Dietes is the plant behind half the commercial landscaping in Australia, and for good reason: it flowers for months, survives on rainfall, tolerates clay, sand, salt and frost, and looks structured all year. It is sold as fortnight lily, fairy iris or wild iris, and gardeners often assume it is a native. It is not, but it behaves like one. The only thing most people get wrong is the pruning, and that mistake is what stops it flowering.
Quick facts
| Attribute | Detail |
|---|---|
| Mature size | Dietes grandiflora 1-1.2 m x 1 m, flower stems to 1.5 m; Dietes bicolor and Dietes iridioides 0.6-0.9 m |
| Position | Full sun to half shade; flowering thins noticeably in deep shade |
| Soil | Almost anything, including heavy clay and coastal sand; prefers drainage but tolerates periodic wet |
| Water | Weekly for the first three months, then rainfall in most climates |
| Climate zones | Subtropical Brisbane, temperate Sydney and Melbourne, Mediterranean Perth and Adelaide, frost-prone inland to about -5 degrees |
| Flowering | Flushes roughly every two weeks from spring through autumn |
| Spacing | 600-800 mm for Dietes grandiflora, 400-500 mm for the smaller species |
Choosing the right variety
Three species are commonly grown in Australia and they are not interchangeable. Dietes grandiflora, sold as fairy iris, is the largest, with broad dark green strap leaves to about a metre and white flowers marked with yellow nectar guides and violet inner segments. Its flower stems stand well clear of the foliage, which makes it the best choice for mass planting where the bloom needs to read from a distance.
The peacock flower, Dietes bicolor, is smaller and finer at 600-900 mm, with narrower foliage and soft pale yellow flowers, each outer segment carrying a dark maroon blotch. It suits narrow beds, driveway edges and pots where the larger species would crowd.
Dietes iridioides is the smallest and the most shade-tolerant. The variegated selection White Tiger carries cream-striped foliage that lifts a dark corner even out of flower. Use it where foliage matters more than flower volume. All three pair well with other tough strappy plants: try lomandra for a contrasting arching form, or read our guide to designing with ornamental grasses before you set out a bed.
How to plant
Dietes establishes best in autumn or early spring, when soil is warm but evaporation is low. Dig a hole twice the width of the pot and no deeper, and set the top of the rhizome level with the surrounding soil. Planting too deep is the failure worth avoiding, because the rhizome sits at or just below the surface in habitat and rots if buried.
Space plants at 600-800 mm centres for Dietes grandiflora and 400-500 mm for the smaller species. That looks sparse in year one and closes into an unbroken mass by the end of year two. Planting closer does not speed it up, it just forces division sooner.
Water in well, then mulch 50-75 mm deep with fine bark or sugar cane, keeping it clear of the crown. On heavy clay, plant on a low mound. In pots, use a free-draining mix and a container at least 400 mm wide, because Dietes builds a dense root mass quickly and dries out fast in anything smaller.
Ongoing care
Watering
Water twice a week for the first month, then weekly until three months from planting. After that, established clumps in Brisbane, Sydney and Perth need no supplementary water in a normal year and only an occasional deep soak in an extended dry spell. In pots, water when the top 30 mm of mix is dry. Dietes tolerates dry soil far better than constantly wet soil, and irrigation that never stops is the usual cause of a collapsing clump.
Feeding
Feed lightly. One application of complete controlled-release fertiliser in early spring is all an in-ground clump needs. Excess nitrogen produces tall, floppy, dark green foliage that falls open in the middle and flowers poorly, which is exactly the look most people are trying to avoid. Container plants can take a second half-rate application in early summer. A layer of compost under the mulch each autumn does more good than extra fertiliser.
Pruning
This is where most Dietes plantings go wrong. The flower stems are perennial, so the same stem produces new buds for several seasons and cutting spent stems back after flowering removes next year's blooms. Snap off only the round green seed capsules, leaving the stem intact. Removing capsules also stops self-sown seedlings appearing in beds and, in coastal Queensland and New South Wales, spreading into nearby bushland. Every three or four years, cut the whole clump to 100-150 mm in early spring; it regrows fully within eight to ten weeks.
Common problems and fixes
No flowers. Almost always too much shade, too much nitrogen, or spent stems that have been cut off. Check all three before doing anything else. Our list of low-maintenance full sun plants shows what else suits the same position.
A clump gone hollow in the middle. Normal after three to four years. Cut it hard to 150 mm in early spring, or lift and divide it into fist-sized rhizome pieces with three fans of leaves each.
Brown, mushy bases after wet weather. Rhizome rot from planting too deep, mulch piled against the crown or permanently wet soil. Pull the mulch back, cut out affected fans and improve drainage.
Ragged, chewed new growth. Snails and slugs feed on emerging fans in wet weather. Iron-based baits or a night patrol after rain will handle it.
Foliage burnt after frost. Cosmetic only, to about -5 degrees. Leave damaged leaves until the frost risk passes, then pull them out.
Frequently asked questions
Why is it called a fortnight lily?
Because the flowers appear in flushes roughly two weeks apart through the warm months rather than in one continuous display. Each individual bloom lasts only a day or two, but a mature clump carries dozens of buds on each stem and cycles through them in waves. It is not a true lily either; Dietes belongs to the iris family, which is why the flowers look like small wild irises.
Should I cut the flower stems off after flowering?
No. This is the single most common mistake with Dietes. The flower stems are perennial and produce new buds for several years from the same stem, so cutting them back removes the next season of flowers. Remove only the round green seed capsules, snapping them off and leaving the stem in place. Cut a stem out entirely only once it has gone brown and woody.
Is Dietes an Australian native?
No. All the Dietes species grown here come from southern and eastern Africa, apart from one species native to Lord Howe Island. They are widely used in Australian landscaping because they suit the climate so well, but they should not be counted as natives in habitat planting, and their capacity to self-seed makes removing the capsules worthwhile anywhere near bushland.
How fast does Dietes grow and when will it fill in?
A 140 mm plant reaches close to full size in 18 months to two years in a subtropical climate, and takes about a season longer in cool temperate areas. Planted at the recommended spacing, a mass planting looks sparse through the first summer, noticeably fuller in the second and completely closed by the third. Watering weekly through that first summer makes the biggest difference to the timeline.
Will Dietes grow in shade?
It survives in shade but flowers far less. In half shade, particularly morning sun with afternoon protection, you get good foliage and a reasonable display. In deep shade under a dense evergreen canopy the clump stays alive and green for years but may never flower at all. If the shaded position is fixed, use the variegated White Tiger for its foliage rather than expecting blooms.
Ready to plant?
Few plants give this much structure and flower for so little input. Mass it along a driveway, use it to hold a bank, or plant three in a large pot by the front door. Browse the full low maintenance plants range, or go straight to the fairy iris for the biggest display and the peacock flower for narrow beds. Our grasses for landscaping collection pairs well with all three species.
