What They Like
Irises have adapted to a wide range of cultural conditions. Most grow best and bloom best in full sun.
In general, the bearded irises are native to Mediterranean and near-desert areas, and grow best in well-drained soils. Preferred pH for most garden varieties is 6.5-7. The soil doesn't need to be particularly nutrient rich, although replenishing phosphorus and other nutrients in the fall can be beneficial. Some varieties do well with a feeding of balanced chemical fertilizer 4-6 weeks before expected bloom time in the spring, and reblooming varieties are more likely to rebloom with supplemental food and water after spring bloom. It is better to underfeed than to overfeed, especially considering the need to avoid conditions favoring bacterial soft rot.
Bearded irises in general should not be mulched. In areas with especially cold winters (USDA zones 5 and colder), some winter covering is advisable, and growers in some desert areas report success with mulch. If in doubt, consult local growers.
Bearded irises of the Hexapogon, Oncocyclus, and Pseudoregelia subsections, known collectively as the arils, can be more challenging to grow because of their requirements for superb drainage and aeration. Arilbreds are hybrids of these with other bearded irises.
The wide variety of types of beardless irises demonstrates adaptation to many environments. Although most grow best in full sun, the "Beefsteak Iris" (I. foetidissima) does well in shade, and some of the Evansias (I. cristata and I. tectorum) grow and bloom in partial shade as well. Some beardless will grow either in moist gardens or completely under shallow water, such as Japanese, Louisiana, Pseudacorus, Versicolor and other Laevigatae. Others, such as some varieties of Spuria, demand a dry period midsummer. Most do best in somewhat acid soil, below pH 6, but some Spurias grow well in alkaline soil. Some are extremely winter hardy, such as I. setosa, flourishing in near-arctic conditions, and others such as the Pacific Coast Irises (e.g., I. californicae) are a significant challenge to grow outside of their native environments.
A rough continuum of irises from water irises to desert irises:
For garden irises the only significant insect pest is the iris borer. Other pests can be a problem for the hybridizer.
A few fungus problems have been identified. The most visible is iris leaf spot, but others, such as mustard seed fungus, can be much more serious in their effects.
The major bacterial disease is bacterial soft rot.
A few viruses also have been identified, although they are rare.
Most irises are propagated vegetatively, preserving the genetic qualities they were hybridized for. This process is called division. Seed pods can be harvested and seeds sown outdoors in the fall or refrigerated, germinated and grown under lights.
Regular MaintenanceWeeding is advised, and will need to be done more frequently for those varieties not mulched. Some growers use Preen, a pre-emergent herbicide, to aid in the task.
Unless hybridizing, as blooms fade, snap them off at the base, taking care not to damage any new buds. When stalks are bloomed out, snap them off close to the ground on a dry day--most can be done without cutting, but do be careful not to pull up the rhizome.
Bearded irises in general are light feeders. Often some bone meal or other phosphorus supplement around the plant but not on the rhizomes is helpful in the fall if not overdone. Plants also benefit from a single feeding of a chemical fertilizer like Miracle-Gro about 4-6 weeks before expected bloom time. Omitting these is preferable to overdoing them.
If the bearded iris is a reblooming variety that you wish to encourage to do so, make sure it continues to be regularly watered after spring bloom time, and provide it with an additional feeding at that point as well.
Siberian irises are moderate feeders. They do well with a balanced fertilizer.
Japanese irises and Louisiana Irises can be considered heavy feeders.