Growing Irises:

For those of you not already familiar with irises, here is our jargon buster for the specialist terminology you’ll find in descriptions of irises and some advice on how to grow them successfully.



who’s who; awards; anatomy of an iris; flower types; cultivation; cut flowers; pests and diseases

The who’s who of the bearded iris world:


Bearded irises are named for the tuft of hair-like filaments in the throat of the flower, which distinguish them from the beardless irises (such as Siberian irises), which lack this structure.  Bearded irises are classified in 6 groups according to size.  Their flowering time broadly correlates with size, the Miniature Dwarf Bearded irises being the first to flower and the Border Irises and Tall Bearded Irises coming last (with further variation in flowering time within each class).  Although individual blooms are fleeting, by choosing irises with different periods of bloom you can achieve a succession of colour through the iris season.

Tall Bearded Irises (TB): Over 70cm, most being around 90cm but some being even taller.  Flowering period ranges from the end of April to the beginning of June.  Within that range, TBs are classified as early (E), mid (M) or late (L) flowering.  How long a given TB will flower for depends on how many flower stalks it throws up and how many buds each carries.  Those that carry many buds, opening in succession, may manage to flower for 4 weeks whereas those with fewer may only flower for 2-3 weeks.

Miniature Tall Bearded Irises (MTB): Under 70cm.  Flower at same time as Tall Bearded irises but have shorter stems and smaller flowers making them more suitable for windy and exposed sites.  Differentiated from Border Bearded Irises by flower style (Miniature Tall Bearded irises are more akin to historic Tall Bearded Irises in having simple but elegant flowers that generally lack the more theatrical features produced by modern hybridisation, such as ruffling).  More delicate and can be swamped in a mixed border unless carefully placed.  Their beauty merits being given their own space in the garden.  MTBs are also known as Table Irises because of their popularity as cut flowers.

Border Bearded Irises (BB):  Under 70cm.  Flower at same time as Tall Bearded irises but have shorter stems and smaller flowers making them more suitable for windy and exposed sites.  As their name indicates, they are suitable for being integrated in mixed borders.  Differentiated from Miniature Tall Bearded irises by flower style (Border Bearded Irises are more akin to modern Tall Bearded Irises in showing features such as ruffling).

Intermediate Bearded Irises (IB): Under 70cm.  Flowering time bridges from the last of the Standard Dwarf Irises through to the earliest of the Tall Bearded Irises. Useful in extending flowering season earlier than TBs.  Can be integrated in mixed borders as long as not too overshadowed.  Differentiated from Border Bearded Irises by flowering time (Intermediates flower earlier) and from Miniature Tall Bearded irises by flower size and style (Intermediates have larger flowers and may show more ruffling than MTBs).

Standard Dwarf Bearded Irises (SDB): Under 38cm.  Early flowering (along with daffodils and tulips). Tough plants creating sheets of colour.

Miniature Dwarf Bearded Irises (MDB): Under 20cm.  Very early flowering (just after crocuses).  Need a definite cold period to flower properly.  Hardy plants but easily elbowed out by aggressive neighbours.  Good in rock gardens or raised beds.

Remontant irises (R): Some irises have shown themselves to be capable of reblooming after the initial flowering season has past.  These are often described as “remontant” or may simply be referred to as rebloomers.  There is a great deal of variation in when and how reliably such irises rebloom.  Repeat flowering makes additional demands on the plant, which therefore needs additional watering and fertilisation and even then reblooming cannot be guaranteed as it depends not just on the genetic make up of the plant but on the vagaries of weather and soil conditions.  It has been suggested that some remontant irises may be somewhat less resistant to very hard winter frosts and for that reason it is worth taking particular care to give them good drainage. However, in principle, remontant Irises may produce a fresh flush of flowers anytime from July through to the first frosts and we have even known one to attempt to do so at Christmas.  This is a welcome bonus which makes any additional care worthwhile.

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Bearded irises have exerted a long-standing fascination.  The wild type was named for a Greek goddess (Iris, the personification of the rainbow).  The history of their selection and development as garden plants goes back many centuries. There are associations devoted to them in many parts of the world.  Initiates into their cult dub themselves Irisarians.  Hybridisers dedicate decades of their lives to raising new cultivars good enough to compete for the top iris awards.  You don’t need to go to those extremes but you might find it useful to know what the main awards are, as they mark out plants that have been found to represent the best of their kind.

Dykes medal: This started in 1927 and has been described as the Nobel prize for irises.  There are separate Dykes medals awarded in the USA, UK, New Zealand and Australia.  These are the top award for the best iris from a hybridiser in the given jurisdiction.  A Dykes medal was also awarded in France but this was discontinued in 1938.

Some other awards:

In the USA: the Caparne-Welch Medal for best Miniature Dwarf Bearded; the Cook-Douglas Medal for best Standard Dwarf Bearded; the Knowlton Medal for best Border Bearded; the Morgan-Wood Medal for best Siberian; the Sass Medal for best Intermediate Bearded; the Williamson-White Medal for best Miniature Tall-bearded; The Wister Medal for best Tall Bearded Irsi; Honorable Mention; and Award of Merit.

