Perennials: Time to Divide & Conquer

Gardening Sweet Spots
By Donna Hessel

It’s that time of year when garden borders begin to look either overgrown or have spots where plants have died out. August is a good time to consider dividing and rearranging perennials. Which ones should you divide now?

It’s a good idea to dig 4 to 6 inches away from the base of the plant. If the plant is large and heavy, simply tilt the mass of soil back on the spade so the roots are visible, or cut the old plant into smaller sections. Remove excess soil so it is easier to see where to cut the divisions. For tough and strong roots, using a hori-hori or other serrated edge knife will facilitate the process.

Every perennial has a preferred way to be divided. A perennial’s root type determines how it likes to be cut and divided.  After you dig out your plant, take a look at the roots before you start separating the plant. Not all perennials have the same types of roots. The dotted lines on the following diagrams illustrate where the dividing cuts should be made on each type of plant. Plants can be divided into small sections with just one stem attached to a few roots. For a bigger impact faster, three to seven stems can be left on each new division.

Three Ways to Make New Plants
Most perennials fall into one of three categories… spreaders, clumpers and those with woody crowns. Each type has a few quirks it’s good to know before it’s dug out of the ground and separated into new plants.

Spreaders
Find the new crowns around the parent plant and slice down between them. Many ground covers, like this bugleweed, spread by modified stems that root where they touch the ground. It’s easy to dig up the newly formed “baby” plants and transplant them.

 

Clumpers
Dividing plants with fibrous or spreading roots like a daylily is easy. The whole clump can be dug up, or the root ball pulled or cut apart. Or, a chunk along the edge can be cut out.

A large clump, such as the one illustrated, can be cut into three or four sections and replanted.

Woody Crowns
Some plants, such as coral bells, have a woody crown. The whole plant should be dug up and the soil shaken off. Sections of crown, each with a few leaves, can be cut off and individually replanted.

When dividing, at least one set of leaves should be left attached to the main root.

Which Perennials to Divide When
Best days for dividing plants are cool and cloudy. If 90 degree weather persists into August, the following tips will assure your divided plants survive. These are also good tips to follow when planting on a more temperate day.

Start by watering the plant and its future home the day before you dig.

1. The next day cut the foliage back by half: This way a smaller root system won’t have to support lots of foliage in the heat.
2. Next, dig the hole where the new divisions will be set.
3. Dig the plant out of the ground and divide.
4. Once the plant division is set into its new spot, fill the hole halfway with soil and soak it thoroughly.
5. Fill the hole the rest of the way with soil, water it again and put down a layer of mulch.
6. Keep the area wet for a few days.
7. If the plant wilts in late evening or early morning, water again. Adding a little protection from the sun will also be beneficial.

The following chart, provided by Garden Gate Magazine, lists common perennials, their root type and when to divide them.

