Gardening with Bearded Iris

Gardening with Bearded Iris: Spring Clean up, Fertilizing Irises, and Iris Borer Control

 

You look out at your iris bed this week and see – not much! My bearded irises are looking pretty dormant. The bed’s main feature is the collection of zinc markers rising up like little headstones in a cemetery. You wouldn’t think the iris garden needs any attention this month. But now is the perfect time to get your irises set for a season of gorgeous blooms and healthy plants!

 

Early spring is the time for three important tasks in the iris garden: cleanup, fertilizing, and borer control. Clear away any winter mulch, old leaves, and miscellaneous debris from your bearded irises. Test the soil, and apply a granular fertilizer for better blooms and more vigorous growth.

 

 

 

 

 

 

 

 

Take steps to control the dreaded iris borer (Macronoctua onusta), if present in your area, at its most vulnerable time of the year.

 

Clean up: when, what, and why?

If you've put Christmas tree branches, straw, or anything else on your bearded irises to prevent heaving during freeze-thaw cycles, clear it away. Any kind of mulch around the rhizomes during spring rains may lead to rot. Floppy leaves and debris can also harbor leaf-munching slugs. Some gardeners trim their iris foliage after the first hard fall freeze. Others leave old green leaves in place to provide a little winter protection and continue feeding the rhizome on mild days. Cleanup is also important for borer control.

 

When temperatures warm to 21°C in spring, borer eggs hatch. Larvae burrow into the leaves. In midsummer, they migrate down to the rhizome. That's when the real trouble begins. Tiny borer larvae become fat grubs that can badly damage entire stands of irises before pupating and emerging as egg-laying moths in fall. Never compost iris leaves. Throw them away as you would any potentially diseased plant material.

 

Some suggest removing only dead or dying foliage to eliminate most of the iris borer eggs that may be present.  When gardening within the borer's range (regardless of whether or not borers have been identified in your particular garden), any foliage which was not removed in fall should be removed all the way down to the rhizome before active growth begins in the spring.

 

 

 

 

 

 

 

 

 

 

As you clean up around your bearded irises, you may notice that some of the rhizomes have heaved right up out of the ground during winter freeze-thaw cycles. There's no need for alarm. Give the rhizome a squeeze to see if it still feels firm and healthy, then just shove it back into place.

 

Occasionally, you may find a rhizome that has developed smelly soft rot over the winter. If some part of the rhizome remains firm, you can try to salvage it. Either cut off the soft portion, dip the rhizome in a bleach solution, and replant it Or, treat it in place by scraping off all soft areas until only potato-like firm rhizome remains, then sprinkle copper or antibacterial powder or a cleanser. Widespread rot can be an indication of borer damage.

 

To fertilize, or not to fertilize?

Many gardeners fertilize their bearded irises in early spring for a fabulous show of summer blooms, using a "bloom booster" product with a high middle number in its N-P-K ratio. However, the only thing irises may resent more than underfeeding is overfeeding. "First, do no harm!" Before applying any fertilizer to your garden, do a soil test to discover what your plants actually need.

 

Your soil test may also reveal a need to correct pH or add trace minerals, so you'll be checking for more than just the need to boost phosphate levels. If you do decide your irises need to be fertilized, granular fertilizers should be applied about six weeks before bloom time.

 

What else can I do to control borers?

If you live in an area where iris borers are a problem, you may wish to practice prevention beyond spring cleanup. Just as irises are breaking winter dormancy, you could apply a granular grub control product containing imidacloprid , also known as Merit. This same systemic insecticide is used in Advantage flea and tick control products. Bayer Advanced Lawn Season-Long Grub Control, and Scotts Grub-X are brands containing imidacloprid. Check the labels to make sure the product contains at least 0.2% imidacloprid.

 

 

 

 

 

It's important to get the product down before warm weather arrives, as imidacloprid is only effective against newly hatched borers. The granular form can be applied directly around each clump (1/2 to 1 teaspoon per clump) and watered in. Imidacloprid is also available as a wettable powder for spray application.

 

 

 

 

 

 

 

Use one form or the other, not both, as double dosing could damage your plants. Follow directions and precautions on the product label.

 

Broad spectrum pesticides may be useful to kill borers present in iris foliage in early to mid summer. Spectracide Triacizide 6 weeks after applying imidacloprid to her irises. She says it also helps against wireworms, Japanese beetles, and other pests.

 

Organic alternatives may also be effective and are becoming more important as concerns are raised about potential groundwater contamination and about the possible effects of pesticides on dwindling honeybee populations. There have been promising studies of treatment with beneficial nematodes. Iris borer larvae can even be eliminated effectively, if tediously, from the foliage during the summer months. Flattening each leaf carefully between your fingers should squash any borer larvae within.

 

Is it time for summer blooms yet?

It's more fun to work among your bearded irises when they're blooming madly. But the time you spend now cleaning up, fertilizing, and taking steps to prevent and control iris borers will pay off tenfold in just a few weeks.