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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.
Cleanup: 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. Iris borers are present in many but not all states in the US, especially in the Northeast, MidAtlantic, and Midwest regions. Check with your agricultural extension service or your local chapter of the American Iris Society to see if they are in your area.
When temperatures warm to 70°F 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. DG's Laurief takes a more ruthless approach to eliminating borers from her irises. Her advice: "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." [1]
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. [2] 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 such as Ajax on exposed surfaces. 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. [3] 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. DG's avmoran uses Spectracide Triacizide 6 weeks after applying imidacloprid to her irises. She says it also helps against wireworms, Japanese beetles, and other pests. [4]
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. [5] There have been promising studies of treatment with beneficial nematodes. [6] 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. Be sure to stop by the DG Iris Forum, and post photos of your garden in bloom!
Photo credits: Thanks to DGer TBGDN for the clear images of iris borers and the damage they do. See the notes on iris borers in his garden journal for more information. Thanks also to JanieJoy for the photo of a borer on a freshly dug iris rhizome. The photos of irises in my garden were taken by me.
[1] Personal communication. Laurie is an avid iris gardener with over 12 years of experience growing both bearded and beardless irises in the challenging climate of northern Minnesota. More great tips and information can be found on her Shadowood Iris Garden FAQ page.
[2] See my 2007 DG article "Planting Iris 101" for bleaching and planting tips.
[3] Contact your local agricultural extension service about free or low cost soil testing services. Read more in this DG article, "Don't Guess, Soil Test"
[4] Personal communication. Anita is an active member of my local AIS chapter and a frequent contributor to discussions on the DG iris forum. For a treat, check out her gallery of iris photos and descriptions at Pilmore Iris Garden.
[6] For further discussion of treatment with beneficial nematodes as well as additional information about iris borers, see this Iris Borer Information page from the Canadian Iris Society and this University of Minnesota Extension Service Yard & Garden Brief: Iris Borers.
About Jill M. Nicolaus
Better known as "Critter" on DG, Jill gardens in Frederick, MD. This week, I've been cutting lavender, putting up pluots (plum-apricot fruits), and pulling weeds (always, LOL). I love being outside in the cooler morning -- coffee cup in one hand, watering hose in the other -- watching the birds and butterflies among the flowers.(Images in my articles are from my photos, unless otherwise credited.)
Posted by dormousehouse (from villennes sur seine
(France)) on March 10, 2008 at 8:46 AM:
hi! i live near paris france which i heard is like zone 9...not sure tho. can i divide now? thanks, lisa
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Posted by critterologist (from Frederick, MD) on March 10, 2008 at 11:39 AM:
Here, I'm told to divide my irises at the end of July or beginning of August. I think the idea is to give them some time to recover and grow after blooming (each rhizome blooms once, so you want to give the "mother" rhizomes plenty of time to form "daughter" rhizomes). Also, and probably not a consideration in your mild zone, you want to plant the divisions at least 6 weeks before frost/cold, if possible. I know this isn't a direct answer to your question, but I'd say your window for dividing irises would be between 7 or 8 weeks after they bloom and 6 weeks before freezing fall weather. I hope that helps!
For more expert advice, and to find other gardeners in your zone &/or region, I hope you'll consider subscribing to DG. A trial subscription would let you look around... The number of forums keeps growing, so its easy to feel overwhelmed, but pick a couple and jump in! Meanwhile, if you're not ready to subscribe, check out some of the forums that are open to both members and subscribers. And welcome to DG! :-)
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Subject: Thank You!
Posted by rosa18 (from Bartlett, IL) on March 10, 2008 at 8:43 AM:
Great article, I wasn't sure when to start applying chemicals,you supplied all the information I needed.
Thanks so much!
