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Thinning and Dividing Flowers
In This Section
In this section, you’ll find discussions, explanations, directions and demonstrations of ways commonly used to thin flower plants, lift plants from the soil, divide their roots, and store bulbs until they should be planted, including:
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- How to thin, lift, and divide flowers that are too crowded.
- How to create new strong plants from old, weak ones.
- Techniques and demonstrations of how to divide roots and bulbs
- Methods of curing bulbs after lifting them from garden soils.
- Proper storage needs and methods for flower and foliage bulbs.
On This Page
Here, you’ll find discussions of the following subjects on thinning, lifting, and dividing flowering plants and bulbs beneath each of the following titles:
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- Reducing Crowding and Renewing Flowers
- Thinning Flowers
- Thinning Groups of Individual Plants
- Thinning Rhizomatous, Tuberous, and Stoloniferous Plants
- Dividing Bulbs
- Why Lift Bulbs?
- Lifting and Storing Bulbs
- When to Lift Bulbs
- Dividing Bulbs, Tubers, and Roots
Reducing Crowding and Renewing Flowers
When flower beds become overcrowded it’s time to thin or lift and divide your perennial plants and bulbs.
Because perennials and bulbs repeat their life cycle and reproduce every year, the number of plants doubles or—for those that produce seeds—rapidly expands in just a few years.
These flowers soon become crowded, and need thinning to reduce the number of plants in your containers or beds. Some must be lifted to divide them. Others are easily divided by working through the group, removing and replanting young flowers and discarding older, less vigorous plants past their prime.
Thinning Flowers
It’s easy to tell when flower plant colonies need thinning. Each colony has dead or dying plants in its center with few flowers, and a surrounding ring of vigorous new plants that produce most of the colony’s flowers.
Crowding, shading, and competition for nutrients and water cause this telltale look.
Another reason to thin is more of a matter of judgment. Plant colonies grow so large that the bed’s appearance becomes unbalanced, with a few dominant flowers. They lack the symmetry that you intended when you planted the flower bed.
Regardless of the reason, thinning is the answer.
To begin, look at each colony and note whether its plants reproduce by seed, by spreading from their roots, or by division from bulbs. We’ll look at bulbs separately. Let’s start with the easiest first.
Thinning Groups of Individual Plants
Thinning seed-producing perennials is quick and simple even for novice gardeners.
Take a sharp trowel, begin at the outside of the colony, and find the growth point of each plant—the spot at the base of its foliage where all its leaves join to the roots.
Drive the trowel vertically down into the soil between the growth point and the rest of the colony to cut roots spreading towards the colony’s center. Then remove the plant and set it aside for later inspection and replanting or disposal.
Repeat this process until you remove the entire colony from the bed. You’ll likely have far more plants than you’ll ever be able to replant in the original bed.
Carefully look and grade each plant for its health, signs of damage or stress, and the condition of its roots. Discard any that are weak, damaged, or dying.
Prepare the bed’s soil in the same manner as you did for transplanting new flowers [see: Preparing Garden Soil for Flowers]. This will replace the organic material and nutrients that were used up in growing the colony.
Return to your bed or containers those plants that pass your inspection, either where the colony once grew or in other areas of your flower garden or landscape.
Thinning Rhizomatous, Tuberous, and Stoloniferous Plants
Thinning these perennials is a bit more work than independent plants.
All three spread outward and creating daughter—or pups—plants that, once established, may be divided from the parent.
Rhizomes are fibrous and intertwined. Tuberous roots are swollen, soft, and much different than rhizomes. Stolons are tough, thick and cord-like.
Plan ahead when dividing such plants. Lay a tarp or sheet plastic on the soil or a nearby lawn or walk. You’ll unearth—lift—the entire colony, often a heavy mass requiring two people to move or carry, and place it on the tarp.
Using a sharp spade, cut down vertically into the bed’s soil about 8–12 in. (20-25 cm) out from the edge of the colony and about 12 in. (25 cm) deep. Repeat until a series of cuts surround the entire colony.
Next, replace the spade with a long-bladed shovel, such as one used to trench. Angle a second series of cuts towards the edges of the colony, each outside the first. Remove the pie-shaped pieces of soil between the two series of cuts, exposing a trench.
When the circle is complete around the colony, use your shovel to pry up toward the center of the colony and break free any remaining roots from the clinging soil.
Lift and carry the colony to the tarp.
Finally, follow our directions and demonstration for dividing it into individual plants [see: Thinning and Dividing Flowers].
Dividing Bulbs
Because bulbs are a special case, and because they consist of true bulbs, corms, bulbs with rhizomatous or tuberous roots, and true tubers, it’s best to read the information that follows and refer directly to our demonstrations showing how to lift, divide, cure, and store bulbs [see: Dividing Bulbs and Roots and Curing and Storing Bulbs].
Why Lift Bulbs?
We lift, divide and store bulbs indoors to protect them from freezing temperatures and from heat in areas with cold winters or in arid-desert climates. Such care is seldom needed in mild-winter areas, but divide bulbs when beds become overcrowded.
Lifting is the careful digging of bulbs after their flower displays have faded and all their foliage has died. At that point, the bulbs are dormant and they’ve stored nutrients for their next growth cycle. They’ve also reproduced, either by dividing or by similar processes.
Lifting and Storing Bulbs
Lift bulbs whenever climate conditions—cold, heat, humidity, or precipitation—cause them to stay damp while dormant, freeze, dehydrate, or suffer overly high soil temperatures. After lifting, separate bulbs or divide rhizomes and tuberous-rooted plants to create new, individuals.
Let bulbs remain in the soil from season to season in climates that that mimic their native growing conditions.
Mediterranean climates—such as those found along the Pacific Coast of the United States—are hospitable to many spring bulbs that originated in southern Europe or the Middle East.
Southern California, the Gulf states, Florida, Hawaii, and other mild-winter, semi-tropical climates support in-ground cultivation of evergreen varieties and summer bulbs. Those same bulbs must be lifted in cold-winter climates.
When to Lift Bulbs
Lift spring-blooming bulbs after bloom, when dormancy’s onset helps them retain their health and vitality.
As foliage yellows, mark each bulb’s position to make finding them easy after their leaves wither or are removed. Dig carefully. Start at the outside margins of the bed. Use a garden fork to loosen the soil, aiming its tines deep below the bulbs and prying up to unearth and reveal them.
Shake each bulb to remove any clinging soil. Lay the dug bulbs onto a canvas tarp or newsprint paper. Let most bulbs dry in a warm, dry spot, out of the sun [see: Curing and Storing Bulbs].
Some bulbs lack onionskin-like tunics. They will quickly dry out and die without protection. After lifting store those species in breathable containers, packed in slightly moist peat moss. The Storage section of the Bulbs guide recommends how to store each bulb species.
Dividing Bulbs, Tubers, and Roots
Most bulbs grow offsets, bulblets, bulbils, new tubers, or root divisions before becoming dormant.
Rearing new plants from small bulblets and bulbils often takes several seasons before they bloom [see: Propagating Flowers and Bulbs]. Retain these bulblets and bulbils if you wish to try growing them to maturity; otherwise, discard them.
Carefully clean and separate larger offsets and tubers from the parent bulbs. Plant them with the harvested parent bulbs the following season. They usually bloom in one to two years.
Plants with rhizomatous and tuberous roots form dense, clumping colonies. Bulbs in the center of these colonies lack sufficient water and nutrients. They are stunted and have small or no blooms at all. When colonies become crowded, it’s time to divide their plants to maintain health and vigor [see: Dividing Bulbs and Roots].