Irises: divide and rule

As a rule no one in their right mind would split and re-plant perennials in July. It's hot, it's dry and they just don't like it. March or September, fine - but July? Well, yes: for irises early July, as soon as they've finished flowering, is the perfect time to give them a fresh surge of energy by digging them up, splitting them and replanting them. The thing is, unlike most perennials, this is when they make a fresh burst of root growth, which helps settle them down after they've been moved.

What's more, over the next couple of months they also start to form the buds for next summer's flowers, and the extra vigour which replanting gives them helps more buds develop. So if you haven't split yours for a while, now is the time.

I'm not talking about all irises, however. Waterside irises are best left until March; it's the bearded irises, also known as flag irises, the colourful and flamboyant irises of the herbaceous border for which now is the time.

First lay a sheet of polythene (an old compost bag is ideal) on the path or grass alongside the iris you're going to divide. Then, using a garden fork, dig up the clump; you don't need to worry about keeping all the soil on the roots but try not to cut the roots off short. Lay the iris on the polythene and shake off the rest of the soil.

Then use a sharp pair of secateurs to cut up the root; to be strictly accurate, those fat pieces are rhizomes and the wiry strands growing off them are the roots. Your aim should be to end up with a series of pieces, each with 7.5-12.5cm (3-5in) of rhizome and leaves at one end.

Don't bother to try to make a new plant out of every piece; keep the young fat growth, and the old and slightly withered rhizomes can go on the compost heap.

You'll notice that the individual rhizomes have very few wiry pieces of root attached: until new roots develop, these few strands have to sustain the plant and with the leaves losing moisture in the hot summer sun they're going to struggle. So, to compensate, cut back the leaves by about half.

Use the secateurs to cut them neatly into an inverted V shape; removing all that foliage will also prevent the wind loosening the roots. Now fork over the site where your irises are to be replanted, mixing in some garden compost or soil improver from the garden centre. Tread it to remove any air pockets, sprinkle in a handful of general fertiliser per square yard and rake it level.

Then set out your irises: tall varieties can go 23cm/9in apart, shorter varieties 15cm/6in apart. They look more attractive if they're all orientated the same way, with the fans of leaves all facing the front of the border.

Using a trowel, plant each one. Firm the soil around the roots with your fingers and try to finish off so that the rhizome is partially buried in the soil but not completely covered. When you've finished, water them in with a liquid feed like Mira-cleGro or Phostrogen added to the water.