Bearded irises only need a few things to really thrive… at least six to eight hours of sun per day during the growing season, good air circulation, and good drainage in a “decent” soil. If you live in an area with very hot summers, choosing a location with a bit of afternoon shade can be beneficial, just not too much shade for too many hours of the day.
Iris rhizomes are NOT bulbs.
Traditional flower “bulbs” such as tulips, daffodils, hyacinths, crocuses and such must be planted several inches deep for proper development. This is NOT the way to plant iris rhizomes.
Depending on your climate and your soil type, bearded iris rhizomes should be planted at or just slightly below the soil surface. If you live in a climate with very cold winters you might want to cover your iris rhizomes with ½” to 1” of soil, but no more than that. And, if you do, don’t be surprised if the newer rhizomes work their way up to the surface over time.
Irises like heat.
Nearly all the bearded irises that we grow in the U.S. are hybrids with Mediterranean ancestry. They like it hot and dry. Too much moisture on the rhizome itself can lead to rot.
If your soil has a high clay content consider creating hills of soil, then plant the rhizome with the roots below ground but with the rhizome sitting right on, or just slightly embedded in, the surface of the soil. This will help keep the roots moist but the rhizome dry.
Bearded irises are also mobile, in a way. As your iris grows the new rhizomes will creep along the surface, gradually spreading and eventually producing a clump.
When you order irises from an online retailer you will usually receive an individual rhizome for each plant that you order. Sometimes these rhizomes will already have small “pups” developing around the “mother” rhizome, referred to as “increases” in the trade.
Look closely at a rhizome and you’ll see the leaves (called a “fan”) are only on one end of the rhizome. As that fan matures and produces a flower stalk, new rhizomes will begin to develop on the same end, and on the sides of that “mother” rhizome. Those new rhizomes will continue growing out in that same general direction.
The original “mother” rhizome will become dormant after its fan of leaves has matured and produced a flower stalk. That mother rhizome will not produce leaves or flowers again. Even its roots will eventually dry up and the rhizome will wither.
Remove Spent Flower Stalks but NOT the Leaves
If you want your bearded irises to multiply more quickly remove the old flower stalk after all the buds on that stalk have produced blooms. You can cut the old flower stalk cleanly right at the base or, just grasp it firmly and “snap” it off. This will prevent the iris from wasting energy producing seeds if any flowers were pollinated by insects.
However, never remove the healthy green leaves on your irises. Let those leaves die down and dry on their own. Those green leaves provide nourishment to the newer rhizomes. Wait until the leaves turn brown and dry on their own. Then cut them back to prevent damage from disease, insects or rot.
It’s okay to trim or remove damaged leaves or those that show signs of disease.
Lift and Divide for More Blooms
Dormant, non-productive rhizomes will accumulate in the center of the iris clump. As old rhizomes build up in the clump and as the “increases” become crowded, the number of blooms will begin to diminish. This is why an old clump of iris will need to be lifted and divided after three to five years.
There are so many leaves, so close together, the newer, productive rhizomes become shaded. Iris rhizomes need the sun on their backs to properly develop and bloom abundantly. Crowding also reduces air circulation which can be another contributor to rot and disease.
The solution is to lift and separate the entire clump of rhizomes. The best time to do this is late summer, a few weeks after blooming has ceased. You want to allow newly-transplanted rhizomes sufficient time to get well rooted before winter.
Do this by carefully digging under the clump with a garden fork. Placing a small piece of 2 by 4 behind and under the fork will give you a bit of leverage to more easily lift the clump. Work your way around the perimeter and you’ll find the entire clump will lift out of the ground, roots and all.
Then you can either break the individual, healthy rhizomes (the ones that have green leaves) away from the clump or separate them with a sharp knife or pruning shears. It’s best to simply discard the older, dormant rhizomes. Those have already produced leaves and flower stalks and, in most cases, will not be productive.
Cut back any long leaves to around four or five inches. This helps prevent wind from rocking the newly-planted rhizome back and forth and allows the new roots to develop properly. It also reduces transpiration of moisture through the leaves, lessening the stress on newly-transplanted rhizomes.
Place the newly-harvested rhizomes in the shade or indoors to dry for a day or two. The freshly-cut ends need to cure and heal before replanting. They can stay out of the ground for a week or so without damage.
To Mulch, or Not To Mulch?
It is best not to mulch around bearded iris rhizomes. Mulching can lead to rot. However, if your soil is very sandy and subject to wind erosion, you might consider “mulching” with a thin layer of pea gravel. This will help prevent erosion while still allowing for good drainage and quick drying of the rhizome.
It’s fine to mulch the flower “bed” that hold your irises. A good mulch can help minimize weeds and retain moisture around the roots. But, unless you use pea gravel, don’t let mulch come into contact with iris rhizomes. Remember, too much moisture on the rhizome can lead to rot.
All in all, irises are tough, disease-resistant perennials that thrive with very little attention. Sufficient sun, good air circulation and good drainage combined with lifting and dividing every few years will provide you with abundant and beautiful iris blossoms.
Don’t Forget Fragrance!
Bearded irises offer virtually endless choices of colors, sizes and patterns. Careful selection of cultivars can provide extended bloom times.
But, of course, I would add that choosing intensely fragrant cultivars will greatly increase the pleasure these gorgeous flowers will provide for many, many years.
Tough, beautiful and intensey fragrant… What more could we ask for in a flower?