Time to divide perennials and ornamental grasses

One item that’s always on my garden calendar for fall is dividing overgrown perennials such as daylilies, daisies, and ferns, as well as some ornamental grasses that have gotten crowded.

Most need it every three to five years, a few become overcrowded more quickly. (Usually because I planted them too closely.)

There’s an old saying among gardeners: Divide spring-flowering perennial plants in fall and fall-flowering perennials in spring. Like most adages, there’s some truth to this. For one thing, it means you don’t forgo flowers.

But most experienced gardeners have proven – through experience – that if you're careful and know what you're doing, you can divide and/or move perennial plants in either season.

For me, what it comes down to is that I have more time in fall than I do spring, when so many other chores are vying for my attention.

And Tracy DiSabito-Aust, in her classic "The Well-Tended Perennial Garden," says always to divide Siberian irises, peonies, and oriental poppies in autumn.

But if you live in a climate where temperatures dip below freezing in winter, you need to divide perennials as early in autumn as possible.

The absolute deadline is one month before your normal hard freeze, but the longer the plants have to become established in their new homes, the more likely they are to survive.

Before you begin the division process, prepare the area where the new divisions will be planted. The day before you plan to start dividing, be sure to water it deeply. Also water thoroughly the plants you'll be dividing.

I like to remove about half of the tops of the plants before I dig them up. But others prefer to cut back the foliage after they've divided and before they replant.

You'll need a spade or other digging tool, a sharp knife, and a garden fork or two. You'll also want to have hose handy. If possible, work on an overcast day.

First, dig up the clump, getting as much of the root mass as you can. (That usually means starting about a foot away for large plants and grasses.)

Small plants you'll be able to pull apart by hand. With large ones, you may need to cut them. Really big clumps are usually a two-person job: Place two garden forks back to back in the center of the clump and pull. Repeat as necessary.

A warning: with ornamental grasses, you may need an ax! They're really tough.

Usually the center of an older perennial plant is dead and should be discarded. Divide the remainder into sections with three to five growth buds. You may  need to wash all the soil off the roots so you can see what you're doing.

If growth buds or eyes aren't evident to you, just make the size clumps you would buy in a gallon pot at a nursery.

Don't let the plants dry out as you work on them. Hose them down occasionally and keep them out of the sun.

All that's left is to replant your new divisions, spacing them so they have room to grow. Water deeply and mulch. Then keep the new plants watered when rain is less than an inch a week.

I always mulch newly divided perennials a bit more heavily in late fall to protect them over winter. Whether you do will depend on your climate and how late you've divided the plants.

But next year you'll be pleased at the vigorous growth of the new plants and how much better they bloom. They make all the work worthwhile.

Here are a couple of videos that illustrate the process:
Utah State on You Tube
Amanda Switzer on About.com