If you want to brighten up your garden next spring, our gardening expert shares what you need to know about these lovely flowers
Irises are essential spring and early summer flowers that are grown from bulbs, offsets and rhizomes in a bewildering number of forms. They have six petals – three upright and three that curve downwards (standards and falls, respectively) – stylised as the fleur-de-lis symbol of France. They thrive in any reasonable garden soil that is well-drained and in the sun.
The bearded irises (main image), from rhizomes, have a “beard” of fleshy projections on their lower falls. They are bold, spectacular, hugely popular border flowers in sumptuous colours for early summer. Theoretically, the ideal time to plant or divide is straight after flowering – as the flowers for next year are initiated in late summer. However, they can be successfully planted at any season, and traditionally late summer has been favoured, which is when bare root divisions are widely sold.
Potted plants are available, but sections of rhizome shipped until October in damp wrapping offer the best value. Specialist nurseries supply a broader range than garden centres and offer more of the remontant types (flower again in late summer), so are useful for smaller gardens and in different heights; tall ones (90cm) suit bigger borders, while the shorter dwarf ones (20cm) are easier to fit into smaller gardens, with intermediate (50cm) ones being useful for windy gardens.
Sections of rhizome 10cm to 15cm long are supplied or taken, each with some coarse roots and a few of the sword-shaped leaves. The rhizomes are planted with the coarse roots pointed straight down and the soil well firmed around the roots to anchor the plant in place with the rhizome on the surface. Purists align the rhizomes so they run north to south, so they get the sun on their backs, with the foliage at the northern end – but this is not essential. Forking in some garden compost or rotted manure before planting to improve soil texture is helpful, but fertiliser is not required until spring.
Given the extreme dryness this year, watering dusty soil before planting will make anchorage more secure, and a post-planting watering will set up the plants to grow new roots. Once established, bearded irises need no watering.
Foliage will die down gradually, but some will remain green over winter, with new growth in spring. Typically, they are so prolific that they need division at least every three years to ensure healthy, freely flowering clumps without dead centres. Allow 25cm between rhizomes.
The Cayeux Iris ‘Oh! Susannah’ at the RHS Chelsea Flower Show 2019 (Photo: Cayeux Iris)
Position behind or among plants that die down in winter and grow up in summer and cast only a light shade to hide the irises which do not look the best after flowering – alliums, hardy fuchsia, lilies or Phygelius for example.
Late summer is also the time to order bulbous irises for autumn delivery and planting on receipt. The little (15cm) Iris reticulata is remarkably inexpensive and brings irresistible colour in late winter when other flowers are scarce. They are most easily grown in small pots, while those grown elsewhere thrive where drainage is good and the soil gets toasted by the later summer sun.
They seem to persist best, and are less prone to splitting, if planted about 10cm to 15cm deep, and grit mulch keeps the blooms free of soil splash. Divide every five years or so.
Dutch irises (50cm) – mostly grown by the cut flower trade – are also reasonably priced, in shades of blue, bronze, purple, white and yellow, and flower in early summer. They need to dry out after flowering, given which they can persist for years, with just occasional lifting and dividing.
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