Growing Irises in Beds and Borders in Tucson and Surroundings
These types of irises have been grown successfully in the area - tall bearded (TB), intermediate bearded, arilbred, spuria, Dutch, Japanese, Iris albicans, Iris unguicularis (Winter iris), and Moraea iris
Selecting a site - Good drainage is crucial, so raised beds are an excellent option. Look for a spot with lots of sun in the cool season, and morning to early-afternoon sun in the hot season. Beware of reflected heat from block walls.
Preparing the soil in a bed or border – Prepare beds a month ahead and beware of hot compost (containing fresh manure). Break up the native soil to a depth of 16-24.” Sift to remove rocks and roots. Add compost to create a 50-50 mixture. Use any high-quality compost, such as mushroom compost. Follow a similar plan if creating a raised bed, but only break up native soil to about 12” beneath the area. Purchase a special soil mix for raised beds and amend as stated above. Compost will likely need to be replenished annually in beds and raised beds. A granular type of 10-10-10 fertilizer may be scatted through the bed before planting.
Planting – Plant rhizomes in the fall after temperatures are consistently below 100° (late September or early October). Place 1 T Triple Super Phosphate about 8” down in the holes when planting rhizomes (since it will stay there for the roots to reach). Irises other than Japanese benefit from a spoonful of bone meal in the planting holes. Plant rhizomes with tops slightly above soil line or slightly below. (Each method has its fan club). Irises will not bloom if planted too deep. Press soil firmly. Water a newly-planted iris once every 3 days for 15 days, then every 4 days for 16 days, and then once a week.
Fertilizing - Irises are heavy feeders. Apply Miracle-Gro Bloom Booster (10-52-10) (or similar fertilizer with a middle number of 50 or higher) every two weeks starting when new growth appears (often in January) and continuing until blooming begins. This can be alternated with fish emulsion. Humic acid can be added anytime. From June to December, apply fish emulsion to soil every two weeks. Reblooming irises need fertilizing to bloom in the fall. Note: fertilizers with too much nitrogen (above 10) can incite rot in rhizomes. If you grow roses, irises enjoy the same fertilizing regime as roses: beginning at the first sign of new growth, add fish meal, Epsom salts, alfalfa meal, rose food, and Milorganite. Bone meal will kill Japanese irises.
Irrigation – Water if top 2” of soil is dry - use a finger or moisture meter to determine this. If temperatures are above 95°, water the soil (not the plant) in evening so as not to encourage rot. Driplines are probably the best way to water irises. Purchase higher-quality ones from an irrigation supply store: in-line drippers every 6” work well. Overwatering will cause rot.
Dividing Irises – Irises will not bloom if they are crowded, and crowding encourages diseases and pests. Divide a clump every 3-4 years. Trim leaves to about 6” and trim roots to conserve the plant’s energy as it begins to regrow. Break or cut apart rhizomes at joints. Allow rhizomes to heal for a few days before replanting.
Shade – In the summer, consider suspending shade cloth (20-40% type) from a scaffold of PVC pipe
Brown leaves - remove? leave on plant? – This is a personal preference. Some individuals leave them on for the shade they provide to the rhizome and the plant, others remove them for the purpose of tidiness.
Grubs in the bed - apply Bayer Advanced Grub Control with its active ingredient, Merit. Note - It is not for vegetable beds!
Common Iris Mistakes - planting in poorly draining soil/clay/caliche, soil too alkaline, soil too hot, over-watering, planting too deep, planting too close to other plants, not fertilizing when planting, failure to fertilize when new growth starts in the spring, failure to keep plants clean, failure to water regularly