In the UK: the Award of Garden Merit (AGM) from the Royal Horticultural Society.

In France: the Franciris awards (Fleur de Lis).

In Italy: the Premio di Firenze (top prize, the Golden Florin).

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The parts of the iris:

Rhizome: Not a root but an adapted stem, which sends roots down into the ground to anchor the plant and produces buds from its sides, from which the fans of leaves and flowering stalks develop.  To develop flowering stalks, the rhizome needs exposure to the sun.  If you think of it as a stem which needs to root itself you’ll find it easier to get planting depth right (i.e. with the back of the rhizome exposed to the sun but its underside in the soil, ready for the roots to reestablish).

Standards: the three upright petals which form a crown at the top of the flower.

Falls: the three drooping or flaring petals which form the lower part of the flower.

Beard: hair-like filaments at the base of the fall.  These can either match the colour of the flower or provide a contrasting accent.  In some irises the beard has developed into a fantastical elongated structure (these are known as Space Age irises, see below).

Style arm: an arching structure inside the flower, under which lurks the anther and on whose lip is to be found the stigma.  If a bee or other pollinating insect successfully introduces pollen from the one to the other your iris may produce a seed pod, which will ripen in the autumn if you leave it but is better removed to encourage the plant to put all its energies into growth.

Fan: iris leaves develop as a fan-like array of sword shaped leaves.  When first supplied, each rhizome will have one trimmed fan of leaves at the end.  Further buds will develop on the side of the rhizome as the plant establishes.  These will develop into new rhizomes each with their fan of leaves.  This growth habit means that irises need periodic division to stop the rhizomes getting overcrowded and that when first planting you should point the fans away from one another. For example, when planting a group of three irises of the same colour, space them 30cm apart and point the “tails” of the rhizomes into the centre of the triangle they make, with the fans facing outward.  That way as the new rhizomes develop they will be good neighbours rather than becoming too quickly congested.

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Terms used to describe Iris flowers:

Flower patterns:

Self – single colour.
Bi-tone – two shades of a colour. Usually the standards are the lighter tone but in a reverse bi-tone the standards are darker than the falls.
Bi-colour – standards and falls of distinctly different colours.
Variegata – a bi-colour with yellow/red combination or variants on those.
Amoena – a bi-colour with white standards and coloured falls. A reverse amoena has white falls and coloured standards.
Neglecta – a bi-colour with blue standards and darker falls.
Blend – two or more colours present in the same parts of the flowers.
Plicata – white or yellow ground with coloured stitching or band around edges.
Luminata – combination of variegata/amoena types and plicata pattern.  Effect is a colour wash over lighter standards and light coloured veining/central light patch on falls.
Petal texture may range from velvety to glossy and some varieties are “diamond dusted” having groups of cells that reflect light.

Flower form:
Historic forms tended to have imperfectly upright standards and falls that hung straight down, often narrow and “pinched in” at the sides.  At their best, however, they have a simple elegance and can be easier to combine with other plants in the garden than the more flamboyant of the modern forms.
Modern forms generally have broader petals, upright standards, and some degree of flare in the angle at which the falls are held.
Many modern cultivars show ruffling (undulating falls and standards) or lacing (frilled edges to the petals) or both, whereas most historic and some modern cultivars (especially MTBs) have a more tailored shape.
“Space age” cultivars (such as Thornbird) have extensions to the beards (horns, spoons or flounces).  Some find these charming, others think them just weird.  You’ll have to decide for yourself.

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Here is some advice to get you started. In truth, irises are tolerant and you can get away with a great deal. However, if you want to get the best out of them it pays to follow some simple rules.  For any terminology you are unfamiliar with, refer to the jargon buster.

When your irises arrive:

Bare rooted iris are freshly harvested and should be planted as soon as possible, ideally within three days and in any event within two weeks of arrival. Soak the rhizomes in water for about an hour first. If you can’t plant them for a few days, plunge them into a tray of moist compost or place them somewhere airy and cool. Don’t leave them in their packaging or where they might overheat.  Don’t worry about any old dead roots that you see. These are spent and will soon be replaced by fresh young ones.  If there are any problems with your order please notify us as soon as possible so that we can rectify the position.  (See terms.)

Soil preparation:

Irises like well drained soil, so if yours is heavy clay, lighten it with the addition of compost. Aim for neutral ph or very slightly acidic.  If preparing new ground plough in grass and cover with clear plastic for at least a month.  Don’t use manure on ground you are preparing for irises as it contains too much nitrogen for irises and tends to encourage soft growth, which is more prone to rhizome rot.


Irises prefer a full day of sun, but will grow and bloom well if given six or more hours of sunlight.  Dig a hole, large enough for the rhizome and roots, mounding the soil slightly if this makes placing the rhizomes easier, but otherwise working the soil back between the roots.  The rhizome should be placed at soil surface on heavy soils (whatever you do, do not bury it in a clay soil or it will rot), but a little below the surface on light sandy soils, as they will work their way back to the surface.  If planting for garden display: replant the divisions in groups, with 30cm (12in) between larger plants and 15cm (6in) between dwarf plants, fans pointing outwards.  This will quickly give you an impact in the garden but will more rapidly result in clumps that need division (after, say, 3 years).

