How to - Techniques

Bring those overgrown perennial clumps to heel

Dividing, moving and reducing the size of overgrown perennial clumps are essential tasks that rejuvenate the perennial garden and result in healthier, better-looking (and more) plants. As a general guide, you can expect perennial clumps to need thinning or dividing every three to five years. Once you've done it, have patience, as some transplanted perennials will take awhile to get re-established and become large enough to bloom again.

Tools you need
For dividing and transplanting perennials

  • A sharp spade with a flat edge or sturdy garden fork (or two) to separate and divide the root ball
  • Sharp secateurs (hand pruners) or a tough, sharp, non-serrated knife to neatly sever large, fleshy roots
  • A watering can or hose
  • A tarpaulin (or old shower curtain) for speedy clean-up, transportation of lighter plant divisions and to provide shade during the dividing task
  • Pots or bags and labels if divisions are being shared
  • A folding handsaw or axe is useful for cutting through dense or woody roots

How do you know it's time?

Perennial clumps that have taken on a doughnut shape are good candidates for renewal division. Most of the vigorous shoots are on the outer perimeter and often there's little growth in the clump's interior. Some pinks (Dianthus spp.), tall bearded iris hybrids (Iris germanica), bee balm (Monarda spp.), Phlox paniculata and lamb's ears (Stachys spp.) are prone to this growth pattern. Other plants must be divided because they have lost their youthful vigour, are producing smaller flowers and fewer leaves, or are congested in the centre of the clump and have grown overly tall, spindly or floppy.

Spring vs. fall
The old rule of thumb is to divide fall-flowering perennials in the spring and spring-flowering plants in the fall. The new thinking says spring-flowering plants should be divided right after they flower so they have the entire summer to settle into their new spots. While this is fine, late summer or early fall, when plants are still actively growing, is still an excellent time to divide most perennials, provided you finish the job at least 30 days before the first hard frost. That way, divisions will have an opportunity to send new roots into still-warm soil. This is especially important in areas with early frosts, harsh winters and unreliable snow cover.

There are exceptions. Some perennials that have fleshy storage roots should only be divided in the fall, regardless of when they bloom. These plants generally expend a lot of energy in the spring and need the summer to build up their reserves, and include peonies, Oriental poppies (Papaver orientale), astilbes, tall bearded irises and Siberian irises (Iris sibirica). Most autumn-flowering plants can be divided in spring, if you prefer.

Split up those bullies
Invasive plants need to be divided regularly and ruthlessly to keep them under control-every year, if necessary. These bullies include a few running ornamental grasses, such as Leymus racemosus, Glyceria maxima and Phalaris, as well as goutweed or snow-on-the-mountain (Aegopodium podagraria), carpet bugleweed (Ajuga reptans), peach-leaved bellflowers (Campanula persicifolia), lily-of-the-valley (Convallaria majalis), plume poppy (Macleaya cordata), bee balm and obedient plant (Physostegia virginiana). Even a few pieces of root left after moving or dividing an invasive plant will quickly send up shoots and fill the area again, so be merciless. Drought, lack of light, root competition or other barriers can also help keep thugs in check.

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