Birthday
HydrangeasThis article was prepared with the support of UFL flower delivery. Irises are perennials that flower for decades when planted correctly. Most flowering problems concern bearded irises, the most popular garden group. Siberian and Japanese irises grow differently, as explained below. For more on groups and varieties see our article on types of irises.
8 reasons irises do not flower
1. Too little sun
Bearded irises need at least 6 hours of direct sun. In the shade of trees or by a north wall they grow leaves but no flower stems. The fix is to move them to an open, sunny spot.
2. Planted too deep
The most common mistake. A bearded iris rhizome needs to bask in the sun: its top should sit at soil level or slightly above, with only the roots underground. A buried or mulched rhizome will not flower and tends to rot. Carefully scrape away soil to expose the top of the rhizome.
3. The clump is overcrowded
After 3–5 years the rhizomes crowd each other and a bare patch of old, leafless sections forms in the middle. Lift and divide the clump.
4. Too much nitrogen
Manure, rich compost and nitrogen fertilisers produce lush leaves instead of flowers and encourage rot. Irises need potassium and phosphorus rather than nitrogen.
5. Recent division or a division that is too small
A division with one small fan needs 1–2 years to build up. If irises were moved in late autumn or spring, they may skip flowering that year.
6. Wet soil and rot
In hollows and on heavy clay rhizomes rot (bacterial soft rot with an unpleasant smell). Raise the bed, add sand and fine gravel, and cut out affected parts back to healthy tissue.
7. Frost-damaged buds
Flower buds form in autumn. A frosty winter without snow or late spring frosts can damage them. Choose hardy varieties that are proven in your area.
8. Pests and diseases
Thrips, slugs and leaf spot weaken the plant. Remove old leaves in autumn and keep plantings well ventilated.
What to do when
How to plant and divide irises
- Timing. July–August, 2–6 weeks after flowering. The division needs 1–1.5 months to root before frost.
- Dividing. Lift the clump, shake off the soil and cut it into sections with a sharp knife: each division needs a 5–7 cm piece of young rhizome with a fan of leaves and roots. Discard old leafless sections.
- Treatment. Let the cuts dry for 1–2 days in the shade or dust them with charcoal. Trim the leaves into a fan 15–20 cm tall and the roots to 10 cm.
- Planting. Make a mound in the hole, set the rhizome on it, spread the roots out and cover only them. Leave the top of the rhizome exposed and point the leaf fan south.
- Spacing. 30–50 cm between divisions, so you will not need to divide again for 3–5 years.
Soil, watering and feeding
- Soil. Light, free-draining, neutral or slightly alkaline. Add wood ash or dolomite lime to acidic soil and sand to clay.
- Watering. Water bearded irises only in drought, at the base. They cope with dryness better than with waterlogging.
- Feeding. In early spring use a phosphorus-potassium fertiliser with a little nitrogen, and after flowering phosphorus and potassium (superphosphate, potassium sulphate) as the label says. Do not add manure or fresh compost.
- After flowering cut the flower stems at the base but keep the leaves until autumn: they feed the rhizome and next year's buds.
Siberian and Japanese irises
Siberian irises dislike drying out, flower in light shade and moist soil, and their rhizomes are planted 3–5 cm deep. Japanese irises need acidic, rich soil that is moist in spring and summer but drier in winter. If these irises are grown like bearded ones, on dry sand, they will not flower either.
Winter care
Established bearded irises overwinter without cover in most of Ukraine. In autumn cut the leaves back to 10–15 cm and clear debris. Young divisions and tender varieties can be lightly covered with conifer branches once the ground freezes; remove the cover as soon as the snow melts. A thick layer of manure or soil causes rot.
FAQ
When do irises flower?
Bearded irises in May–June, Siberian irises in late May–June and Japanese irises in late June–July. Reblooming varieties flower again in September.
Can I move irises in spring?
You can, but they will probably not flower that year. The best time is after flowering.
How long can an iris stay in one place?
Bearded irises 3–5 years, Siberian irises up to 10 years without dividing.
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Author: Maria Svitlovir
Graduated from Taras Shevchenko National University of Kyiv, Institute of Biology and Medicine (2001). BSc in Biology, MSc in Horticulture. Writes about plants, works at a garden centre selecting and repotting plants, and contributes to plant reference guides.
