How to Grow Bearded Iris in Australia

Bearded iris (Iris germanica hybrids) is a hardy perennial in the iris family (Iridaceae), grown for tall spring spikes of ruffled flowers. It grows from a rhizome (a thick, creeping stem at the soil surface) that flowers once, then carries the clump on through offsets. It thrives on hot, dry summers and cold winters, and kindness kills more plants than neglect: too deep, too wet or too shaded. Dutch iris, a bulb, and peacock iris (Moraea), a corm, are different plants.

Three mistakes that matter

1. Burying the rhizome. A rhizome planted deep stays too cool to flower, and one smothered in mulch is prone to rot. Set it with its top at the soil surface, or under no more than a thin skin of soil, as shown under Planting Rhizomes.
2. Keeping it wet. Summer watering, heavy soil and damp mulch over the rhizome bring on soft rot, the usual way bearded iris die. Plant in free-draining or raised soil, keep mulch off the rhizome and water sparingly once plants are established.
3. Planting in shade. With less than half a day of summer sun, a clump makes leaves but few or no flowers. Choose an open bed in full sun, clear of shrubs and groundcovers that would shade the rhizomes.

When to plant

Plant rhizomes from late summer into autumn, once the worst heat has passed, so new roots settle before winter. Cold winter nights set the flower buds, so the southern and cool zones suit bearded iris best and the tropics do not.

Clumps divided straight after flowering, in November or December, can be replanted then, with only light watering over summer.

Site, soil and setup

An old clump exhausts its patch, so replant divisions into fresh ground or refresh the soil with compost first. A soil test kit settles the pH question in minutes; see soil pH explained.

Container growing

Tall bearded iris grow in pots with drainage holes, provided the mix drains freely and the watering stays light. A wide, shallow pot suits better than a deep one. One 28 to 33 cm wide and 20 to 26 cm deep, about 12 to 18 L, holds three rhizomes. Divide them every couple of years, before they crowd the pot. Fill it with a free-draining premium potting mix that is not high in nitrogen. The shortest varieties in the range, 'Dragons Drum' at about 86 cm and 'Bahloo' at about 89 cm, are the least likely to topple.

• Planting. Set the top of the rhizome level with the mix, which should sit at least 2.5 cm below the rim.
• Watering. Water only when the top 5 cm of mix is dry. Take care in heatwaves, when wet mix rots rhizomes fast; see watering pots through summer.
• Feeding. Dissolve sulphate of potash in water at the label rate and give it once a fortnight from the first buds until the last flowers fade.
• Position. Full sun out of strong wind, and away from a north-facing wall that keeps the pot too warm on winter nights.

Planting Rhizomes

Each rhizome is planted whole, with its fan of leaves and its roots, into a prepared bed in full sun.

Choosing among the varieties

All twelve varieties in the range are tall bearded irises of about 86 to 104 cm, but a few details change how they are grown.

Planting in the ground

• Soak a dry rhizome for an hour or two before planting.
• Dig a shallow hole and build a low mound in its centre, then sit the rhizome on top with the roots spread down each side.
• Face the rhizome north, with the fan of leaves at the back, so the sun reaches it.
• Firm soil over the roots so the base of the rhizome sits snugly on the soil below, at the depth in the table below.
• Space rhizomes 30 to 45 cm apart, or plant three in a triangle, toes pointing in, to form a clump quickly.
• Water in well. New leaves follow once the roots are growing.

Planting depth by district

Care by growth stage

Water decides the outcome: overwatering kills far more bearded iris than drought. Bearded iris are not hungry plants: in a bed, compost at planting and one potassium feed in spring are enough. Keep nitrogen low, because soft, leafy growth rots easily and flowers poorly.

In poor soil, a controlled release granular lower in nitrogen than potassium, around NPK 6-10-10, can replace the compost. Mix it into the planting spot at the label rate, clear of the rhizome.

Division and Maintenance

Every three to five years a clump crowds itself and flowering falls away, which is the signal to divide it. Lift it soon after flowering or from late summer into autumn, as under When to plant.

