Germination temperature
Needs cold first: 1-5 °C (34-41 °F) for at least 6-8 weeks, then germinates as spring warms.
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Iris seed can be grown at home, but it needs a winter of cold, moist conditions before it sprouts and two to three years before the first flowers. Plan for patience, and expect seedlings that differ from their parents.
Needs cold first: 1-5 °C (34-41 °F) for at least 6-8 weeks, then germinates as spring warms.
About twice the seed's diameter, roughly 1 cm (0.4 in).
Usually the first spring after chilling; some seed waits another year.
Full sun for growing on.
Cold, moist stratification is essential.
Sow in pots or trays, plant out young plants.
Sow seed fresh or soon after purchase, and label each batch, because germination can be spread over more than one season.
Not usually needed. The hard seed coat and natural inhibitors are dealt with by soaking and a cold, moist period rather than by nicking.
Essential. Either sow in autumn and leave the pots outdoors through winter rain and frost, or mix seed with damp paper towel or damp vermiculite in a sealed bag and keep it in the refrigerator at 1-5 °C (34-41 °F) for 6-8 weeks or longer; Minnesota iris growers chill seed for at least 60 days.
Optional but helpful. Some growers keep seed in water for several weeks before or during chilling, changing the water weekly to wash out germination inhibitors; they report higher germination in the first year.
After chilling, germination starts as temperatures rise in spring, roughly from 10 °C (50 °F) days up to about 21 °C (70 °F). Where winters are warm, as on the Indian plains, chill the seed in the refrigerator instead of relying on outdoor cold, and sow in the cool season.
Cover the seed to about twice its own diameter, about 1 cm (0.4 in). Seed left just on the surface can sprout too if it never dries out.
A free-draining seed compost in pots or trays; Minnesota iris growers use a mix of about 60% peat moss with garden soil.
Keep the compost evenly moist through the cold period and during germination; seed that dries out may not sprout until a later season.
No special light is needed for germination. Once seedlings show, give bright light and then full sun.
Most seed comes up in the first spring after chilling. Some seed waits until after a second winter, and slow aril irises have been recorded taking years. Keep pots that show nothing for at least another season before discarding them.
Plant seedlings out when they are about 15-20 cm (6-8 in) tall, spacing them 20-25 cm (8-10 in) apart in a nursery bed; space mature clumps about 45 cm (18 in) apart.
Keep young plants weeded and watered in their first summer. Some seedlings may flower in the second year; most flower by the third.
Cause: no or too short a cold period, or the seed dried out. Fix: give at least 6-8 weeks of cold, moist conditions and keep unsprouted pots for another winter.
Cause: bacterial soft rot, entering through damage and favoured by wet soil. Fix: grow bearded irises in well-drained soil, dig out and destroy rotten rhizomes.
Cause: iris borer larvae (in North America). Fix: clear and destroy old leaves and debris in autumn or winter, where the eggs overwinter.
Cause: fungal leaf spot, which overwinters on old foliage. Fix: remove dead leaves and keep air moving around plants.
Cause: seed of hybrid irises does not come true. Fix: treat seedlings as a mix and keep the plants you like; increase favourites by dividing the rhizomes.
Bearded irises need sun, good air movement and well-drained, limy soil; beardless types such as Japanese and Siberian irises prefer moist, slightly acidic soil.
When planting out bearded irises, keep the rhizome just below the surface in medium soil, slightly above it in heavy soil and about 5 cm (2 in) deep in light soil.
Divide established clumps every two to five years in late summer or early autumn after flowering. Divisions of a seedling you like are identical to it.
Irises are long-lived perennials; a well-placed clump can stay in one spot for many years if divided when crowded.
Give iris seed a cold, moist winter and expect the first flowers in two to three years.
Most seed comes up in the first spring after a cold period of at least 6-8 weeks. Some seed waits until after a second winter, so keep pots for another season.
Usually not. Garden irises are hybrids, and their seed produces varied seedlings. To get an identical plant, divide the rhizome of the plant you like.
Yes, if you chill the seed in the refrigerator for 6-8 weeks before sowing and sow in the cool season. Keep seedlings out of waterlogged soil in hot, wet weather.