Some of the most spectacular flowers in the garden are irises. With development and improvement that the iris has experienced over the last 50 years there are now thousands of cultivars of bearded, Siberian and Japanese irises in many colors, sizes and forms. Although not a native of Minnesota, many of the over 300 species worldwide are suitable for our Minnesota gardens. Tall bearded irises with stems of 28 inches or more are the most common, but Siberian irises are less common but actually easier to grow. Shorter bearded irises are also available. Japanese irises require special care and are for gardeners who like more of a challenge.
Bearded irises get their names from the fuzzy patch on the down turned petals (also called the “falls”). The upward arching petals are called the standards. Bearded irises bloom from early to late spring in Minnesota. Some may rebloom in late fall if left undisturbed. Irises grow on rhizomes which are a thick fleshy part of the stem which grows horizontally underground and produces both the shoot of the plant and roots. Rhizomes store both carbohydrates and proteins and allow the plant to survive underground in the ‘off’ season. A bearded iris rhizome will have a fan of leaves at one end and thick roots coming out underneath. The best time to plant them is from late July to Labor Day, so that they will have ample time to develop a good root system before winter.
Choose a sunny, well drained spot to plant irises. Irises do not like ‘wet feet’ which means they do not like to sit in standing water. Make sure the spot does not hold water after a heavy rainfall or watering. To assist with drainage mix peat moss into the soil. Dig a shallow hole for each rhizome leaving a ridge of soil in the middle. Place the rhizome on the ridge with roots draped over each side. Make sure the rhizome is no more than an inch below the soil surface; it is even OK to have the top of the rhizome showing, irises do not like to be planted deep. Plant rhizomes at least eight inches apart to allow for growth. Bearded iris need the same care as other perennials in the garden. Deep watering in dry periods, regular weeding and fertilizing once or twice a year. Do not fertilize after August, to allow the plant to prepare for winter.
Bearded iris can stand to be thinned out and divided every three to four years. The best time to do this is late July through early August. Lift the entire clump of iris with a spading fork, being careful not to break off the fat feeder roots. Wash off the soil and using a sharp knife cut the rhizome into individual fans. Discard the centers of the rhizome which has no fans. Cut the leaves back to about 6 inches. You can store your remaining fans for a few weeks or plant them back in the soil immediately. Just be sure to plant in plenty of time for them to develop a strong root system and store up more carbohydrates for the winter.
The most destructive pest of irises is the iris borer. It can destroy the iris rhizome by eating it, and if that is not enough the feeding damage allows the entry of bacterial soft rot which finishes the job. The life of the iris borer begins as an egg which over-winters in old leaves and plant debris at the base of the iris plant. In the early spring it hatches into a tiny caterpillar which works its way up the new iris foliage, tunneling into the leaves leaving only pin prick size holes which are very hard to detect. As it eats its way down the leaves to the rhizome it leaves streaks that are tan or water-soaked looking. The leaf tips may turn brown as the little critter sucks the life out of them. Generally the caterpillar will reach the rhizome by mid-summer (July in Minnesota). It continues to feed on the rhizome until late July or August when it moves to the soil where it pupates. In late summer or early fall it emerges from the soil in its adult form – a moth with a 2 inch wing span, chocolate brown wings in the front and yellow brown hind wings. The moth is seldom seen since it flies at night, but within a few weeks the females begin the cycle again by laying eggs on the leaves and plant debris at the base of the irises.
Inspecting irises in July is very important. If you see brown leaf tips or dying leaves dig up the rhizome and inspect for iris borer. Discard any rhizomes that have the borer or are soft, mushy or tunneled. In the fall clean up any iris leaves, stems and nearby plant debris to remove and kill eggs thus reducing the chance of iris borer next year. After a hard frost cut back your iris leaves to about 6 inches. Mulch your irises with leaves, straw or light compost. Remove the mulch gradually in the spring and enjoy your iris blooms again.