• Lift the whole clump gently with a garden fork, working in from the outside.
• Snap off young rhizomes, each with a fan of leaves and roots. Discard the old centre, because a rhizome that has flowered never flowers again.
• Trim the leaves by about half and shorten long roots, so each division settles without wilting.
• Replant within a couple of weeks. Rhizomes kept longer go in a cool, dry place, never damp, and are soaked for a couple of hours before planting.
• Snap each spent flower stalk off at its base once its last bud has finished.
• Pull away brown, dead leaves through the year, since they harbour rot and leaf spot.

Companion planting

The best neighbours share full sun and a dry summer without spreading over the rhizomes.

Grows well with

• Roses, which want the same sun, provided their regular summer soak stays off the iris rhizomes.
• Ornamental grasses woven through a sunny border, for their upright, open habit.

Keep apart

• Violets, evening primrose and other creeping perennials, which spread over and shade the rhizomes.
• Thirsty plants that need regular summer watering or deep mulch, which keep rhizomes too wet.

Harvesting Blooms

Cut stems for the vase in the cool of the day, when the first bud is still closed but shows colour at its tip.

• Cut low on the stalk with clean secateurs, leaving the leaves to feed the rhizome.
• Carry stems upright with open flowers apart, because the petals bruise and crease easily.
• Stand stems in clean water; a coloured bud usually opens in a day or two.
• Pick off each faded flower so the next bud on the stem opens.

Cut stems usually last about 3 to 7 days in a vase. A single tall stem makes a vase on its own, or stands above lower spring flowers.

Common problems

No flowers in the first spring. A newly planted rhizome often spends its first year settling in, then flowers the following spring.

No flowers in a warm-winter garden. The rhizomes miss the winter chill they need, as on the Adelaide waterfront, in western Perth and eastern Sydney suburbs and most of Brisbane. A north-facing wall that holds heat overnight has the same effect.

Clump flowering less each year. Crowding, tired soil or too much nitrogen fertiliser are the usual causes. Divide it as under Division and Maintenance. Read more

Brown leaf tips in summer. Dry soil or salt building up from bore water burns the tips. Trim them off if they bother you.

Marked or misshapen flowers. Frost on open flowers or full buds leaves them limp and see-through, and glyphosate drift in the months before flowering deforms the blooms.

Stems kinked or fallen over. Wind dries and bends stems while they grow fast. In warm, wet springs, water trapped where a stem meets a leaf rots the joint.

Pests and diseases

Most trouble comes from wet weather rather than insects, so good drainage prevents most of it.

Bacterial soft rot (Pectobacterium carotovorum). The rhizome turns to a slimy, foul-smelling mush and the fan of leaves yellows and collapses. The bacterium enters through wounds and thrives in wet, poorly drained soil. Cut away all rotten tissue, leave the cut surface in the sun for a few days, then replant in a drier spot.

Sclerotium crown rot (Sclerotium rolfsii). Most common in warm districts and under winter mulch, it leaves the rhizome brown, soft and crumbly, with white fungal threads in the crown. Tan to reddish-brown balls the size of mustard seeds form on the plant and the soil. Bag and bin the plant with the soil around it, and never compost it.

Iris leaf spot (Cladosporium iridis). Small brown, water-soaked spots with yellow margins spread and merge along the leaves, worst after rain and on hail-damaged leaves. Pick off badly spotted leaves and clear away leaf debris. Orange-brown pustules inside pale spots are iris rust (Puccinia iridis), recorded from Tasmania to Queensland; pick off and bin those leaves the same way.

Garden snail (Cornu aspersum) and slugs. They leave ragged, torn leaves, mostly on young growth in cool, damp weather. Hunt them by torchlight a couple of hours after dark, especially after rain, and check under upturned broken pots set out as traps. Read more

Prevention first

• Give rhizomes the drainage and dry, mulch-free surface set out in Three mistakes that matter.
• Avoid nicking rhizomes when weeding or dividing, because wounds let soft rot in.
• Keep nitrogen low, as under Care by growth stage.
• Bin spotted leaves and spent stalks rather than leaving them on the bed; see garden hygiene.

Tips

1. See varieties in bloom at an iris society show, held from mid October to early November; Victoria's falls around Melbourne Cup weekend.
2. Bin or dry out unwanted rhizomes rather than composting them, because tiny offsets regrow, and garden iris has gone wild in parts of southern Australia.
3. Label each clump at planting, since varieties look alike when out of flower.

Due to quarantine restrictions and our shipping policy, we cannot send bearded iris rhizomes to Western Australia or Tasmania.