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

Planning For An Established Garden

Maybe you recently purchased a new home with existing plants, or you’ve finally decided to tackle that project of making your yard less of an eyesore.  Those plants were there before you were, and you really have no idea what they are or where to start? Or, maybe you even planted some of them at some point, and you thought you would remember what they were, but life got busy and you had several other things to remember.  Suddenly the name of the perennial Aunt Martha gave you 10 years ago as a housewarming gift is not one of the things you recall.

If it’s the first year in your home or just the first year you are actively paying attention, the best thing to do is to slowdown and start a journal of plant notes.  That may sound like a tedious project in itself, and in your mind taking out every plant and starting fresh sounds less like a root canal than a journal of plant notes. Here's a video about garden journaling.  However, ripping out your entire landscape is equivalent to burning down your house because it hasn’t been cleaned in a long time, and it’s a lot more work than you might anticipate.  It’s likely that project will also wind up partially completed, and you’ll have an overgrown bed adjacent to a bare naked bed.  If you do actually pull everything out, you’ll forget you haven’t decided what you’re replacing it with, and each trip to the nursery to pick out plants will remind you how expensive those replacements are which could mean completion takes longer than you expected anyway.

For the first year, you’ll want to keep a plant journal of each season.  Keeping a gardening journal doesn’t mean you are at a standstill or you aren’t making progress toward the yard of your dreams.  Don’t worry about not doing enough, because you can always clear leaves and anything you are positive is a weed, and that will keep you busy year round. 

The plant journal is a great place to record what you have now and also to brainstorm what you might like in its place or in addition.  Otherwise, you’re likely to plant on top of something that is already growing there and just hasn’t shown itself yet.  It may be a good time to pull out the plat map of the property that came with your title report when you purchased your home because it can be provide a good map of the property.  Or you can make your own map of the area in your journal. 

Take photos of the garden at different times of the day as well as different seasons, and keep them in your journal.  You may want to label each photo with the type of plant or care that is necessary to keep it healthy, but to start at least keep photos with your garden notes so you can remember the changes.  You’ll also begin to notice some areas of the garden that don’t fill in and need to have something planted there.  Or, you may discover taller perennials located at the front of the bed, when they should be in the back so they don’t cover up everything behind it.

You’ll also want to start identifying what you already have to keep a master plant list, and see how that plant is doing in its current location.  It may be that when it blooms you decide you love it and won’t want to remove it after all, but you realize it’s overgrown or just not quite resembling the photos of what you think it could look like.  It may mean you relocate the plant to another area of the yard where it does better, or maybe it needs divided or trimmed.  You can use perennial finders online  to help identify plants, or download the app for tree identification leafsnap   to figure out what trees are in your yard.  Of course, for one-on-one assistance, your local University of Idaho Extension Office. can help you identify plants and trees and provide tips to caring for them also.

 

After you’ve survived 4 seasons, you’ll start to have an idea of what you like and dislike about your existing plants.   You’ll be able to decide which ones you dislike or what you might change about the location.  Plus, you’ll have seen so many plants at this point in your plant identification search that you’ll start to have your own ideas about what you would like.  You may have already written down possibilities and then selected a plant that will do well in that location as well as what you find attractive.  In the end, your garden is ever evolving and changing, but a little planning will make the actual planting a lot less painful. For even more how-to information, check out this article.