Gadenin日本語Get the appOne name, two entirely different plants
The word iris covers a very large group, and most of the confusion in growing them comes from advice written for one part of it being applied to another. The practical split is between the bearded irises, which have a fat rhizome that must sit exposed on the soil surface in dry sun, and the beardless irises, which have fibrous roots or a slender rhizome, are planted properly buried, and in several groups actively want moist ground.
You can tell them apart by looking into the flower. The bearded kinds carry a dense fuzzy strip, the beard, running along the middle of each of the three drooping outer petals. The beardless kinds have no beard, usually showing a smooth signal patch or a network of veins in that position instead, and their flowers tend to look lighter and more delicate.
The leaves give the same answer. Bearded irises have wide, flat, gray green sword shaped leaves arranged in a stiff flat fan. The beardless groups have narrower, softer, greener leaves that often arch over, and they generally form a looser clump. If you are unsure what you have inherited, scrape the soil at the base: a thick surface rhizome as fat as your thumb means bearded, and thin roots running down mean it is not.
Everything on this site about leaving the rhizome exposed, keeping the ground dry and never mulching applies to the bearded group only. Applying it to a Siberian or Japanese iris, which want to be buried and kept moist, will kill the plant.
The bearded group, sorted by height
Within the bearded irises the useful distinction is size, because height determines both when a plant flowers and where it belongs in a bed. The smallest kinds open first, at the very start of the season, and the tall ones finish it several weeks later. Planting a range of heights therefore stretches the display from early spring into early summer without needing anything unusual.
The small kinds, roughly 8 in (20 cm) and under, are tough, wind proof and quick to multiply, and they suit the front of a bed, a gravel area or the top of a wall. The middle sizes, from about 8 to 27 in (20 to 70 cm), flower next and are the most useful all rounders, being large enough to read from a distance but rarely needing staking.
The tall bearded kinds, above roughly 27 in (70 cm), carry the big, heavily ruffled flowers most people picture, several to a branched stem. They are the most likely to need support in an exposed garden, the most sensitive to being crowded, and the ones that resent shade most sharply. They also travel outward fastest and so need dividing on the shorter end of the three to four year cycle.
A note on reblooming. Some bearded irises will produce a second flush in late summer or autumn in a long, warm season. They need more water and more feeding than the once flowering kinds to manage it, and in a short or cool season they simply will not rebloom at all, which is a matter of climate rather than a fault in the plant.
| Group | Typical height | When it flowers | Where it suits |
|---|---|---|---|
| Smallest bearded kinds | Up to 8 in (20 cm) | Earliest, at the start of the season | Front edge, gravel, tops of walls, containers |
| Small to medium bearded kinds | 8 to 15 in (20 to 40 cm) | Shortly after the smallest | Front of a border, edging a path |
| Intermediate bearded kinds | 16 to 27 in (40 to 70 cm) | Between the small and the tall | Middle of a border, rarely needs staking |
| Tall bearded kinds | Above 27 in (70 cm) | Late in the bearded season | Back of a sunny border, needs open space |
| Reblooming bearded kinds | Varies with the group | Main flush plus a second in a long warm season | Sunny bed where extra water and feed are easy |
The beardless groups and what they want instead
The beardless irises are worth knowing about precisely because they thrive where a bearded iris will not. The narrow leaved kinds often grown in cool temperate gardens form dense grassy clumps, flower a little after the tall bearded season, are planted 2 in (5 cm) deep, and want soil that stays reliably moist. They are also far more shade tolerant and much less troubled by borer and rot.
The large flat flowered kinds associated with waterside planting go further still, wanting acid, permanently damp soil and even standing water in the growing season, and they resent lime. Other beardless groups grown in warm climates spread by slender rhizomes through rich, wet ground and flower after the bearded season has finished. All of these are buried at planting, and all of them are divided in a different season from the bearded kinds.
There is also a set of small bulbous irises that flower in the depths of late winter, only a few inches tall. Those are true bulbs, planted in autumn a few inches deep like any other small bulb, and they belong to a different calendar and a different set of rules altogether.
Choosing what to plant where, and what to record
Match the group to the site rather than trying to change the site. A hot, dry, sharply drained bank in full sun is bearded iris ground and nothing else will do it better. A damp corner where water sits after rain, or a pond margin, is where the beardless kinds earn their place, and putting a bearded rhizome there simply produces a rotted rhizome within a season or two.
If you already have a clump and do not know what it is, do not guess from the flower color. Look for the beard, look at the width and stiffness of the leaves, and scrape at the base for a fat surface rhizome. That three point check takes a minute and decides everything that follows about depth, mulch, watering and when to divide.
Write down, for each clump, which group it belongs to, its height, and the week it flowered. Height and flowering week together are what let you spread the season across a bed deliberately instead of by accident, and the group tells anyone working in that bed later, including you in three years, whether the rhizome is supposed to be showing above the soil or buried out of sight.