Garden math: Divide and multiply

Who doesn't love free plants? While dividing perennials helps to keep your garden neat, healthy and blooming at its best, the prospect of making more plants motivates many gardeners to do the math -- and dig in.

Right now is a great time to expand your landscape without a single trip to the garden center. Many perennials can be divided in early spring, when tender shoots emerge from the ground with lots of stored energy in their root systems. That, combined with abundant soil moisture and cool weather, will allow plants to recover from the dividing process during the growing season.

Recommendations from the University of Minnesota Extension Service call for perennials to be divided about every three years, but your plants may tell you when it's time. Plants that are crowded, showing signs of fungal disease, flopping, dead at the center or producing smaller blooms will benefit from dividing.

How to divide

You can add to your dividing success by doing a little planning:

A few days before, water -- but don't drown -- the plants you intend to divide. If you can, wait for an overcast day. (That will help prevent damage to young leaves from evaporation.)

Have a pail of water handy and keep divisions moist until you can replant them. (You can store divisions for later planting by wrapping them in damp newspaper and keeping them in a cool, shady place.)

Once you lift the plants out of the ground, shake or hose off the extra soil so you can see the root structure. That will help determine how best to divide each plant.

Plants with fibrous, spreading roots can be teased apart by hand or cut with garden shears with minimal damage. This kind of division works well with sedum, sweet woodruff, lamb's ear, monarda and many groundcovers.

On large plants with tougher, more tangled roots (yarrow, catmint or rudbeckia), use a sharp knife or insert two garden forks, back to back and apply leverage to pry them apart. Clumping plants such as hosta, astilbe and ornamental grasses with dense root systems may require a shovel or saw to separate them.

Once you've divided the plant, trim and remove the dead centers and any rotting or diseased sections from each division. It won't hurt to groom and weed them while you're at it.

Make sure that divisions are viable and have a good balance of roots to shoots, with at least three to five eyes, or growing points, on each. Buds or shoots growing at the outside of the clump will have the most vigor.

When transplanting, place plants at the same depth at which they previously grew and water them in well. It's tempting to plant close together for a fuller look, but be careful to space them properly to avoid overcrowding. Continue to water until the plants are well established.

When to divide

Daylilies, Siberian iris and peonies can be divided in spring or fall. Bearded iris and Asiatic lilies should be divided in late summer or early fall. (If you trim about half of the foliage, the plants will be easier to handle.)

Peonies need to be divided infrequently, because they take a long time to recuperate and bloom again.

Some plants, especially those with long taproots such as butterfly weed and false indigo, shouldn't be divided.