Everything you want to know about growing irises. (And then some.)
Easier than you might think
Even for beginners, irises are one of the easiest perennials to grow. Just plant them where they’ll get the six or more hours of sunlight a day that they need to grow and bloom well. That location should also be well drained, since standing water can lead to problems.
How irises grow
Irises send out thick underground stems called rhizomes (pronounced rye-zomes), that store up food that the leaves produce, spread out just beneath the soil’s surface, and embed many small roots in the soil.
When to plant them
Since irises are perennials, you can plant or move them practically anytime, so long as you’re careful. Even so, some times are better than others. The best time to plant or move them is during their dormant period from late summer to early fall (usually July and August). That way, the rhizomes will have time to develop a good root system before freezing sets in.
You should also plant new clumps every three to five years, when the existing ones start getting crowded.
When you remove a rhizome with one bud or growing point, it will produce a large fan of leaves and a flower stalk. And it will breed true, since rhizomes produce irises by vegetative propagation.
How to plant them
Pick a spot with enough sunlight and drainage (see above). Plant your new rhizomes 18-24 inches apart to leave room for clumping. Dig and loosen your soil, and mix in your fertilizer. If your soil’s a bit acidic, now’s a good time to add lime. Dig a hole, and form a mound in the center. Set the rhizome on top of the mound, then spread the roots around it and fully cover them with dirt. Leave the top of the rhizome bare or just barely covered. Firm the soil tightly around the rhizome to hold the plant in place. Plant the rhizomes all facing the same direction, so they’ll all extend in the same direction.
How and when to water
Water new irises thoroughly, soaking the roots. Established irises need little or no watering (depending on your climate, of course).
Keeping your irises healthy and beautiful
If you do everything you’ve read about so far, you’ll have healthy, beautiful irises. But there’s one more thing you’ll need to do to keep them that way.
Whoever first said, “Cleanliness is next to godliness” must have had gardeners in mind, because weeds and grass growing with your irises are an invitation to trouble. Planting your irises in rows rather than beds makes keeping them weed- and grass-free easier. So does applying a pre-emergent herbicide when you cultivate (if you want to use chemicals).
When you cultivate, do it shallowly, so you won’t damage the roots and rhizomes just beneath the surface. If you’re not hybridizing, cut off the bloom stalks close to the ground once they’re finished blooming. Don’t cut the fans back before fall, since they’re needed for new growth. Different growers will tell you different times in the fall to do it; I wait until late fall/early winter.
Do be sure to discard all the garden waste. And don’t mulch over the rhizomes; this can cause rotting in wetter climates or seasons.
Separating and transplanting
To assure maximum growth and bloom, iris clumps need dividing every 3-5 years, preferably in July or August, a month or two after the irises have bloomed. This timing gives the newly planted rhizomes time to form roots before the winter freeze. Just dig up the rhizomes you want to move, cut off a rhizome with a growing point, and follow the planting steps above.