Iris offer superb value and are perfect for both borders and containers, but the phrase how to grow iris covers three quite different plants. Some grow from bulbs and some from rhizomes. They go in at different times of year, at different depths, and one of them is barely planted at all. Nearly every iris that disappoints has been treated as though it were the other sort.
WHICH IRIS HAVE YOU GOT
A bearded iris, grown from a rhizome, May and June. See all bearded iris
Look at what you are holding. A bulb is rounded, papery and about the size of a marble, and that is a dwarf iris or a Dutch iris. A rhizome is a flat knobbly finger of root with a fan of leaves at one end, and that is a bearded iris, also sold as flag iris or iris germanica. They go into the ground in different months, at different depths, and one of them is not really planted at all.
The three also flower at different times, which is the useful part. Dwarf iris come in February, the bearded kinds in May and June, and Dutch iris in June and often again later. Buy across all three and you have iris in the garden for five months of the year rather than three weeks.
HOW DEEP SHOULD YOU PLANT IRIS BULBS
Deeper than most people plant them, and deeper than the packet usually says. Current advice for bulbous iris is up to 15cm, and the reason is not stability but reflowering, because a bulb that sits too shallow puts its energy into splitting rather than into next year’s flower. Space them about 5 to 7cm apart in ground that drains.
This one figure explains most reticulata disappointment. They flower beautifully the first February, then produce leaves and nothing else ever after, and the cause is a bulb that has broken into a cluster of small daughter bulbs, each of which needs a year or two to reach flowering size. Planting deep slows that down. Sharp drainage slows it further.
A dwarf iris, grown from a small bulb, February. See all dwarf iris
HOW DEEP DO BEARDED IRIS RHIZOMES GO
Barely at all, and that is not a mistake. A bearded iris rhizome wants to sit in a shallow scrape with its back exposed to the sun, roots spread downwards into the soil beneath and the top of the rhizome visible. Bury it properly and it rots. This is the single most common way a bearded iris is killed with kindness.
The exposure is doing a job. The rhizome needs a summer baking to set flower buds for the following year, which is why a bearded iris slowly disappearing under a spreading geranium stops flowering long before it looks unwell. Keep the neighbours off its back, literally.
WHY DO BEARDED IRIS STOP FLOWERING
Four reasons, and three of them are fixable this year. It was planted too deep, it has been shaded out, the clump has grown congested, or it was fed something high in nitrogen which produces leaves at the expense of flower. Congestion is the usual one.
A clump wants dividing every three to five years, once flowering starts to fall away or the rhizomes have grown over each other. Leave it longer and the middle of the clump goes woody and bare while the outside carries on, which is the ring of leaves with no flower in it that people describe when they write to us.
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WHEN SHOULD YOU DIVIDE BEARDED IRIS
July and August, straight after flowering, which feels early but is not. Dividing then gives the new rhizomes enough time to develop and to make roots before winter, whereas an autumn division goes into the cold with nothing holding it down.
Throw the middle away. Lift the whole clump, break it into pieces by hand, and keep only the young outer sections that carry a healthy fan of leaves and firm white roots, cutting the leaves back to a fan of about 15cm so the wind cannot rock them. The old woody centre will never flower again and is not worth replanting.
DO IRIS WANT SUN OR SHADE
They want different things, which is why one page of advice never fits all of them. Dwarf iris are suited to a semi shaded position, in full sun they will go over more quickly, so a little shade buys you a longer February. Bearded iris are the opposite and want as much sun as they can get.
What all of them share is the need to dry out in summer. It is important that the bulbs are allowed to ripen when dormant, so a sunny aspect is important, and the same is true of a rhizome that needs baking. A wet summer position kills more iris than any winter ever has.
CAN YOU GROW IRIS IN POTS
Dwarf iris are the ones for pots, and they are better in a pot than almost anywhere else. As well as select spots in the garden they are ideal for containers, where their beauty can really be appreciated close up, and at ten centimetres tall that closeness is the whole point of them.
There is a second reason that nobody mentions. Dwarf iris are scented, and it is a real scent rather than a polite one, but you will never notice it at ankle height in a border in February. Put them at table height by a door and you get the half of the plant most people never meet.
Soft blue, and one of the earliest of all.
Pale and finely marked, best seen close up.
Deep purple, and strongly scented for its size.
ARE IRIS GOOD AS CUT FLOWERS
A Dutch iris, also a bulb, June and good for cutting. See all Dutch iris
Yes, and Dutch iris are what the trade cuts. They flower in June on long straight stems, they last well in water, and unlike most bulbs you can cut them fairly hard without wrecking next year, because the bulb has already done most of its building by the time the flower opens.
Cut them just as the bud is showing colour but before it opens. A Dutch iris cut in tight bud will open indoors over a day or two and last several days longer than one cut wide open, which is the difference between a vase that lasts a week and one that lasts three days.
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WHAT TO PLANT WITH IRIS
Something low that does not lean on them. Dwarf iris sit well with crocus, chionodoxa and the taller muscari, all of which flower at the same moment and none of which will smother them, and evergreen dwarf periwinkle makes a spreading mat that works as a foil for underplanting.
For the bearded kinds the rule is different and it is about space rather than colour. Anything that flops over the rhizome in July costs you flower the following May, so keep a hand’s width of bare soil around each one and put the spreading things further back.