Plant name Botanical name	What season to divide	Root type	Special tips
Astilbe Astilbe spp.	Spring, Fall	Clump	Divide in early spring or after it stops blooming in late summer to early fall; will bloom better with division every 2 to 3 years
Bachelor’s button Centaurea montana	Spring, Fall	Clump	Easy to divide; needs to be divided every 1 to 2 years to keep plants vigorous
Bearded iris Iris hybrids	Summer	Rhizome	Divide in summer when flowering is over, at least 6 weeks before frost so plants have time to get established
Bee balm Monarda spp.	Spring, Summer	Rhizome	Spreads quickly; easy to divide; for ease of handling, divide in spring as foliage is emerging
Black-eyed Susan Rudbeckia spp.	Spring, Fall	Clump	Easy to divide; divide every 3 to 4 years; for best appearance next season, don’t make divisions too small
Blanket flower Gaillardia x grandiflora	Spring, Fall	Clump	Easy to divide; roots usually pull apart easily by hand; divide every 3 to 4 years
Bleeding heart, old-fashioned Lamprocapnos spectabilis	Spring, Summer, Fall	Clump	Divide as foliage goes dormant in the heat or mark plant location; roots are brittle, handle carefully
Brunnera Brunnera macrophylla	Spring, Summer	Clump	Divide in late spring to early summer, after blooming; will reseed but divide to keep specific cultivars true
Bugleweed Ajuga reptans	Spring, Summer, Fall	Clump, spreads by stolons	Plants root as they spread; no need to dig whole plant; cut off rooted outer pieces and replant
Butterfly weed Asclepias tuberosa	Spring	Clump	Difficult to divide because of taproot; for best results, purchase new plants
Catmint Nepeta x faassenii	Spring, Summer	Clump	Doesn’t need division often; cut back whole plant by half so roots have less foliage to support
Columbine Aquilegia hybrids	Summer, Fall	Woody crown	Divide every few years to keep this short-lived perennial around; reseeds but seedlings may not look like parent plants
Coneflower Echinacea purpurea hybrids	Spring, Summer, Fall	Clump	Easy to divide; divide every 4 to 5 years for healthier plants; reseeds and seedlings are easy to transplant
Coral bells Heuchera hybrids	Spring, Summer, Fall	Woody crown	Don’t worry about getting roots with each piece, they’ll sprout from the stem; cut individual leaves back by half to conserve moisture
Coreopsis Coreopsis grandiflora	Spring, Summer	Clump	Division every 3 to 4 years helps this short-lived perennial stay around longer; cut plant back by half
Daylily Hemerocallis hybrids	Spring, Summer, Fall	Clump	Divide every 5 to 7 years; rebloomers are best lifted in spring before they flower
Delphinium Delphinium spp.	Spring	Clump	Blooms best if divided down into small sections; division every 2 to 3 years will prolong plants’ life
Dianthus Dianthus spp.	Spring, Summer	Clump	Mat-forming types root as they grow; cut a rooted piece from the edge and replant
European wild ginger Asarum europaeum	Spring, Summer, Fall	Rhizome	Easy to dig and split; likes moist to wet soils, so keep new plants well watered
Fall anemone Anemone spp.	Spring	Woody crown, spreads by runners	Doesn’t like to have main clump disturbed; sends out underground runners so dig small new plants around the edges to transplant
False indigo Baptisia australis	Spring	Clump	Difficult to divide because of taproot; for best results, purchase new plants; slow to recover from being moved
Feather reed grass Calamagrostis acutiflora	Spring	Clump	Divide in spring; lift clump and cut into sections; you may need a saw or hatchet to divide roots
Garden mum Chrysanthemum hybrids	Spring	Woody crown	Easy to divide; divide every 2 to 3 years to keep plants blooming and vigorous; discard dead central portion
Garden phlox Phlox paniculata	Spring, Summer, Fall	Clump	Divide every 3 years or when flowering diminishes; discard the woody center and plant the edge pieces
Geranium, hardy Geranium spp.	Spring, Summer, Fall	Woody crown	A ring of foliage around a dead center tells you it’s time to divide; usually every 3 to 4 years is sufficient
Goat’s beard Aruncus dioicus	Spring	Clump	Rarely needs to be divided; tough roots make dividing difficult; leave plenty of room around new transplants because the plant gets large
Goldenrod Solidago spp.	Spring, Fall	Clump	Divide every 3 to 5 years to keep plants vigorous
Hellebore Helleborus spp.	Spring	Clump	Rarely needs to be divided; if you do divide it, do it in early spring while plants are in bloom or right after they stop flowering
Hosta Hosta hybrids	Spring, Summer, Fall	Clump	Easy to divide in early spring when foliage is emerging; dividing large clumps later won’t harm plants but leaves them lopsided for the summer
Japanese painted fern Athyrium niponicum pictum	Spring, Fall	Clump	Rarely needs to be divided, so divide only for new plants, every 7 to 8 years
Joe-Pye weed Eupatorium purpureum	Spring, Fall	Woody crown	Divide in early spring for easy handling; for fall division, cut back flowers; may need a saw or hatchet to cut apart woody crown
Lady’s mantle Alchemilla mollis	Spring, Fall	Clump	Divide in spring or fall; easiest in early spring as leaves begin to emerge; doesn’t need frequent division
Lamb’s ear Stachys byzantina	Spring, Summer, Fall	Clump	This hardy plant roots along the stem; cut a piece off the edge and replant; cut leaves back by half; don’t overwater
Lily-of-the-valley Convallaria majalis	Spring, Summer, Fall	Rhizome	Rhizomes pull apart easily; make sure each new division has a leaf and a cluster of roots
Lungwort Pulmonaria hybrids	Summer, Fall	Clump	Big leaves wilt easily; cut each leaf back by half and keep summer transplants well watered
Maiden grass Miscanthus spp.	Spring	Clump	Divide in early spring; use a hatchet or saw to cut clump apart; can dig whole clump to divide or chop out segments
Oriental poppy Papaver orientale	Summer	Woody crown	Divide in midsummer; foliage goes dormant quickly after flowering so mark the plant’s location; keep new transplants watered
Peony Paeonia spp.	Summer, Fall	Clump	New plants will take a few years to flower; set new plants at same depth as they were previously or they won’t bloom; keep watered; does not need to be divided often
Russian sage Perovskia atriplicifolia	Spring	Woody crown	Very difficult to divide because of taproot; for best results, purchase new plants
Salvia Salvia spp.	Spring, Fall	Clump	Divide every 5 or 6 years or when plant dies out in the center
Sedum (tall) Sedum spp.	Spring, Summer, Fall	Clump	Easiest to divide in spring when foliage is still small; dividing clumps later won’t harm plant but may reduce bloom for the year
Shasta daisy Leucanthemum x superbum	Spring, Summer, Fall	Clump	Easiest to divide in spring when foliage is still small; dividing clumps later won’t harm plant but will reduce bloom and leave plants lopsided for the year
Speedwell Veronica hybrids	Spring, Summer	Clump	Easy to slice into pieces; make sure to get plenty of roots with each new section
Turtlehead Chelone lyonii	Spring, Fall	Clump	Divide every 4 years or as needed; if you divide in fall, wait until the plant is done flowering
Yarrow Achillea spp.	Spring, Summer	Clump	Easy to divide; most types spread quickly so divide every 2 to 3 years

