Irises give the late-spring garden an unmistakable splash of colour, with flowers ranging from deep purple and blue to yellow, cream and almost black. They're also relatively easy to maintain once planted in the right conditions. The catch is that not all garden irises like the same treatment. Bearded irises want their rhizomes warmed by sunshine, while Siberian irises cope with moister soil.
Bearded and Siberian irises are different
Bearded irises, commonly Iris germanica hybrids, have fleshy rhizomes sitting near the ground's surface and fans of blue-green leaves. Their flowers often carry an elaborate ruffled appearance and a fuzzy 'beard' on the lower petals.
Siberian irises, Iris sibirica and its hybrids, have narrower, grass-like leaves and generally enjoy somewhat moister conditions. They grow as clumps and are usually bought as container-grown plants.
Check the label before planting. Instructions to leave the rhizome exposed are intended for bearded irises, not every plant that happens to have 'iris' in its name.
Where should border irises grow?
Most border irises flower best in full sun. Some Siberian varieties tolerate dappled shade, but persistent deep shade reduces flowering. Bearded irises especially benefit from sunshine reaching the upper surface of their rhizomes.
Good drainage is important for bearded irises. Heavy, waterlogged soil can lead to rhizome rot. A raised bed or sunny slope may provide better conditions than a damp hollow.
Siberian irises are more adaptable to soils that remain moist, provided roots aren't permanently submerged. This makes them useful where bearded types might struggle.
When should you plant irises?
According to the RHS, new bearded-iris rhizomes are best planted from July to October. This gives them time to establish after flowering and before the following season.
Container-grown border irises can generally be planted from March until mid-November when conditions allow. Avoid planting into frozen soil, a waterlogged bed or the middle of a serious summer drought without adequate watering.
If you buy bare rhizomes, plant them promptly. Don't leave healthy roots drying in a hot shed while you wait for a convenient weekend.
How to plant bearded iris rhizomes
Prepare a shallow planting hole and spread the roots outwards over a little mound of soil. Place the rhizome horizontally, with the leaf fan upright, and work soil between the roots.
Leave the top of the rhizome partly exposed so sunlight can warm it. This differs from the deep planting used for many spring bulbs. Burying a bearded iris under a thick layer of soil or mulch can discourage flowering and encourage decay.
Allow around 30cm between larger plants, or roughly 15cm for dwarf types, following variety advice. Firm the soil around the roots and water to settle everything in.
For a tidy display, arrange groups with their leaf fans facing the direction you want them to grow. Plants gradually spread and eventually need dividing.
How to plant Siberian irises
Siberian irises are often sold in pots. Water the rootball, remove the container and set the plant at its previous growing depth. Backfill and firm gently, then water thoroughly.
They are good candidates for a border with moisture-retentive soil. Do not copy the exposed-rhizome technique used for bearded irises. Instead, treat the plant like a clump-forming herbaceous perennial.
Leave enough space around young plants for the clumps to expand. Once established, their upright leaves can make an attractive screen even after the flowers finish.
Watering and feeding iris plants
Newly planted irises need adequate water during their first growing season. Once established, bearded irises tolerate fairly dry conditions and generally need extra watering only during severe drought.
Siberian irises prefer more consistent soil moisture. Container-grown plants of either kind dry out more quickly than those in the border, so check their compost regularly during warm spells.
Irises do not need heavy feeding. A modest layer of well-rotted garden compost around the plants in spring can improve soil condition. Keep thick mulch off exposed bearded-iris rhizomes.
An overfed iris can produce leaves at the expense of flowers. More fertiliser won't correct lack of light or a congested clump.
Why does an iris have leaves but no flowers?
Insufficient sunlight is a common cause. A bearded iris planted under overhanging shrubs may have plenty of green foliage without enough energy to develop flowering shoots.
Crowded clumps are another problem. Older bearded iris rhizomes may become tangled and exhausted in the centre, needing division to restore flowering.
Check planting depth too. A rhizome hidden beneath soil or deep organic mulch misses the warming sunshine it prefers. Finally, consider whether the plant was divided too recently and is still establishing.
Dividing bearded irises after flowering
The RHS recommends dividing established clumps every three or four years when they become crowded. Summer, after flowering, is generally a suitable time.
Lift the clump carefully and shake off loose soil. Select healthy young rhizome sections with roots and a fan of leaves, discarding rotten or badly damaged pieces. The RHS suggests useful sections around 10–15cm long.
Shorten excessively long leaves to reduce wind resistance and moisture loss, then replant the selected sections with their upper surfaces exposed. Water while roots establish.
Dividing the clump is often more useful than simply adding fertiliser around old, congested plants.
Dividing Siberian irises
Siberian iris clumps can also be divided, although their root system is different. Lift the plant and separate suitable sections, each with healthy roots and shoots. A sharp spade may be needed for a large mature clump.
Replant the divisions at their normal depth and water well. Avoid allowing exposed roots to dry while you work. A freshly divided Siberian iris may need a season to regain full flowering vigour.
Caring for irises after they flower
Remove faded flowering stems by cutting them near the base, taking care not to damage healthy leaves. Those leaves continue feeding the plant and should generally remain until they yellow naturally.
Remove occasional dead or diseased leaves to keep the border tidy. Don't cut all green foliage to the ground immediately after flowering; the plant still needs it to replenish its stored energy.
Iris leaf spot can cause brown markings. Improve air circulation and clear badly affected dead material rather than allowing a damp mass of leaves to build up around the clumps.
Can you grow irises from seed?
Yes, but it is much slower than dividing established plants. Seeds collected from named garden hybrids may produce offspring that differ from their parents in colour and shape.
The RHS advises collecting ripe seeds in autumn, soaking them for 48 hours and sowing into seed compost covered with grit, then keeping the pot sheltered outdoors. Some may germinate the following spring, while others take another year; flowering can take several years.
For a predictable display of a favourite variety, buying a rhizome or dividing a known clump is simpler.
Common iris problems
Rhizomes feel soft: Poor drainage may have caused rot. Remove damaged material and improve growing conditions before replanting healthy pieces.
Flowers fall over: Tall stems may need temporary support in exposed borders.
Leaves have brown spots: Iris leaf spot is a possibility; remove affected dead debris and improve airflow.
Clumps flower less each year: Look at congestion, shade and planting depth before increasing fertiliser.
Building a reliable iris display
Choose bearded irises for a sunny, well-drained spot and Siberian irises for a border that holds more moisture. Plant at the correct depth, keep the leaves healthy and divide overcrowded clumps every few years. With those basics in place, irises can become some of the least troublesome flowers in the garden.
Source
Royal Horticultural Society: How to grow border irises.