Does your iris have a beard?2020 Spring
by Kaleena Miller
Kaleena Miller is an MSU Extension agent in Madison- Jefferson Counties.
That spike in the flower garden, with curved, ruffled-petal tongues of color at the top, is an iris. These hardy, herbaceous perennials are named after the Greek goddess of rainbows. They are one of the first to bloom in Montana’s spring, peaking in June.
With over 300 species in the genus Iris, the most familiar to us fall into the tall, bearded category (Iris germanica). The beard is named for soft hairs along the center of the falls (the outer, hanging petals). Iris species are separated into two major groups, rhizomatous and bulbous. Claiming to be one of the oldest garden flowers, irises come in nearly every color, including blue, yellow, pink, orange, and even black. Seemingly endless varieties provide options for height and bloom periods. The smallest cultivars can be eight inches tall, with the largest ones reaching almost 48 inches. One native iris in Montana, Iris missouriensis, showcases a light blue to deep violet color and only requires moisture in spring. Irises are known for being rugged, reliable and easy to grow in Montana. Their six-petal formation contains three outer, hanging petals called falls and three inner, upright petals called standards. Their roots are rhizomes from which flowers regenerate annually. Not all irises can be considered good: Yellowflag iris, Iris pseudacorus L, is a Priority 2A noxious weed in Montana and its large yellow flowers can be spotted in riparian or riverbank areas where they are difficult to control.
Planting and Siting Tips
Most irises will grow best with full sun for six to eight hours a day. In very hot areas, afternoon shade helps keep flower colors from fading in the heat. They perform best in very well-drained, neutral to slightly acidic soils. In absence of ideal soil, amend it with organic matter or build raised beds for irises. Consistent drainage all year is important for irises, this will ensure optimal growth. When planting, loosen soil with a garden fork to a depth of about 12 inches, then mix in several inches of compost. Beware of adding too much nitrogen as that can trigger the iris to focus growth on leaves at the expense of flowers. Set irises five to nine inches apart. They will grow from the end with the leaves, so point that end in the direction of intended growth. Lightly cover the rhizomes with soil and water the entire bed. It’s easy to make the mistake of planting irises too deeply, where they won’t thrive. The rhizomes of these plants should be partially exposed to the elements.
The best time to dig irises for planting is after bloom in late July or early August, allowing enough time for establishment before winter. If you receive bare rhizomes or irises in a container earlier in the year, plant them as soon as convenient. It’s better to get them in the ground rather than wait until the ideal time. When considering bulbs, pay close attention to shipping dates. Aim to get bulbs in the ground before frost, by early October.
When feeding established iris, do not let fertilizer touch the rhizomes. Remove old blooms and stalks promptly after flowering to allow the plant to devote energy to growth rather than seed. Removing old blooms and stalks also encourages repeat flowering on reblooming iris (though a rebloom is not likely in Montana). Fertilize in early spring and be careful to not overwater irises; too much soil moisture can cause rhizomes to rot. Water consistently and deeply, especially during summer drought. After hard frost in fall, cut flower stalk to the ground and remove any foliage that appears spotted or yellowed and dispose of all debris in the trash. Every three to five years, dig up, divide, and replant healthy rhizomes in fresh soil. You don’t need to dig too deeply compared to other perennials to access the rhizomes. To keep track of flower color for relocating, label stems prior to digging. One idea is to write on a leaf with a felt tip pen. Each rhizome clump grows outward from the center, and the old rhizomes die. Since rhizomes keep extending and branching, save the youngest ones.
Disease and Pests
Irises should be planted in an area with good air circulation to help prevent disease problems. Bacterial soft rot is the most serious iris disease. Soft rot causes rhizomes to become mushy and have a disagreeable odor. Remove any yellowing leaves promptly to help prevent disease spread. If there are signs of rot in the rhizome, dig it and remove the affected parts. Manure can also encourage soft rot, so make sure it is aged for at least one year before incorporating. Iris leaf spot, caused by a fungus, is a common disease. Remove all leaf and other debris in fall, since diseases and insects often overwinter on old foliage. Iris borer is the most serious insect pest of iris. Iris borer overwinters as eggs in spent leaves. If vertical streaks are seen in the leaves, then look for these pests and squash them.
Learn more about growing irises and perennials in Montana from your local Master Gardeners, or look online for regional American Iris Society clubs.