When Should You Thin Out Iris Plantings?

Irises can be kept healthy and full of flowers by dividing clumps before they get congested. This is also a good way to increase stocks of plants.

Rhizomes are creeping swollen root-like structures that are actually adapted stems. Roots, stems with leaves and flowers are produced along its length. See plants such as Anemone nemorosa, bamboo, canna, border iris.

These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes.

Irises are perennial flowering plants that grow from rhizomes – thick, underground stems that store nutrients Over time, irises multiply and form large clumps which can lead to overcrowding and reduced blooms. Thinning and dividing congested irises helps rejuvenate growth and flowering But when is the best time to tackle this garden task?

Signs Your Irises Need Thinning

Watch for these indicators that your irises are overgrown and due for division

Clumps exceed 2 feet in diameter with crowded, intertwined rhizomes
Fewer blooms and smaller flowers compared to previous years

Thick foliage with poor air circulation
New growth emerging in the center of the clump rather than the edges
Overall decline in plant vigor and health

Plants spreading out beyond their intended space in the garden

Thinning and dividing overgrown irises every 2-3 years keeps them healthy and flowering at their best. This routine maintenance renews the planting and prevents congestion issues.

When to Thin Iris Plants

For bearded irises, the optimal time for thinning is late summer, typically July – August. Here’s why this season is ideal:

Allows time for new root growth to establish before winter dormancy. Dividing in fall doesn’t allow adequate regrowth before frost.

Prevents loss of next year’s flower buds, which form in fall. Spring thinning removes buds.
Avoids active growth periods when iris is more stressed by disturbance.
Warm weather enables quick recovery after division.

Lower risk of diseases like bacterial soft rot that thrive in cool, moist conditions.
Leaves start yellowing, indicating the plant is going dormant. Energy is moving to the rhizomes, which aids regrowth after dividing.

The exact timing depends on your climate and the bloom period. Use these guides:

Wait 4-6 weeks after blooming finishes to divide. This ensures nutrients get channeled back to the rhizomes.
End dividing 6-8 weeks before first expected fall frost so plants can establish.

Adjustment may be needed for certain iris species that don’t follow typical growth patterns. But for most bearded irises, thinning in mid to late summer is ideal.

The Thinning Process

Here are some tips for properly thinning congested iris clumps:

Use a garden fork to dig under and loosen the entire root mass, lifting it out. Shake off excess dirt.
Slice off foliage to about 6 inches above the rhizome using sharp pruners. This reduces transplant stress.

Gently pull rhizome sections apart with your hands, breaking them at natural dividing points. Don’t cut them apart.
Inspect rhizomes and roots as you divide. Remove any that are diseased, damaged, or haven’t leafed out.
Replant vigorous rhizomes that are around pencil-thickness. Discard excessively large, old rhizomes.

Trim leaves further to 4-6 inches on replanted rhizomes. This promotes root regrowth over foliar growth.
Space rhizomes 12-24 inches apart depending on variety size to avoid re-crowding.
Water thoroughly after replanting to settle soil. Avoid mulching which can cause rotting.

If needed, you can further divide the healthiest rhizomes for propagating or sharing extras with fellow gardeners. Replant the youngest, most vigorous sections.

Alternatives to Summer Thinning

Some situations may call for dividing irises at other times:

Transplanting existing plants – Divide when digging up irises to relocate them.
Emergency thinning of struggling plants – If iris shows severe decline, thin anytime to save it.

Post-bloom thinning – Dividing immediately after flowering is less ideal but still works.
Fall or spring thinning – Where winters are very mild, fall division is possible. Spring thinning is risky due to flower bud loss.

While not optimal timing, iris plants are quite resilient to being divided at less favorable times if needed.

Enjoy Vigorous, Flowering Irises

A routine thinning schedule keeps your iris plantation healthy and overflowing with blossoms. By dividing when rhizomes become overcrowded, usually every 2-3 years in mid to late summer, you’ll enjoy vigorous irises and have extras to share. Time division properly and you’ll be rewarded with a flush of refreshed blooms.

RHS Malvern Spring Festival

Irises can be kept healthy and full of flowers by dividing clumps before they get congested. This is also a good way to increase stocks of plants.

Both clump forming irises and those with

When to divide irises

Rhizomatous irises, which have fleshy stems at ground level, are split just after they flower.
Irises that grow in groups are split up in early fall or early spring.

How to divide and thin out your irises.

FAQ

What time of year do you divide irises?

Ideally, though, late summer is the best time. Dividing in late July or August allows the semi-dormant plants to become well-established and to reserve plenty of carbohydrates. Iris divided in late summer are more likely to flower the next spring. Divided in fall or winter, they may not flower until the second spring.

When should you thin your iris?

Iris need to be thinned or divided before they become overcrowded, generally every 3-5 years. If Iris are allowed to become too crowded the bloom will suffer, some varieties may crowd others out and disease problems may be aggravated.

What happens if you don’t thin irises?

If you don’t divide Bearded Irises this is what happens You will eventually have fewer flowers and by and by the rhizomes will die or they have already rotted through diseases. By not dividing bearded Iris and not keeping your garden hygienic you are inviting dead decaying plant eaters to your garden.