Some perennials should not be divided. If a plant has a tap root or brittle roots, it’s easier to propagate it by another method, such as by seed, cuttings or layering.

Don’t Divide These Perennials

Plant name Botanical name	Best propagation method
Baby’s breath Gypsophila paniculata	Seed or cuttings
Butterfly weed Asclepias tuberosa	Seed
Candytuft Iberis sempervirens	Seed or cuttings
Evening primrose Oenothera macrocarpa	Seed
False indigo Baptisia australis	Seed
Foxglove Digitalis spp. and hybrids	Seed
Gas plant Dictamnus albus purpureus	Seed
Lavender Lavandula spp. and hybrids	Seed, layering or cuttings
Lavender cotton Santolina chamaecyparissus	Seed or cuttings
Lupine Lupinus spp. and hybrids	Cuttings
Mediterranean spurge Euphorbia characias	Cuttings
Rosemary Rosmarinus officinalis	Seed or cuttings
Russian sage Perovskia atriplicifolia	Cuttings

You’ll likely have some flowers you had to cut off to reduce the size of the plants you want to divide. Make yourself a bouquet to enjoy indoors…and imagine how beautiful your gardens will look next year when all your new divisions have taken root and blossomed into new plants!

Text Sources:
“How to Divide 45 Favorite Perennials” – Garden Gate Magazine Online Newsletter – August 18, 2023
by Garden Gate Staff
Published April 16, 2019

“Don’t Divide These Perennials” – Garden Gate Magazine Online Newsletter – August 18, 2023

By Sherri Ribbey
Published May 2, 2012; Updated August 12, 2020

“3 Simple Ways to Divide Plants” – Garden Gate Magazine Online Newsletter
by Garden Gate Staff
Published April 18, 2019; Updated September 15, 2021

Photos courtesy pexels.com

Illustrations courtesy of Garden Gate Magazine