Questions on: Iris

Ron Smith, Horticulturist, NDSU Extension Service


Q: I dug up my flower bed a few days ago and now have hundreds of iris plants. We cut the leaves to about 2 inches and stood them up in a lawn wagon. I gave a garbage can full to a friend. If I don't get around to replanting, how do I store them during the winter? (e-mail reference)

A: Store them in a cool, dark location. Keep only the healthiest rhizomes and dust them with sulfur, which helps prevent rot and insect problems. You would be better off making an extra effort to get them planted this summer or early fall.


Q: I have a large iris bed of 90 percent white and 10 percent lavender sprinkled among the white irises. I want to replant all the lavender in a group. If I wait until August, I won't be able to identify the lavender plants. What are the drawbacks to replanting now while I can see the color? (e-mail reference)

A: Moving them now will not kill them, but will keep them from blooming next year. I would suggest marking the leaves with a "W" or "L" now and moving them late in the summer or early fall.


Q: Several of my bearded irises have rotting leaves and some have bore holes. Is there something topical I can apply to cure the problem? (e-mail reference)

A: Once the borers get into the iris rhizomes, all you can do is dig them out and throw away the affected plants. Dust the rest with Sevin insecticide and replant.


Q: I am always fighting grass growing in my irises. What can I use to kill the grass and not destroy the irises, and where can I get the product? (Ada, Minn.)

A: Look for a grass control product that contains sethoxydim. There are several on the market. If label directions are followed, the product will selectively take out the grass without harming the iris.


Q: Please tell me if I can deadhead my irises and cut the stocks or do I have to let them die naturally. (e-mail reference)

A: Deadhead the iris flowers, but leave the foliage alone.


Q: One of my irises developed root rot this spring. I have destroyed it, along with as much of the soil as I could. What should I put in the soil to prevent this from reaching other iris plants? They are planted 15 to 18 inches apart. I had this problem in a different part of the yard about 20 years ago. I think there was some sort of powder that I worked into the soil. I don't want to lose my iris plants because they are special plants that I spent a great deal of money on last year. I hope you can help me. (New Salem, N.D.)

A: You can use a fungicidal drench of Subdue, Terrachlor, Fore, Formec (mancozeb) and even Funginex. Powdered sulfur mixed into the soil also will help thwart root rot and other diseases.


Q: I have strange problem. I planted yellow bearded irises given to me years ago. The irises are in a bed on the south side of my house. They did great for years. Then I got some violet irises and planted them in a section next to the yellow plants. I gave them the same proper feeding, watering and care. Then I noticed that the violet irises were doing well, but the yellow plants were dying. The violet irises still are doing fine, but the yellow irises are not blooming. The same thing happened when I planted a batch of white irises given to me when a relative moved and planted them next to a batch of violet irises. They were not intermingled. In a few years, the white irises gradually died out and the violet irises took over. What is the problem? I can’t find any information about this problem in my books or on the Web. (e-mail reference)

A: I certainly don't consider myself an expert on growing irises, but do know that purple is the naturally dominant color. It might be the other colors are not as robust or hardy as the purple irises in your environment, so eventually they die out. We have mixed irises in our home garden with no apparent problems. If an iris expert reads this column, I'm sure I'll get a correction or addition to this statement, which I would certainly welcome!


Q: My first question is about iris corms. My sister and I purchased some new plants yesterday and have them in cold storage. Can they be planted outdoors at this time? Iris winter OK once they are planted, so we think they could be planted when the ground is thawed enough to dig. What is your opinion? My second question is about tulips. I have several that are in clumps, so the original bulb must have several bulblets on it. I want to dig these after blooming, then separate the plants and replant to get more tulips. Most of what I have read says this doesn't work. Would I be wasting my time or will I get new plants? I figure they must do this somewhere to get new bulbs to sell. I live in zone 4. (e-mail reference)

A: You can plant the iris after the frost is out of the ground. The tulips can be dug up and replanted after they finish blooming and the foliage dies down. While they will not produce large flowers the first year or two (likely remain vegetative), they should do well after that. This practice is done all the time, so I don't know where you found information to the contrary. Have a good spring!


Q: We have hundreds of beautiful irises, but we are moving. In a panic the last warm winter day (March 3), I dug up a bunch of them. Is it OK to store the plants in crates until I am ready to replant in late summer? My concern is that there are some green stalks and leaves. Could I temporarily store them in some sort of soil mixture? (e-mail reference)

A: Pot the irises for the time being. Once they get started, it is hard to stop the forces of nature.


