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Thinning and Dividing Perennials
Time to Separate, Divide and Multiply
Thinning and dividing keeps perennial flowers healthy so your landscape and containers will bloom with beautiful blossoms.
Perennial flowers become crowded and require thinning or dividing because they live and multiply for years.
The only difference between thinning and dividing perennial flowering plants is how you finish.
In thinning, you’ll remove and discard specific weak or crowded plants from a crowded colony. In dividing, you’ll remove both healthy and weaker plants, then discard those in poor health while saving the rest to plant again.
They are specific techniques also used in propagation to make new plants [see: Rooting, Layering, and Cuttings].
Step-by-Step Instructions
It’s time to thin or divide colonies of perennials whenever the group of plants becomes unsightly or flower production becomes uneven.
It is common for crowding to cause plants on the outside of the colony to thrive while those in the center die out or become unhealthy.
Less common reasons to thin or divide are for aesthetics, to restore balance and order to the bed or border’s design, or to plant another area of your landscape with flowers.
Only simple gardening tools, materials, and implements are needed to think and divide flowers.
You’re ready to learn how to thin plant colonies and divide roots into new plants.
Step-by-Step Instructions
Because thinning and root division create stress for plants, they are activities usually performed at the end of the growing season. That’s when many perennial flowering plants slow their growth and enter dormancy after they end their blooming cycles.
Exceptions are some early-season bloomers that produce their flowers on second-year growth. For them, it’s best to prune as soon as flower production stops. If you prune their seasonal growth away in autumn, few flowers will appear the following year.
That gives them time to produce the new foliage spurs, stems, and branches that will bear flowers the following spring.
Gather a garden fork, a fabric tarp or piece of burlap and choose the method your plants require to divide and replant them:
Three Ways to Divide Roots
Tuberous- or Fibrous-Rooted Plants
Carefully dig up the colony and gently shake soil off the roots.
Using two hand forks, pull the plant apart into sections bearing complete roots, crown, and leaves.
Discard any incomplete or withered sections.
Replant divisions into amended soil, setting them at same depth as when dug. Firm the soil.
Apply water mixed with half-strength fertilizer to settle the soil around the divisions’ roots.
Heavy-Rooted Plants
Use two spading forks to lever apart the plant’s roots and foliage.
Divide the plant or colony of plants into two or more sections, each bearing roots, crowns, and foliage.
Discard incomplete or withered sections.
Remove yellowing foliage.
Replant divisions into amended soil, setting them at same depth as when dug. Firm the soil.
Apply water mixed with half-strength fertilizer to settle the soil around the divisions’ roots.
Dividing Rhizomes, Tubers, and Corms
Divide after flowers fade, carefully digging up each rhizome, tuber, or corm.
Remove shriveled rhizomes and roots plus any plant with symptoms of pest damage or disease.
Use a sharp knife to divide each rhizome into V-shaped pieces with two growing tips. Leave foliage intact.
If desired, dust the root edges with fungicide powder to discourage disease.
Replant the rhizome sections on the soil surface, with only their roots buried underground.
Water to settle the soil. Avoid applying high-nitrogen fertilizer.
Dividing Container Foliage and Flower Plants
Divide plants in containers when they become root bound or overgrown.
Root division of container plants requires removing the plant from its container, dividing it, and replanting each root division.
Gather a sharp, long-bladed garden knife, a shovel, two garden hand forks, gloves, and a tarp or large, shallow basin, and follow these easy steps:
How to Divide Rootbound Container Plants
Use a garden knife to cut the outside edge of the rootball free from the sides of the pot.
Carefully lay the container on its side, supporting the plant’s foliage with your open palm.
Gently slide the plant’s container off the rootball, avoiding pulling the plant free by grasping its foliage or stem.
Fibrous-Rooted Plants
Use two hand forks to pry the individual plant clusters apart.
Divide the plant’s rootball into separate sections, each containing a complete root, crown, and leaves.
Colonies of Separate Plants
Use two hand forks to pry the individual plants of the colony apart.
When divided, each plant will have its own roots, crown, growth point, and foliage.
Rhizomatous Plants
Use a sharp garden knife to cut or separate each rhizome into V-shaped pieces.
Each division should have at least two growth points.
Thick-Rooted Plants
For large plants with thick, heavy roots, use sharp trowels to cut between complete plants, dividing them into smaller groups or individual plants at each section.
Each section should bear one or more crowns and roots.