraniums, lamb’s-ear, primulas, astilbe, rudbeckia, Shasta daisies, blanket flower, coreopsis, lady’s mantle, Jerusalem artichokes, dianthus, and hostas.
Once the dividing and transplanting are done, it’s also a good time to plant spring bulbs.
West is more cautious about moving Oriental poppies, bearded iris and peonies, which she describes as “fussy plants” that dislike being moved.
Gardeners should also be careful about what they share. Aggressive spreaders such as sweet woodruff, lily of the valley, periwinkle and lamium can quickly move beyond their allotted space. “These plants have to be handled carefully,” says West. “They are best kept between a rock and a hard place.”
How to dig and divide
West tries to divide plants when rain is expected. If the garden bed is dry, water it thoroughly the day before so the soil lifts cleanly rather than crumbling off the roots.
Once you’ve chosen the plant to divide, push a spade or garden fork into the ground 10 to 15 centimetres from its base and work your way around it before lifting the entire clump. “It’s easier on your back that way,” says West.
The best transplant material tends to come from the outside edges of the clump, where growth is newest. Shake or gently tease away enough soil to see the roots and crown, then pull or cut the clump into sections, making sure each has healthy roots.
Check each division for signs of rot and remove any weeds that have worked their way into the roots.
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Replant the divisions at same depth at which they were previously growing and put transplant fertilizer or bone meal into the newly prepared hole. Carefully press the edges of the soil down with the heel of your boot so that all the roots have contact with the soil. “You really need to push down hard so the roots have a chance to establish and won’t pop out when you’re raking in the fall,” says West.
New divisions also need protection as temperatures fall. West covers hers with a 7-to-10-cm layer of leaves she collects from around her property. “We used to be able to predict when the killer frost would come. Now it’s unpredictable, so it’s best to give these fledgling plants lots of protection.”
Practice patience.
Not every transplant will immediately reward your efforts. Some perennials are more sensitive to being disturbed. Peonies, in particular, can take a few years to bloom again after being moved.
Occasionally, a division won’t survive but West cautions not to panic. “You can do everything right and a plant still won’t make it. That’s not your fault. It’s just nature.”
