Fall is prime planting time for many bulbs and plants, including Iris, which like an early- to mid-fall planting in order to establish a strong root system prior to hard freeze. Planting irises and other bulbs in fall is quite simple, so there is no reason not to do it!
The first step to successful planting is choosing a variety (or several) that grows well in your planting zone. Though most bearded and Siberian iris are quite hardy and can survive with little problem in Zones 3-9, some Dutch and Louisiana iris varieties may be a bit more tender, thriving only in Zones 6 and up. Choosing an iris that can thrive in your zone will assure you many years of spring color!
Next, choose a good spot. Iris love sun, so it is best to plant them in an area that will receive 6 or more hours of direct sunlight per day (unless you live in an exceptionally hot climate, where they will benefit from a bit of shade, especially in the afternoon). Iris will spread annually, increasing and blooming more prolifically each year, so a good rule of thumb is to plant them 12 to 24 inches apart. (You can plant them more closely if you would like a more immediate fill, but be prepared to thin them earlier and regularly.)
Iris are quite adaptable to a wide range of soils, so you can likely plant them with success in many different growing mediums, but their preference is a well-drained, slightly acidic soil. And, like many plants, iris will benefit from a generous layer of mulch in fall and again in spring.
When planting iris, place them so that the tops of the rhizomes are exposed, while the roots are spread out facing downward into the soil. In very hot climates, you may wish to cover the top of the rhizome with soil, but in more temperate climates this is not necessary. The main thing to remember is that there is no need to plant iris too deeply.
The onset of December means that planting season has ended for most of us here in the colder zones. But if you missed planting some new iris varieties this year, it’s never to early to plan for fall 2011!