Q: I am moving to another house, so I want to dig up some iris bulbs and take them with me. Do I need to replant them right away or wait until spring? (e-mail reference)

A: Wait until spring to replant. Go to www.ext.nodak.edu/extpubs/plantsci/landscap/h113w.htm for complete information on growing and planting iris.


Q: I want to thin out some iris bulbs and transplant them to a different location. What is the proper procedure for dividing bulbs? (e-mail reference)

A: Let me give you a technical correction. Iris are rhizomes, not bulbs. They should be cut, trimmed and free of any weedy rhizomes, such as quackgrass or thistle, and then set into the new location as soon as possible. Plant just below the soil surface and water in. Go to my publication on iris care at www.ext.nodak.edu/extpubs/plantsci/landscap/h113w.htm for more detailed information.


Q: I have transported some iris bulbs from my mother-in-law’s iris garden. They are blooming wonderfully, but they are tipping over from being top heavy. Is there a solution to this problem? Both my neighbors are having the same problem. I want to make sure they are healthy because these bulbs hold a sentimental value for my wife. They are the offspring from her grandmother’s original iris garden. (e-mail reference)

A: When the blooms are large, heavy with dew or rainwater or a little weak from shade or nitrogen fertilization, it is necessary to stake them. I find that peony hoops work quite well on large clusters that tend to droop or use individual bamboo stakes for single plants. If they are in any significant shade, such as a half day, try to relocate them, after flowering, to a full-sun location.


Q: I planted some Iris bulbs in late summer last year. We’ve only had one flower so far. Do they need time to get established or are they hopeless? My neighbor says that if I take the dead flower off and plant it, the dead flower will become a bulb. Is that true? I have enjoyed reading the questions and your answers on the Web. You are amazing! (e-mail reference)

A: Me, amazing? You have no idea how many people will be laughing at your very nice compliment. Thank you! The dead part of the flower very likely will contain iris seed, which in due time will produce new corms (you refer to them as bulbs) and then flower. As to the flowering of the plants from last year, assuming they came from a neighbor – they eventually should flower.


Q: I planted some bearded irises around my house last year. Only one is blooming. The leaves have little holes in them. What could be eating the leaves and what can I do about it? (e-mail reference)

A: Any number of insects could be feasting on your iris plants! Spray with Orthene because it has systemic and residual action. That should take care of the insects that are dining on your plants.


Q: Last spring my husband and I landscaped our new yard using potentellas, a pink spirea, a rose bush, barberrys, salvia, Stella d' Ora daylilies, irises and day lilies. In the fall my mom cut everything down to the ground except for the potentellas. This spring the only plants that came back were the potentellas and the Stella d' Ora daylilies. The local nursery said barberrys and irises, daylilies, etc. are very hardy and don't usually die like that. I told them how my mom cut them down and they thought that might've been the problem. This year we re planted and used another pink spirea, more barberrys, irises, daylilies, salvia, a rose bush and a wiegela. Should I leave them alone this fall and winter? I'm scared to trim them. (E-mail reference)

A: Leave them alone and take the pruners away from your mom. Save the trimming for next spring.


Q: I have an Iris bed that is getting very dense. Can I divide the bulbs now or should I wait until fall? (E-mail reference)

A: It’s best to do it late summer or early fall.


Q: I have some nice large purple Irises. Two years ago I thinned them out because they were pushing out of the ground. Now those that remain come up green but do not flower. What might be the problem? (Moorhead, Minn.)

A: They just need a year to rebuild carbs for flowering.


Q: A friend recently brought me extra iris that she had. Do I still need to get them in the ground this fall? (Chancellor, S.D.)

A: You still have time so get them planted now. You need to mulch since the ground is beginning to freeze. Mulching will prevent heaving.


Q: My husband made me a beautiful brick iris bed. We filled it with good soil and added bone meal and fertilizer. The bulbs came up last year but this year they came up wilted and not healthy. Usually the leaves last all summer but they went brown and dried up. Can I save anything and what happened? (E-mail reference)

A: At this point, your guess is as good as mine. Your problem could be leaf spot disease, rust, downy mildew, iris borer, powdery mildew, crown rot or bacterial soft rot. If they all succumbed as you suggest they did, it isn't likely that anything can be saved. I would dig them up and examine the bulbs, throwing away any that do not appear healthy. Save the rest to replant somewhere else next spring.


Q: Is there an effective preemergent herbicide for an over the top application in my iris fields (4 acres), as Surflan is not available ? (Bismarck, N.D.)