Watering at planting will help to start root growth but regular irrigation should not be needed.

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Ongoing care:

Don’t cut back leaves after flowering.  They are needed to fuel development of next year’s flower buds.  Remove spent flower stalks unless you want the plant to produce seeds.  Remove dead leaves in winter as they harbour pests and diseases.

Don’t mulch irises, as their rhizomes need to be exposed to the sun and air circulation and will rot if buried under mulch.

Fertilise in early spring and again after flowering. (Ideally, with seaweed or mix of 4 parts bonemeal, 2 parts superphosphate of lime, 1 part each of sulphate of ammonia and sulphate of potash at a rate of 60 to 75g per square metre. Avoid excess nitrogen as it may cause rot.)

Only 60-75% of Iris bloom the first year after planting. Sometimes they need an extra year to become established. Unusual weather conditions or late spring frosts can also harm iris blooms.  Some types of irises (see Remontant, above) have the capability of reblooming later in the summer or autumn.  However, subsequent blooms are not as reliable as the initial bloom, and depend greatly upon the quality of the soil, climate, and geographic location. Remontancy is therefore not guaranteed.

Irises are generally drought tolerant but there are three times when your irises will need water. Keep the soil moist but not waterlogged after first planting until the roots have taken hold. You will know when the roots have taken hold by the new center leaves coming up. This usually takes 2 to 4 weeks. Once established you should be able to reduce the watering until the autumn and winter rains take over.  Secondly, during dry spells (over three or four weeks long) you will need to give your irises a good deep watering every 3 to 4 weeks depending on the temperature. It is particularly important that your iris gets water during early Spring but generally speaking the climate will take care of that without any need for intervention from you.  Thirdly, reblooming irises need more water in order to develop stalks and flowers in the summer and autumn seasons.  Therefore, if you want to encourage reblooming you may need to water this stype of iris more frequently during dry spells than is necessary for the non-reblooming types.

Lift and divide rhizomatous bearded irises every three to five years.  Doing this maintains the flower power of your irises, which will otherwise diminish as the clump gets congested.  It also gives you the opportunity to rethink their position in the garden and to give lucky friends some rhizomes.  Division is ideally carried out six weeks after flowering (@July), to give sufficient time for the plants to produce new growth for the following season before they enter winter dormancy.  However, you can divide in early Autumn (late August/Sept) provided that you are somewhere with a longer growing season.  Indeed, in areas with hot summers and mild winters, September or October planting may be preferred to avoid moving plants at the peak of summer heat.

Cut away each fan of leaves from the clump, using a sharp knife. Some writers recommend breaking off rather than cutting, the theory being that a break creates less of an entry point for disease as the break runs between rather than across plant cells.  Each fan should have a portion of young rhizome (up to 15cm/6in long for tall bearded irises, smaller for miniature tall bearded irises). Select the largest fans with the healthiest rhizomes. Discard smaller fans and old, withered looking rhizomes.  Shorten the leaves to about 15cm (6in) above the rhizome and trim the roots to shorten them.  Trimming the leaves prevents wind rock while the roots are developing, whilst trimming the roots makes replanting easier and avoids the plants being pulled up by birds (who mistake the roots for worms!) The old roots will die back anyway after flowering and new roots will form for the next season.

Follow planting instructions above for replanting.

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Irises as cut flowers:

One of the pleasures of having irises in the garden is to cut some for the house, where the beauty of their form and their perfume can be appreciated at close quarters.  Cut your irises early in the day with the buds just opening. Place them in a bucket of tepid water and re-cut the stem end underwater at an angle 2 to 3 cm up. Note that cut iris may drip a sap-like substance and in the case of dark coloured iris this can stain porous surfaces, so it is best to place the vase on a non-porous surface.

Pests and diseases:

Irises are tough.  They are not prone to many pests and diseases and those that do affect them are for the most part easily controlled.

Rhizome rot (Bacterial soft rot – toppling fans, mushy rhizomes, foul odour).  Thrives in wet weather/heavy soil or if plants are over fertilised with nitrogen (so is best avoided by maintaining the right soil conditions). Scoop soft tissue away (you can use a teaspoon dipped in chlorine solution for this).  Ideally, drench the wounds with a 10% bleach solution (1 part bleach to 9 parts water) and allow them to dry for several days. Then replant and the plant should recover.

Fungal diseases (sclerotia and fungal leaf spot) – yellow tips to leaves, plants look moldy or brown rims around yellow spots on leaves.  Treat with systemic fungicide.  However spores remain in soil so prevention is best – avoid overcrowding and clean up dead foliage.

Iris borer – white caterpillar, turning into brown pupa, grey moth. Remove dead foliage on which eggs are laid.  Watch for notches on centre fans indicating caterpillar activity.  Pinch fan to crush caterpillar hidden inside.  There are nematode treatments for borers.  (You can also use a systemic insecticide containing dimethoate when the iris leaves are about 4 to 6 inches tall, repeated 10 days later.  However, our own preference is to avoid insecticides.)

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