Divide and Conquer: A Guide to Dividing Perennials

by Brett Kerley

Dividing perennials is a fundamental horticultural practice. It is used to maintain perennial plant health, manage plant density, and propagate new specimens. In northern climates zone 3 & 4, the short growing season, cold winters, and heavy soils common in the region make proper timing and technique especially important. Understanding when and how to divide perennials allows gardeners to rejuvenate mature plantings, maintain vigorous growth, and sustainably expand their landscapes.

This guide outlines the biological reasons for division, the indicators that a plant requires it, and practical techniques appropriate for prairie garden conditions.

Division is more than a method of plant propagation—it is an important maintenance practice that supports long‑term plant performance. Many herbaceous perennials naturally expand outward from a central crown, forming progressively larger clumps over time. Without periodic division, these clumps can become overcrowded and physiologically stressed.

Prevent Overcrowding
As perennial crowns expand, root systems compete intensely for soil moisture, nutrients, and oxygen. High root density can limit access to these resources, leading to reduced plant vigor. Dividing the clump redistributes growth points and restores a more balanced root-to-soil ratio.

Rejuvenate Aging Plants
Many perennials exhibit reduced productivity as their central crown ages. Division removes senescent or woody portions of the plant and allows younger, more vigorous outer growth to be replanted. This process often restores flowering performance and improves overall plant structure.

Vegetative Propagation
Division is one of the most reliable forms of clonal propagation. Because each division contains genetically identical tissue, the resulting plants maintain the characteristics of the parent plant—an advantage when propagating cultivars or particularly successful garden specimens.

Improve Airflow and Reduce Disease Pressure
Dense plantings reduce air circulation within the canopy. Poor airflow increases humidity around foliage, which can encourage fungal pathogens such as powdery mildew or leaf spot diseases. Thinning plant clumps through division improves microclimate conditions around the plant and reduces disease susceptibility.

Daylilies no longer blooming

Experienced gardeners often recognize when plants are physiologically stressed by overcrowding. Several visual and performance-based indicators suggest that a perennial would benefit from division.

Declining Flower Production
A noticeable reduction in flower quantity or size often indicates resource competition within the root zone. As crowns become crowded, fewer carbohydrates are allocated toward reproductive structures.

Center Dieback
One of the most common indicators of an overgrown perennial clump is the development of a hollow or weakened center. In many species, the oldest portion of the crown eventually becomes woody or depleted while newer growth continues outward. Dividing the healthy outer sections restores plant vigor.

Reduced Plant Size or Weak Stems
Crowded plants may produce smaller leaves, thinner stems, and weaker flowering stalks. This occurs because individual shoots receive fewer nutrients and less root space.

Dense or Matted Root Systems
When lifted from the soil, an overcrowded perennial often reveals tightly intertwined roots with little visible soil. Such root congestion limits water infiltration and nutrient uptake.

Loss of Structural Stability
Perennials that once maintained an upright habit may begin to flop or splay outward. This can occur when the central crown declines or when shoots compete for light within an overly dense clump.

Recognizing these signs early allows gardeners to intervene before plant health declines significantly.

The timing of perennial division is typically based on flowering period and plant growth cycle. The general horticultural guideline is:

• Divide spring‑flowering plants in late summer or early fall.
• Divide summer and fall‑flowering plants in early spring.

This approach ensures that plants are not actively investing energy into flowering while undergoing root disturbance.

In Edmonton’s climate, fall division should ideally occur from late August through mid‑September, providing sufficient time for root establishment before soil temperatures decline. Spring division typically occurs April through May, once the soil has thawed but before rapid vegetative growth begins. Having said that, I wrote an article last year about splitting perennials during the summer heat. Check out what can be done then.

Spring‑Blooming Perennials Suitable for Fall Division

These species complete their flowering cycle early in the growing season and benefit from division once flowering has finished and root growth resumes.

• Peonies (Paeonia)
• Bearded Iris (Iris germanica)
• Bleeding Heart (Dicentra spectabilis)
• Primrose (Primula species)
• Garden Phlox (Phlox paniculata)
• Bergenia
• Columbine (Aquilegia)
• Creeping Phlox (Phlox subulata)
• Rock Cress (Arabis)
• Pasque Flower (Pulsatilla)

Summer and Fall‑Blooming Perennials Suitable for Spring Division

These plants initiate vegetative growth in spring and flower later in the season, making early spring the ideal time for division.