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Posted by critterologist (from Frederick, MD) on March 10, 2008 at 11:43 AM:
You're welcome. I figure if you're going to use a chemical control, it's important to use it when it'll be most effective... :-)
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Subject: Just the info I needed
Posted by Lindawalkabout (from Holden, MO) on March 6, 2008 at 11:26 AM:
I was just looking at my Iris patch a few days ago and wondering what I should do, its been about 5 or six years that I planted them and just left them alone. Maybe I should start to divide and look at them, I mean really look at them 8 ). Critter your artical will help me know what the heck I'm doin.lol , thanks for the pics and links, lots of info : )
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Posted by LTilton (from Glen Ellyn, IL) on March 6, 2008 at 11:46 AM:
Excellent advice about iris borer control!
I do constant battle against those things.
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 11:50 AM:
Glad you found it helpful! I've only been growing irises for a few years, so I was glad to get expert advice to be sure I'd covered the basics -- both for myself & for the article.
Linda, dividing should wait until later this summer, probably around the end of July in your area. Check my first article about irises for some information about "mother" and "daughter" rhizomes that will help when you're dividing your clumps.
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Posted by Lindawalkabout (from Holden, MO) on March 6, 2008 at 11:58 AM:
Yes I will Critter, TY
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Posted by Seandor (from Springfield, MA) on March 6, 2008 at 1:34 PM:
Thanks, critter!
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Posted by MaryE (from Baker City, OR) on March 6, 2008 at 2:02 PM:
A good article with lots of links to follow up information, thank you! I have iris with little holes in the rhizomes but I have now eliminated iris borers as the pest, and instead need to look for information on wireworms.
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 2:05 PM:
When I was first dividing some irises I'd been given, I was really worried about the little holes I saw in some of the older rhizomes... They were just riddled with them! Then somebody explained to me that those were just where the roots had been, perfectly normal to see on old "mother" rhizomes. You can replant the mother and it may continue to produce more daughter rhizomes, or you can just break off the larger daughter rhizomes and replant those.
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Posted by Indynannyof8 (from Brownstown, IN) on March 6, 2008 at 2:07 PM:
I sometimes fine spots on my iris foliage and am wondering what causes that kind of damage.
Thanks for some great information, I have to admit tha I don't always know quite what to do for my irises.
Veronica
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Posted by MaryE (from Baker City, OR) on March 6, 2008 at 2:20 PM:
Root holes! Whoda thunk? Surely not me. Thanks.
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 2:24 PM:
If you want to post a picture (you need to start a new thread... I think only the first post on an article response thread can have a photo), I'll see if those holes are the same as what I was seeing... but I'll bet that's what you're looking at.
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Posted by Indynannyof8 (from Brownstown, IN) on March 6, 2008 at 4:05 PM:
Now I know what those holes are. I thought they were bug or worm holes but they always look so neat and clean..... I learn so much here
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 4:25 PM:
I *knew* I was going to forget to mention something in the main article! LOL I'm glad it came up here.
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Posted by oriole (from Mifflintown, PA) on March 6, 2008 at 4:59 PM:
Thanks, lots of great info.
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Posted by bthyme (from Buena Vista, VA) on March 6, 2008 at 6:08 PM:
Would systemic rose food such as Bayer's be effective against borers and wire worms? Since roses are heavy feeders and this article tells me that irises are not, I suppose the amounts would have to be modest.
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 6:50 PM:
For what it's worth, the most effective (still on the market) chemical treatment for borers seems to be imidacloprid. You might get some of the borers with a general systemic (and I don't know about wireworms), but getting the larvae while they're young seems to be the best bet.
Some people do say that irises are heavy feeders, but other people tell me that over-fertilizing is far worse than under-fertilizing... and I've seen gorgeous stands of irises in clay soil that hasn't had attention in years. Granted, those were probably older varieties (some of the modern hybrids seem more picky to me), but still that argues against heavy fertilizing being necessary. Soil testing is a good idea. :-)
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Posted by planolinda (from Plano, TX) on March 6, 2008 at 7:20 PM:
a few iris bloomed over the weekend--now it is snowing!! a lot! all of my iris are up and out of the ground--i wonder if it will hurt them--
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 7:50 PM:
Oh, goodness! I think if you've got any with fat buds, you might cut them and bring them in so you'll get to see the blooms... I seem to remember that the buds and blooms will freeze if it gets too cold, although the foliage and the plants should be just fine.
