Anchorage Daily News

Q. I have four dependable perennials that have returned for six years: a yellow spike ligularia, a blue hosta, yellow trollius and lady's mantle. I need to divide all of them, but I fear limiting their current season's performance at best or killing them outright in a worse-case scenario. When is the best time to divide such plants?

A. The ideal time to divide most perennials is early in the season when they start to poke out of the ground. If you plan to divide plants this season, you better hop to it fast. Perennials that bloom in May and early June, like trollius, can be divided after they finish flowering.

There are two reasons to divide plants. You divide perennials because you want more of the same. One of the joys of gardening is being able to propagate plants you already have growing.

However, not all perennials need to be divided to produce more of a good thing. Lady's mantle is a notorious self-seeder. Simply let the plant flower, and next season you'll have plenty of seedlings. Don't divide lady's mantle because you want a smaller plant. If the plant is too big, replace it with dwarf lady's mantle. Alchemilla erythropoda makes a nice clump but is about half the size of its big sister.

I don't recommend dividing hosta unless you have the patience of a saint. Hosta is slow to emerge from the ground in spring and takes years to attain the big, beautiful clumps you see in gardening magazines. Divide hosta only if you've lost its label and need more of the same cultivar.

Horticulturists classify this type of propagation as asexual, but the term that pops into most gardeners' heads is cloning. If you're propagating plants without involving the birds and the bees, you get an exact replica of the parent.

The other reason to divide a plant is because it's not growing well. Some plants lose the center of their clump during a difficult winter. Others have a tendency to crowd themselves to the point of reduced vigor: Iris that need to be divided stop flowering; the magnificent leaves of ligularia get puny. If your ligularia has so many stems that its leaves are less than 14 inches across, it's time to divide. After all, like most perennials, ligularia blooms only for a few weeks. It's spectacular leaves that make this plant worth growing.

Be ruthless in your dividing. A spade, ax or chicken cleaver can be employed if you can't gently tease the crown of the plant apart using two garden forks. Be sure not to make divisions too small. Sections should include a sizable clump of roots and more than a single shoot. You won't kill plants unless the roots dry out.

Keep newly divided plants well-watered until they are established. Alaska gardeners are lucky. Most of the perennials we grow don't need to be divided often to flourish.

Julie Riley is the Anchorage horticulture agent for the University of Alaska Fairbanks Cooperative Extension Service.

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