How to Plant Bearded Iris With the Rhizome Left Above the Soil

The rhizome is a sunbather, not a bulb

The thick, knobbly thing you hold in your hand is not a bulb. It is a rhizome, a horizontal stem that stores food, and it behaves like a stem in every way that matters. It wants light and dry air on its upper surface. In the wild these plants grow on stony hillsides where the rhizome lies half exposed, roasting through the summer, and that summer roasting is what triggers the flower buds you will see the following season.

Almost every case of an iris clump that produces plenty of strong green leaves and no flowers comes back to the same cause: the rhizome is buried. Sometimes the gardener planted it too deep. More often it was planted correctly and then buried later, by mulch, by compost spread over the bed, by soil washing down a slope, or by a groundcover creeping over the top of it. The plant survives that perfectly well. It just stops flowering.

So when you plant, aim for the top of the rhizome to be visible. In a normal garden bed the upper surface should sit level with the soil or just above it, like the back of a fish breaking the surface of a pond, with only the roots buried. On very light sandy soil, or in a climate with fierce sun, you can cover it with a bare skim of soil, no more than half an inch (1 cm). On heavy clay, err the other way and leave it proud.

If you inherit a bed of iris that has not flowered for years, scrape the soil and mulch back from the rhizomes before you do anything else. That alone often restores flowering the following season without any digging or dividing.

Pick a spot with full sun and nothing leaning over it

Bearded iris needs at least six hours of direct sun, and eight is better. Anything less and the clump leans, the flower stems get thin and topple, and the rhizomes stay damp for longer after rain, which is the opening that soft rot needs. A spot that was sunny when you planted may not be sunny five years later, so look up as well as down before you choose.

Just as important is what grows next to it. Iris will not tolerate being crowded. A shrub that has spread, a clump of tall perennials flopping across the bed, a hedge whose roots have reached under the planting, all of these reduce flowering long before they actually shade the plant out. Leave a clear foot (30 cm) of open ground around the outside of the clump and keep it clear.

Drainage decides whether the plant lives. A rhizome sitting in ground that stays wet through winter will rot. If water stands on your soil after heavy rain, build the bed up. A raised bed, a bank, or simply mounding the planting area 4 to 6 in (10 to 15 cm) above the surrounding level all work. Digging grit into heavy soil helps, but raising the planting level helps more.

Soil richness is a minor concern next to drainage and light. A neutral to slightly alkaline soil suits them, and ground that has been heavily manured is actively bad, because soft, nitrogen fed tissue rots and attracts trouble. Ordinary garden soil that grows other perennials adequately will grow iris well.

Setting the rhizome in the ground

The planting hole is an unusual shape, because you are burying roots while leaving the rhizome on top. The easiest way to get it right is to dig a shallow hole and build a ridge of soil down the middle of it, then sit the rhizome astride the ridge with the roots hanging down either side. Late summer to early autumn is the best planting season, while the soil is still warm enough for new roots to grow before winter.

1. Dig a shallow hole and raise a ridge in it Make the hole roughly 10 in (25 cm) across and 4 in (10 cm) deep, then mound the loose soil into a low ridge running down the center. The top of the ridge should sit level with the surrounding soil surface. This ridge is what holds the rhizome up while the roots go down.
2. Work out which way the fan is pointing A rhizome grows forward, away from the fan of leaves, and it will keep traveling in that direction for years. Set it so the fan points where you want the clump to stay and the growing end points into open ground, not into a wall, an edging, or another plant.
3. Straddle the ridge and spread the roots Sit the rhizome on top of the ridge and comb the roots down both sides so they lie out rather than coiling. Fill soil in around the roots and firm it with your fingers, keeping the upper surface of the rhizome clear of soil the whole time.
4. Firm hard so it does not lift A newly planted rhizome has no anchorage and frost or heavy rain will heave it out of the ground. Press the soil down firmly on either side. If the plant rocks when you nudge the fan, it is not firm enough, and a short cane pushed in beside it for the first few months holds it steady.
5. Water once, then leave it alone Give it one thorough soaking to settle the soil around the roots. After that, water only if the ground goes bone dry for two or three weeks. Newly planted rhizomes rot far more often from kindness than they suffer from drought.

Depth, spacing and direction at a glance

The numbers below cover a standard tall bearded iris. Dwarf and intermediate kinds use the same planting method with tighter spacing, and the beardless irises such as the Siberian and Japanese kinds are the exception to everything on this page: those have fibrous roots rather than a fat surface rhizome and are planted properly buried, 2 in (5 cm) down, in ground that stays moist.

What to write down at planting time

The single most useful record is a simple sketch of the bed showing where each fan sits and which way it points. Bearded iris disappears from view for part of the year and marches forward a few inches every season, and in three years, when it is time to divide, you will not remember what was where. Note the planting season, the spacing you used, and whether the ground was raised or level.

Add one line about the position of the sun. Write down roughly how many hours of direct sun the spot got on a clear day when you planted. If flowering falls off in a few years, that note is what tells you whether a shrub has grown up and stolen the light, which is a completely different problem from a buried rhizome and needs a completely different fix.

Finally, record the date you first see flower stems appear in the season after planting. Many rhizomes planted in late summer skip their first spring or produce a single stem, then flower fully the year after. Knowing that this clump is simply young saves you from digging up a healthy plant that was doing nothing wrong.