Late Summer Perennial Division: Give Your Plants a Fresh Start

By mid- to late August in Boise, most of our early summer blooms have finished their show. The garden settles into a slower, late-season rhythm: greens deepen, seed heads form, and those long, golden evenings hint at autumn’s arrival. It’s the perfect time to give some of your hard-working perennials a fresh start by dividing them.

Division is like a spa day for plants—it reinvigorates growth, prevents overcrowding, and even gives you extra divisions to plant elsewhere or share with friends. In our high-desert climate, with hot summers and fast-cooling autumn nights, late-summer division is a smart move. The warm soil helps roots reestablish, and cooler evenings reduce transplant stress. As the University of Minnesota Extension points out, dividing perennials stimulates new growth, encourages more vigorous blooming, and improves airflow to help reduce disease pressure. 

The best candidates for this timing are spring- and early summer-bloomers. Day-lilies, bearded iris (divided right after their flowers fade), cheerful Shasta daisies, and peonies all respond well to late August or early September division. Creeping phlox, shade-loving hostas, and sturdy yarrow also benefit from this seasonal refresh. Fall bloomers like asters and mums are best left until spring so their floral display isn’t interrupted.

Some of our native perennials can be treated the same way. Coneflowers and goldenrod both respond beautifully to late-summer division, sending up vigorous new shoots the following spring. Blanket flower and prairie smoke also divide well now. Others—like penstemon, globe mallow, and showy milkweed—tend to resist root disturbance and are better propagated from seed or cuttings rather than being dug and split.

Your plants will often tell you when they need attention. A thinning or bare center in the clump, smaller or fewer blooms, or an enthusiastic spread that’s crowding out its neighbors are all signs that a division is in order. When you’re ready, start by watering thoroughly the day before so the roots are well-hydrated. Work carefully with a spade or fork to lift the entire root ball, keeping as much root intact as possible. Then gently tease the clump apart by hand or use a sharp knife to create new sections, each with healthy roots and several strong growing points. Prepare the planting area with compost to give your divisions a nutrient boost, and replant promptly at the same depth they were growing before. If you’re not ready to place them in the garden, you can also pot up divisions in containers, keeping them watered and sheltered until you’re ready to plant, donate, or share them. Water deeply, and keep the soil consistently moist until you see new growth.

For the smoothest recovery, work in the cool of morning or evening, and if the sun is intense, offer a bit of temporary shade for a few days. A light mulch will help regulate soil temperature and hold in moisture.

By tending to this now, you’ll set the stage for stronger, healthier plants next spring—and perhaps even have a few extras to share with friends or donate to a community plant sale. Sometimes, the best way to help a plant thrive is to give it the gift of space and a fresh beginning.

We've included links to show you more:

• University of Minnesota Extension – Dividing Perennials

• University of Illinois Extension – Dividing Perennials (PDF)

• https://www.rhs.org.uk/plants/types/perennials/dividing