A: The following herbicides are listed as cleared for iris: Acclaim Extra, Barricade, Dimension, Fusilade II, Gallery, Pennant, Snapshot, Treflan, and Vantage. Be sure to read and follow label instructions carefully.


Q: I had a problem with my iris this year. One of the buds would open nicely, but the others would dry up and had a papery texture. Can you tell me what happened and how to keep it from happening again? (Jud, N.D.)

A: It could be aphids or thrips that caused the blossom problem, especially if the rest of the plant appears healthy. In the future - next spring - spray with a systemic insecticide like orthene.


Q: Could you please tell me when is the best time to plant iris and peony? (Medina, N.D.)

A: Both should be planted in late summer, August to early September. That way, there is ample time for root development before winter arrives.


Q: The first bud on my iris would open nice, but the second ones would then be "mushy." Our weather has been really good up here so I doubt excess moisture would be the cause. Any suggestions? ( Mohall, N.D.)

A: Possibly thrips. They are known to attack iris flower buds. Or it could simply be botrytis, which is a common blossom blight. It doesn't take much for this to get started; cool nights leading to heavy dew that sits for hours on secondary flower buds could cause the problem. I'd put my money on that without seeing anything up close and personal. Control is by removal of the infected parts, and spraying with captan, chlorothalonil, or maneb.


Q: I have quite a large iris flower bed. I seem to be losing a battle with the dandelions taking over. Is there anything I can use to get rid of the dandelions that will not affect the Iris? We get our lawn sprayed each year for dandelions and it is pretty well clear of them. My husband thinks we should spray the iris with the same type of weed killer late this fall and see if it would stop the dandelions from growing. I am afraid it will kill the iris. What do you suggest we do? (E-mail reference, Geneva, Iowa)

A: I'm afraid that the only post-emergence herbicides I am aware of that can be used in an iris bed are for grassy weeds, not broadleaf ones like dandelions. Goal is a restricted use pesticide (RUP) that is cleared for iris. Find out if your lawn care operator is qualified to make such an application.


Q: Do I need to dig up iris bulbs in the winter? (Napoleon, N.D.)

A: Actually, irises grow from rhizomes--underground stems. More than likely, you shouldn't have to dig them up, as most are hardy in our region.


Q. When is the best time to plant tulip bulbs, and how deep should they be planted? I also would like to plant some iris. Would I plant them the same as tulips? (Donnelly, Minn., e-mail)

A. Generally, tulips are planted about 6 inches below the soil surface and planted en masse at a site rather than individually. Think in dozens rather than individuals.

Iris, on the other hand, are planted just below the soil surface--about 2 inches. With iris, you are likely better off planting them in the spring to allow the roots to get established in the new location. I'm afraid they could be winterkilled from heaving if planted in the fall.


Q. Can you tell me how to control quackgrass in my iris bed? I also would like to know if there is anything I can spray on my crabapple tree to prevent suckering? (e-mail)

A. Quack in iris beds is a tough one to get under control. Poast can be used, but I am finding out it is difficult to find. The only other option is dig the bed up and remove the quack by hand.

No, there is nothing you can spray on the roots of the crabapple to prevent suckering. You might try cutting the suckers off with a weed cutter that goes below the soil surface and raising the mower height to 3 inches if it isn't already there. Sometimes that procedure will at least reduce the number of suckers that have to be dealt with!


Q. My iris leaves have little round spots on them, with some of them so bad they are turning brown and curling up. I have applied malathion 50-percent spray twice since May 1, and it isn't any better. Even seems worse. What is it, and what can I do? (Aberdeen, S.D., e-mail)

A. It sounds like your irises are suffering from a common malady known as Didymellina leaf spot. If we are not too late, try controlling it with Zineb, also known as Dithane. These materials should be readily available in any garden store or nursery. Hope you can catch this in time!


Q. Last fall I repotted some house plants in some new potting soil. Now when the soil on the top gets dry it starts to get white like the enclosed sample. Can you tell me what the problem is?

I also am wondering if I can spray my iris bed with anything this spring to control the weeds. (Tripp, S.D.)

A. The white you are seeing is salt crystals. Nothing to worry about, unless the plants' quality starts to decline.

Once weeds get started in an iris bed, they are difficult to control. I've dug a couple up in my lifetime to get rid of weeds, and it is not fun.


Q. I would like to know what I can spray on my strawberry plants, rhubarb, and iris bed to help control quack grass. I can't move it to another location because it is everywhere. Once I read an article you wrote about Poast or something like it. (Monsfield, S.D.)

