Tall bearded iris are the one plant that can be divided and transplanted at this warm time of the year! Blooming from late May into June, these plants enter dormancy in the hot summer. The spreading rhizomes are easily dug up with a spading fork, separated, roots trimmed, and replanted in a gritty soil mixture in full sun. We often bring a bag of builder’s sand along for the purpose of “unimproving” rich garden soil.
Keep iris away from your irrigation system, as they are quite drought tolerant and don’t like to be overwatered. You can easily hand water any newly set divisions with a watering can until they “take.” That moment comes when you can’t pull them up by lightly tugging on their tops. If you’re about to be vacationing now, don’t despair – this work can be undertaken through the end of September. Iris need a period of time to root and settle before winter.
There is one pest of bearded iris that can be very destructive: the iris borer. You’ll be able to quickly identify trouble if you smell something nasty as you handle the rhizomes. Then you’ll notice a pinkish worm (the iris borer larvae) moving in and out of tunnels in the rhizome. Immediately bag and discard these plants, as this is the easiest way to contain the problem. Leave the area empty during the hot summer, and don’t replant with new iris until next spring, because any larvae left in the soil will only invade the new rhizomes! A better idea would be to rotate the position of your bearded iris to a new full sun spot and try a new perennial in the old location.
We also trim the fronds of tall bearded iris into low fans during the summer and fall to discourage adult iris borers from recognizing the plant and laying their eggs. In addition, we clean up browned out foliage that is a result of the hot weather.
If you’ve had a problem in the past with iris borer, it can be effectively controlled in the spring by spraying insecticidal soap on the plants after the first two days of 70 degree weather. This is a good preventative that is timed when the larvae begin to emerge.
Please let Kim know <firstname.lastname@example.org> if you’d like our help with maintaining your patch of bearded iris.
And if you want to acquire some new varieties of Bearded Iris for your garden, show up at Verrill Farm in Concord on Saturday, July 27 for the annual sale organized by the Iris Society of Massachusetts. Hours are 11 am – 3 pm.