• Hostas
• Daylilies (Hemerocallis)
• Asters
• Black‑eyed Susan (Rudbeckia)
• Coneflower (Echinacea)
• Sedum / Stonecrop (Hylotelephium)
• Shasta Daisy (Leucanthemum × superbum)
• Goldenrod (Solidago)
• Bee Balm (Monarda)
• Yarrow (Achillea)

Avoid dividing during extreme summer heat or shortly before winter dormancy, as plants require time to re-establish root systems after disturbance.

Not all perennials respond well to division. Species with deep taproots, woody root systems, or sensitive crowns often perform poorly when disturbed.

Peonies (Paeonia)
Peonies develop large, fleshy storage roots that can extend deep into the soil profile. Disturbing these roots often results in several years of reduced flowering while the plant re-establishes its root system.

Poppies (Papaver)
Many poppies develop delicate taproots that are easily damaged during transplanting. Because the plant relies heavily on this root structure, division frequently leads to plant failure.

Lupines (Lupinus)
Lupines possess a long taproot that anchors the plant and allows access to deeper soil moisture. This structure does not regenerate well after disturbance.

Butterfly Weed (Asclepias tuberosa)
Like many milkweed species, butterfly weed forms a deep and brittle taproot. Once established, it strongly resents root disturbance.

False Indigo (Baptisia)
Baptisia develops a large, woody root system that can extend more than a meter into the soil. Mature plants are extremely difficult to divide successfully and are best left undisturbed.

In these cases, propagation through seed or cuttings is often more successful than division.

Using appropriate tools helps minimize root damage and increases the likelihood of successful plant establishment.

Perennial splitting fork

Garden Spade or Digging Fork
A sharp spade or sturdy digging fork is used to loosen the soil surrounding the plant and lift the root mass from the ground. A fork is often preferred for dense clumps because it reduces the likelihood of cutting major roots.

Dividing Iris

Sharp Knife, Soil Knife, or Pruning Saw
Dense perennial crowns may require cutting tools to separate sections cleanly. Sterile, sharp blades create smoother cuts and reduce the risk of disease introduction.

Water Source or Bucket
Roots should remain moist throughout the division process. Exposure to dry air, even for short periods, can damage fine feeder roots responsible for water absorption.

Compost or Organic Matter
Incorporating compost into the planting area improves soil structure, increases microbial activity, and supplies slow‑release nutrients that support root recovery.

Mulch
Organic mulch helps stabilize soil moisture and temperature around newly divided plants, reducing transplant stress and suppressing weed competition.

1. 

   Pre-water the plant

   Pre‑Water the Plant
   Water the plant thoroughly the day before division. Moist soil reduces root damage during digging and helps maintain root hydration.

2. Loosen the Surrounding Soil
   Insert a spade several inches outside the plant’s drip line and gently loosen the soil around the root ball. Work around the entire clump before attempting to lift it.
3. Lift the Plant Carefully
   Lever the plant out of the soil while preserving as much of the root mass as possible. Shake or rinse away excess soil if necessary to reveal natural divisions within the crown.
   

   Separate the crown

4. Separate the Crown
   Divide the plant into sections using your hands, a knife, or a spade. Each division should contain multiple healthy shoots (or “eyes”) and a substantial portion of the root system.
5. Remove Dead or Woody Sections
   Discard any diseased, woody, or non‑productive portions of the crown. Replant only vigorous outer sections.
6. Replant Immediately
   Plant the divisions at the same soil depth as the original plant. Firm the soil gently around the roots to eliminate air pockets.
7. Water Thoroughly and Mulch
   Provide a deep watering after planting and apply a layer of mulch to stabilize soil moisture and temperature while the plant re‑establishes.

• Divide plants during cool, overcast weather whenever possible.
• Replant divisions promptly to prevent root desiccation.
• Water consistently for several weeks after transplanting.
• Label newly planted divisions to track cultivar placement.

Dividing perennials is an excellent way to keep your garden looking lush and vibrant while preventing overcrowding and maintaining plant health. By following proper timing, techniques, and care, you can ensure that your perennials continue to thrive year after year. Whether you’re looking to rejuvenate aging plants, share with friends, or simply expand your garden, dividing perennials is a rewarding practice that contributes to a flourishing outdoor space. Take the time to assess your plants each season, and enjoy the benefits of a well-maintained garden. The Edmonton Horticultural Society holds a Spring and Fall Perennial Exchange. Bring in your extra perennials to get tickets which allow you to select from the many perennials brought in by others. It’s a fantastic way to share with your fellow gardeners, and a lot of fun.

A little care goes a long way—happy dividing!