Bearded Irises, such as this heirloom purple bearded iris, are not hard to grow. They grow from rhizomes, fleshy roots that will reproduce over the years. Eventually, these roots start to crowd each other and must be divided.
The rhizomes are planted in July, August or September. The heat of the summer forces them into dormancy so that they can be safely planted. This is also the best time to divide them, since they can become established before the first frost. Plant or divide the rhizomes at least four to six weeks before the first frost.
Irises require at least six to eight hours of sun a day and prefer full sun. Light shade in the afternoon can help irises manage in extremely hot climates, but most irises do just fine in the heat.
Irises require well drained soil and prefer a pH of around 6.8, but are forgiving about the pH if the soil is well drained. If you have heavy clay soil then work in about three inches of compost into the top six inches of soil.
To plant irises, it is important to remember not to dig too deep a hole. The top of the rhizome should just be visible when the planting is complete. Dig a hole and build up a small mound of soil in the center of the hole. Spread the roots out on the mound and then firm soil over the roots until the rhizome stands upright and is just visible. Water the rhizome thoroughly.
It will take two to four weeks for the first leaf to show on the newly planted iris. Depending on the rhizome, you may or may not get blooms the first spring after planting.
Rhizomes should be spaced twelve to twenty-four inches apart. They are often planted in a triangle formation in groups of three. All the rhizomes should be pointed inward towards the center of the triangle when planted this way.
Rhizomes should be watered when first planted until the first good rain. Overwatering the rhizomes will rot them. For this reason, after the rhizomes are established, they should only be watered when the top three inches of soil are dry. Deep watering of an inch all at one time is better than frequent shallow watering.
Every three to four years, the rhizomes become too crowded and the blooms decline. At this point, the entire clump needs to be dug up. Replenish the soil with compost worked into the top six inches of the soil.
Discard the old center division that has no leaves. Separate the rhizomes into separate pieces with two to three leaves each on them. Cut the leaves to four to six inches in length. Replant some of the rhizomes into your space, remembering to leave twelve to twenty-four inches between plants. You will need to follow the guidelines for watering these replanted rhizomes, as well. Either plant the other rhizomes in another space or give them away.