Answer: There are many kinds of irises; bearded irises are the most commonly grown and they grow from rhizomes; beardless irises grow from bulbs. The beardless kinds should be planted in the fall; the bearded kinds can be planted from July through October.
Either type iris will perform best in a well-prepared bed. Begin by amending the bed with organic matter to facilitate drainage. The easiest way to do this is to spread a 3-4" layer of compost or shredded leaves over the top of the soil surface and then dig it in to a depth of 6 to 8". Rake the bed smooth and then plant the rhizomes and bulbs. Set the bulbs 4" deep and 3-4" apart; plant the rhizomes just beneath the soil surface and 1-2' apart. You can leave the bulbs and the rhizomes in the ground all year around, but you'll want to cut the dead foliage away in late fall and mulch over the top of the bed with straw or other organic mulch to protect them from heaving up through the soil during alternate freezing and warming winter weather.
If you plant your bulbs and rhizomes this fall, and cover over them with an organic mulch, they'll emerge in early spring and flower in early summer. Cut off the flowering stalk when the flowers fade but leave the foliage alone. It will collect and store energy in the roots for the following year's flower display. Late in the fall, after the leaves have withered and died, cut them away at soil level and cover over the bed with organic mulch.
Enjoy your irises!
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