Last call for dividing perennials

Our story this issue on growing fall asters talks about dividing them as soon as the fall bloom has finished. That same advice applies to garden mums, many of the salvias, and several other fall-flowering perennials.

Add to that list the names of spring- and early-summer-blooming perennial plants and you’ll probably have a majority of the perennial species used in Texas landscapes. These are all plants that need to be dug and divided in the fall. As in “NOW.”

Here’s a partial list in rough chronological order of spring flowering: sweet violets, candytuft, thrift, Louisiana phlox, oxalis, iris, St. Joseph’s lilies, daylilies, Shasta daisies, purple coneflowers, gloriosa daisies, and closer to the date of the first freeze, cannas and mallows (once their tops have died to the ground). Hostas, ferns, and other foliar perennials could also be put onto that list even though we grow them for foliage, not flowers.

All these plants establish strong roots over the winter so that they can shoot out with new growth as the soil begins to warm up in in late winter and early spring.

If you have any of these plants, and if they’ve been in their same beds for several years, odds are good that they’ve become crowded. That will hinder future growth and bloom, so you need to dig and divide them before the first frost kills their tops back to the ground.

I use a small spading fork or nursery spade to lift my perennials. My perennial gardens grow in soil that’s been heavily amended with organic matter and expanded shale, so if the ground is moist, I’m usually able to break the clumps of plants apart rather easily with my thumbs and the heels of my palms. In a few cases I must use the blade of the spade to cut them in halves or quarters.

Once I have all the plants lifted and their parts of the beds emptied, I use a mini-tiller or the spading fork to work several inches of fresh organic matter (compost, rotted manure, sphagnum peat moss and finely ground pine bark mulch) and another inch of expanded shale back into the soil. I’m then ready to replant back into the same spaces, or if I’m changing my landscaping layout, this is the time to do so.

One of the big temptations when you’re replanting is to use every one of your transplants. If you’re putting them right back where they were growing before, you’re going to have them badly over-crowded. Either create a new and larger space for them or give them to friends. Just don’t pack them back into the same space.

For daylilies, iris, St. Joseph’s lilies and similar plants I normally trim off about half of the plants’ foliage. It’s going to die with the first frost anyway, and by trimming it, I reduce the amount of stress it will put on the roots as they try to regrow.

I apply a high-nitrogen fertilizer, since most of my planting soils already have enough phosphorus, but if you’re gardening in a sandy soil, you could use a high-phosphate root-stimulator now and pick up the nitrogen come spring.

Finish by watering deeply. I usually top off my perennial beds with a lightweight winter mulch of tree leaves I’ve run through the lawn mower. Most perennials you’ll be dividing won’t need mulching for protection from cold. They’ve already been through several winters and survived just fine.