Late summer is a time when you sit back and enjoy the fruits of your planning and labor in the garden. Plants are usually mature by now and many are about done flowering and fruiting for the season. But this can also be a great time of the year to divide herbaceous perennials and expand your collection. By now, many of these plants are large and often outgrowing their space.
There are several reasons to consider dividing plants now. First, if they’ve been in the same spot for a while, they may be getting too large and overgrown and they may be out-competing their neighbors. Or, they may be a plant species that you’d like to have more of, and dividing is a method of propagation. Dividing is a great way to expand your landscape. Dividing and replanting in late summer gives the plants time to get re-established before the temperatures drop. This helps them survive the winter and be prepared to grow in the spring.
Daylilies are the perfect plant to discuss when thinking about summer plant division. They are a natural. They’re usually finishing their summer show by now, and you’re not going to sacrifice flowering. They can be dug in mid- to late August and are easily divided. I’d start by digging the plant (usually a large clump) and setting it on the ground. Grab a spade (sharpening it helps). Use the spade to cut the clump into smaller clumps. You can divide Hemerocallis into individual fans (leaf fans) if you really want lots of that cultivar. Cut the foliage down to three to six inches long and replant the clump. Hosta can be treated the same way, using a spade to cut the clumps into smaller pieces. Hosta, too, can be divided into small clumps as small as a single eye of growth.
Other species that respond well to late-summer division include Echinacea (Coneflower), Heuchera (Coralbells), Phlox, Pulmonaria (Lungwort), Dicentra (Bleeding heart) and many species of Iris. All of these species can be dug and divided. In most cases I’d use a large knife rather than a spade for these species, as they are a bit more delicate.
It’s important to remember that your newly divided plants should be replanted as soon as they’re divided, and water them as soon as they’re in the ground. In most cases the plants should be cut back to reduce the foliage area and reduce stress on the plant. Keep them well-watered from now until fall, as they will need to grow a new root system before frost.
There are a few plant species that you should not divide in the summer, including most ornamental grasses. They’re just starting their late summer flowering and seed set and don’t like to be disturbed in the summer. Many will not survive the dividing. In general, most grasses will respond much better to spring dividing and replanting.
Now can be a great time of the season to dig and divide many herbaceous perennials. Doing so before the cool weather sets in gives the plants time to re-establish themselves. They can develop a good root system and be ready to grow in the spring.