I got outside for a bit this afternoon and managed to clear out about half my iris bed. Tomorrow I'll pick up some Grub-Ex to put down. :-)
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Posted by planolinda (from Plano, TX) on March 6, 2008 at 8:02 PM:
crazy weather--up to 70 last weekend and now heavy snow!! when it was 70 i was working in the yard and had some very big weeds--i used my shovel to get some and up came what i thought was a big rock--it was one of the turtles that live in the yard hibernating--i was so scared that i had hurt him so i layed him on the flower bed and continued to work in the yard--he finally came out of his shell and walked off--poor guy will have to get back in the ground again--he must be confused!
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Posted by critterologist (from Frederick, MD) on March 6, 2008 at 8:04 PM:
ROFLOL! I've dug up plenty of worms and startled an occasional toad or snake... but I've never dug up a turtle. I'm glad he was OK, and I'm sure he's happily dug in again somewhere in that nice loose soil you created for him. :-)
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Posted by planolinda (from Plano, TX) on March 6, 2008 at 8:12 PM:
we've had turtles living in our yard for at least 10 years and are attached to them in a strange sort of way--we don't take care of them -they are on their own and sometimes we don't see one for months--but we always get excited when we do see one!
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Posted by pajaritomt (from Los Alamos, NM) on March 7, 2008 at 2:25 AM:
Thanks, critterologist for the great advice on iris borers, fertilization, etc. After all the stuff I have heard about the death of bees, I won't be using imidacloprid, not that I ever did. There are good manual ways to get rid of iris borers. No need to kill off the helpful insects in order to save our irises.
Loved your article and was happy to finally see a picture of you.
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Posted by critterologist (from Frederick, MD) on March 7, 2008 at 11:49 AM:
You're right -- there are alternatives to imidacloprid. I mentioned them and included links for more information as well as links about imidacloprid use. Beneficial nematodes and squashing borer larvae by hand seem to be the most promising nonchemical controls. As with any pesticide, you should consider alternatives and weigh the pro's and con's before using it.
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Posted by pajaritomt (from Los Alamos, NM) on March 8, 2008 at 1:15 AM:
I have no experience in this department, but from what I read, if one watches the iris, one can see where the iris borer enters on the leaf and where it begins boring down to the rhizome. From what I hear, all you have to do is grab the leaf below where the borer has come in between thumb and forefinger and move it right up the leaf until one has squashed the borer. It doesn't even dirty your fingers!
I like this method because imidacloprid.seems to be such a threat to the bees which we so desperately need.
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Posted by critterologist (from Frederick, MD) on March 8, 2008 at 1:22 AM:
I'm not sure they're that easy to spot... I've read that they can be eliminated by hand, but that if you think they're in your irises, you need to go through them leaf by leaf, running each leaf through your fingers to squash any larvae.
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Posted by pajaritomt (from Los Alamos, NM) on March 8, 2008 at 2:08 AM:
Yes, that is what I have heard as well. If you have a huge number of iris it is a lot of work. On the other hand, I could do about 100 per day, I think. Anyhow, just a thought.
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Posted by figaro52 (from Oak Lawn, IL) on March 10, 2008 at 11:59 PM:
Thanks for such a timely article. I've noticed some heaving of newly planted rhizomes during the winter even though the roots tend to be firmly anchored. I'm not so worried after reading your article.
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Posted by critterologist (from Frederick, MD) on March 11, 2008 at 1:29 AM:
I was outside just this afternoon, stepping on a few rhizomes that looked like they needed to be planted more firmly. (I didn't really stomp on them, just nudged them down into the soil/mud with my toe.) I'm glad you found useful info about something other than borers in the article... :-)
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.
Is it time for summer blooms yet?
