June 22, 2012 7:34 pm

An eye for an iris

Now is the moment to split up old under-performing clumps and replant them
Cayeux Iris ‘French Cancan’ and Campanula persicifolia©Clive Nichols

Cayeux Iris ‘French Cancan’ and Campanula persicifolia

The iris season is now over. Despite the British rainstorms there have been some magnificent displays of flower. There have also been some non-performances, especially in my garden. Once the irises are over it is so easy to forget them. In fact this next week is a time for planning and action.

If you want flowers next year off ever more irises, now is the moment to split up old under-performing clumps and replant them. They like to have another three or four months in which to build up strength. Experts recommend an early replanting, but amateurs usually forget to act until early winter or even the following spring. So it is worth sizing up the season so far and responding to it.

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Robin Lane Fox

The big border irises give us colours that are irresistible and totally at odds with fashions for “meadow purple” or stale pink, seen through a shimmer of invasive grasses. Black-purples, deep blues, marmalade orange and all the wondrous bicolours are superb selections from Iris germanica and make a nonsense of subtle colour planning or the “rules” of colour charts and that old colour “wheel”. They are unmatched in late May but not all of them flower freely in British conditions. One of the major iris exhibitors at the Chelsea show is now Cayeux Irises from southern France but the exhibitors are admirably frank about the suitability of many of their loveliest forms for an English garden north of Watford. Without the summer heat they will often not flower at all.

Iris ‘Mer du Sud’©Garden Collection

Iris ‘Mer du Sud’

In the relative warmth of walled Oxford irises have had an excellent year. Out in the colder zone of my own garden, in the now-notorious Chipping Norton set, they have put on a mixed display. My soil is not so rich and the weather is not so hot. I give them lots of love, or LOL as our prime minister abbreviates it for selected Chipping Norton locals, but only a few of the best varieties respond with masses of flowers.

Of the enviable French varieties of Iris germanica the most floriferous are the wonderful deep blue Mer du Sud and the pale blue and white bicolour Alizes. I cannot recommend them too highly and a few rhizomes will soon multiply into a divisible clump. Among strong deep yellows I remain unshakeably loyal to Iris Starshine, a fine American selection of the early 1960s, which I first noted when on day-release from school to the Chelsea Flower Show nearly 50 years ago. I had a sharp eye for an iris at an age when other boys believed they had a sharp eye for a girl. Starshine is still a superb, free-flowering variety that remains in my top five.

Iris ‘Before the Storm’©Marianne Majerus

Iris ‘Before the Storm’

So do any of the irises with the softer yellow word Lemon in their names. They all flower magnificently and their lemon yellow is contrasted prettily with white. One called Wabash is a different matter, a unique mixture of white and velvet blue-purple that would be at home on an exotic orchid. It is another free flowerer even when the conditions are tough. The darkest of all tall black purples is now Iris Before The Storm, but I stay loyal to the excellent Black Swan that is closer to indigo and flowers even more freely. Among whites, the proven White City flowers well but begins with a shade of pale blue before ageing to the icier white. Others have white flowers from the start and among them my best success is Iris Lacy Snowflake.

The choice between modern varieties is bewildering but I recommend beginning only with varieties that are described as “free-flowering.” They have now been out of flower for about two weeks and in the next fortnight they can be best divided. The aim is to lift the mat of iris-rhizomes and discard the weakest, often those that have flowered best. Choose the fans of leaf that show most healthily from younger, fatter rhizomes, or “root-stocks”, often on the outer edges of the total mat. Split them off from the main clump, discard older exhausted rhizomes and replant the young thrusters on the same site. The important point is to be sure that the rhizome lies flat on the surface of the soil with its upper surface exposed to the sunlight. Never bury a rhizome or saddle of roots with its nose or totality pointing downwards or worst of all, completely under the soil. The best plantings place each rhizome on a ridge of soil immediately beneath it while its roots are spread out more deeply into the soil on either side of this ridge. Replanting is easily done if you remember that the aim is to expose the upper side of the rhizome or “root” to direct sun and help it to develop strength for next year. Do not follow the old, neat practice of former “trained” gardeners and cut the fan of leaves on each replanted rhizome down to a tidy height of three or four inches above soil level. This cut looked very neat and professional but is now considered to weaken the plants needlessly.

Cayeux Iris ‘Chocolate Marmalade’©Clive Nichols

Cayeux Iris ‘Chocolate Marmalade’

Should the replanted irises be fed? Older advice is to leave them alone but on poorer soil I always dust bonemeal under the replanted rhizome by scattering it on the ridge of soil beneath and also by mixing it into the soil around the side roots. On acid to neutral soils it also helps to add some lime from time to time as the forms of Iris germanica love alkaline soil. In the great garden at Sissinghurst Castle the irises are treated to dressings of crushed limestone sold as “Dolo-dust” in honour of the limestone parts of the Dolomite mountains. I do not need Dolodust on my alkaline soil but I give a light dusting of powdered Osmocote fertiliser to enrich the transplanted rhizomes even when I have put the slower-acting bonemeal underneath.

Can anything be grown among irises in order to maintain the interest when the season of the taller varieties is over? The expert Anglo-American designer, Lanning Roper, used to encourage his clients to grow the peach-leaved bell-flower, Campanula persicifolia, among their irises in order to give a second season of blue and white without roots that would upset the irises’ prospects. He would then follow on with annual salpiglossis, bedded into any bare earth. I saw these ideas working very well, as they still would. They involve plants that will not vex the irises’ recuperating rhizomes, but they assume that the irises are not infested with weeds and in need of constant weeding.

If you set about your best irises now you can expect flowers next May from ever greater areas. If you wait, you will probably miss the following season of flower. I need hardly add that there is no reason to be too frightened by the cost of parent plants of the best named varieties. Properly loved, they will outperform any other reasonable investment and multiply until the base cost seems trivial. Second rate forms, as ever, are a false economy.

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