Irises for moist soil (beardless iris)

For gardens that do not dry out completely in summer, beardless irises in their various forms will give a great display through the season.

Iris 'Caesar's Brother'

Iris 'Caesar's Brother'

Quick facts

Common name Bearded iris
Botanical name Iris
Group Herbaceous perennial
Flowering time Spring to summer
Height and spread Various
Aspect Sun
Hardiness H7 Hardy
Difficulty Easy

Cultivation notes

Cultivation

Beardless irises thrive in a soil that is rich and moist. It is important that the soil does not dry out in summer. They benefit from an annual mulch of bark or garden compost. The two exceptions are Iris unguicularis and I. foetidissima which like slightly drier conditions. Others, such as the Siberian irises are well-known for their tolerance to most soil conditions. Some species, such as Iris ensata and I. laevigata are at home in or by water.

  • Plant in spring and apply mulch
  • In subsequent years apply a balanced organic-based fertiliser such as fish, blood and bone or Vitax Q4 at 70g per sq m
  • After flowering, cut down flowered stems to near ground level
  • Divide every three or four years – flowering starts to decline as clumps become overcrowded
  • In autumn or as foliage deteriorates later in the season, remove affected material to near ground level with sharp secateurs

See our page on bearded iris for irises suitable for dry soils.

Propagation

The usual and easiest method of propagating beardless irises is by division in summer.

  • Use two garden forks placed back to back and prise the clump apart into pieces of no less than four to five rhizomes or about the size of a 13cm (5 in) pot
  • Plant 3-5cm (1-2in) deep and water well until established. Cut back the foliage of the plants by two thirds to reduce transpiration and wind rock

You can also collect seed as soon as it is ripe. The pods will change colour to brown and start to split open. Sow early in the year, but bear in mind that the seedlings from cultivars do not ‘come true’ and will produce a range of flower colours.

  • Soak the seed for 48 hours and sow in John Innes seed compost with 25% extra grit by volume. Cover the seed with a thin layer of grit
  • Place seed tray in a cold frame or in the shelter of a wall
  • Germination may be erratic, so don’t give up - more seed may germinate in the second year

Cultivar Selection

Beardless irises are so called because they lack the line of coloured hairs on central line of the ‘falls’ (lower petals).

The following irises are available from a range of nurseries in the RHS Find a Plant;

Iris ‘Silver Edge’ (Sib) AGM: Rich blue standards (upright petals) finely edged white, and deeper blue falls with a dark violet tinge, edged with white, and yellowish white at the base. Narrow foliage and stems to 1m. Late spring-early summer

Iris pseudacorus ‘Roy Davidson' AGM (yellow flag): Light yellow standards with light brown veining. The falls have a crescent-shaped brown central flash. Stems can reach 1.2m (4ft) in height. It can become invasive. Early summer flowering

Iris laevigata (water iris): In June and July purple-blue flowers are produced in groups of two to four plants reaching 80cm (32in) in height.

Iris ensata 'Rose Queen' AGM (Japanese water iris): More unusual and likes a very wet soil. Flowering in midsummer, blooms can be up to 25cm (10in) across. Height 80cm (32cm)

Iris ‘Green Spot’ (SDB) AGM: Ivory blooms with an olive-yellow mark on the falls (lower petals). Late March-April flowering over upright blue-green foliage. Height 25cm (10in)

Links

AGM Iris

Problems

Beardless irises are generally trouble-free, grown in the right conditions.

They can be affected by a few diseases. Iris leaf spot is unsightly but there are no controls, so remove affected leaves as you see them.

Iris sawfly can strip foliage and should be picked off as soon as seen.

Non-flowering is an indication that the plants are in too much shade, or that they need dividing.

References

Books

Irises: RHS Wisley Handbooks by S Linnegar & J Hewitt (Octopus Publishing Group 2003), ISBN-13: 978-1845333836 ISBN-10: 1845333837)

The Iris by B. Mathew (Batsford 1981, ISBN-10: 0713460393, ISBN-13: 978-0713460391)

These books are made available through our RHS Lindley Library.


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