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Divide Perennials Before They Conquer

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Robert Smaus Add to Journal

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Many perennials grow by spreading, and if they’re not dug or divided, they’ll soon shove other plants out of the way in a bid to take over the garden. Equally disturbing is how their old centers die out as they deplete the soil underneath.

If these plants are divided, though, you can toss the old dead sections onto the compost pile and replant the more vigorous chunks in improved soil. As a bonus, you’ll have enough extra plants to pot up and give to friends. (One wonders how rampaging asters can even be sold by nurseries since so many are given away by homeowners.)

That said, not all perennials behave this way, so make sure of your plant’s spreading habit before you start digging it up. To make certain a perennial can be divided, look closely at the base to see the spreading roots, rhizomes or stolons. There should be many stems coming out of the ground, not just one or two. If you see many, you’re safe to proceed.

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Use a sharpened spade
Photo Credit: Robert Smaus
Use a garden spade with a long, straight, flat blade for dividing perennials.
Many common flowering perennials do spread and need dividing, including Alstroemeria, Armeria, Aster, Astilbe, Bidens, Campanula, Coreopsis, Geum, Helenium, Hemerocallis, Lychnis, Rudbeckia, Stachys and Veronica. Some will actually perform poorly if they’re not divided and restarted in good soil as often as every two years.

 

To divide a perennial clump, use a sharpened flat-bladed spade, not a shovel. (You can sharpen the flat blade with a rough “bastard” file [available at hardware stores], so it’ll slice cleanly though the root mass and stems.)

True spades, especially those with usefully long blades, may take some looking for. Though common in England, these tools can be hard to find here since we haven’t honed perennial gardening to the degree the English have. Some spades can be sharpened, but others have serrated blades (which work well if you’re not the kind of person who keeps things sharp).

Warnings
  • Lavandula and Santolina are examples of plants often called perennials, but they’re more a “sub-shrub” of sorts and, though their centers die out in time, they cannot be divided. They don’t even like to be cut back by more than a few inches.
Definitions
  • Crown: The very base of a plant, where the stems enter the ground or become roots. It’s often swollen or large. The crown is a critical part of many plants because it can rot or become diseased. (You’ll often hear advice about keeping the crown dry or not burying it too deep.)
 
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