A. You remembered correctly—it is Poast (sethoxydim). You apply it to actively growing grasses, but you cannot apply it within 7 days of harvest. For other information, refer to the label.


Q. Please tell me how to get grass out of iris beds--fast. Thank you. (Rosholt, S.D.)

A. It all depends on the grass. If it is quack, it may prove too difficult to remove chemically, but give it a try anyway.

Use Poast or a similar product that will selectively remove most grasses. If it is a heavy infestation of quack, the only way may be to dig everything out and begin all over.


Q. I always read the write-up you have on plants in the Weekly Peddler, but somehow I missed the one on quackgrass in iris. This is a regular pain for me as I have a lot of them. Could you please send me some information on this matter.

Your write-ups are welcome to those of us who need help. Keep them up. (Lakota, N.D.)

A. Quackgrass in iris can be a real problem, in that I have found the rhizomes of the quackgrass can penetrate through the stout rhizomes of the iris.

My suggestion is not widely embraced by many gardeners, but if you are wishing to grow those iris where they are, you must be determined in your attack on this obnoxious weed. Here it is:

1.Carefully dig out the entire iris planting. Discard any rhizomes that have been penetrated by quackgrass.

2.Going down 4 to 6 inches, remove every quackgrass rhizome you can find. Be sure there are no quack rhizomes along the border of your planting bed. They will only reinvade once the iris are reset.

3.Once you are sure the area is clean, reset the iris and keep future invasions to a minimum with carefully sprayed Roundup.


Q.Could you please tell me what is wrong with this Iris plant?

Also, our evergreen trees have started to grow. The bud broke, but the growth is real small. The needles from old growth are brown. They were transplanted last summer. They are about four feet tall. What could be the problem with them? Do they need fertilizer, and if so, what kind? (Warwick, N.D.)

A.The iris leaves appear to have a couple of problems: a leaf spot fungus and bacterial soft rot. These problems are usually brought on by too much water and/or poor drainage. I suggest gging the remaining healthy ones and moving them to a better cation, or improving the drainage dramatically.

The transplanting operation for a tree is bound to damage some roots. This, along with the plant's re-orientation in the environment is going to produce the symptoms you describe. Providing a light fertilization sometime before Aug. 1 with Miracle-Gro will help them along.


Q: I have written before asking about quackgrass in my Iris flower bed. Your advice worked. I now have a nice clean Iris bed. Thank you.

I now need some more help. Last year I grew some beautiful dahlias. I dug them after the frost and put them in the coolest place in the basement. This spring when I was ready to plant the roots, they were all dried up.

I now have some dahlias again and they are starting to bloom. What can I do to keep them for next spring? I can't afford to buy new ones every year. I hope you can suggest some things I can do this fall to save my dahlia bulbs. (Ellendale, N.D.)

A.You might want to try "storing" them outdoors. Dig up the roots after the first frost and place them in a pit or dugout in the ground in a 50/50 mix of peat moss and sand that is dampened. Mulch heavily with leaves before the ground freezes and encourage snow collection.

This should work if we don't have an open, dry and very cold winter.

Glad my earlier advice worked.


Q: I have iris plants from my mother's yard in California. Last year they bloomed. This year, only one of them bloomed. They are full of leaves, but no flowers. One clump is between peonies, and the second is next to a flaming bush and daisies--all of which are next our house, which we had painted last year. What's the problem? (Britton, S.D., e-mail)

A: The problem of iris not flowering could be due to damage the painters inadvertently caused. If they had spilled any cleaner or paint on them, I'm sure they would be dead now. The painters probably covered the iris up with drop cloths, which could have had a detrimental effect on the flower bud set. As long as the foliage remains healthy, they should be OK in the future.


Q: I have an iris bed that has become infested with dandelions, some growing in between the rhizomes so they can’t be dug out. Is there any spray that I might use that won’t harm the iris? (Dent, Minn.)

A: I know of no herbicide that will selectively take out dandelions in an iris bed. Best thing to do is dig everything up and re-set the iris in clean soil.


Q: We dug out bearded iris bulbs today. I would like to know if there is anything special we need to do to them before we replant them. We had been told that you need to soak them in bleach but that seems kind of harsh. (E-mail reference, Indiana)

A: Soaking in a bleach solution is standard if disease was noted in the digging. Of course, the diseased material was discarded, so the soaking is to nuke out any remaining spores that may be existing on the rhizomes. If everything seemed clean, then simply replant at your earliest convenience. I've done that for years and have never had any problems!


Q: Why doesn’t my reblooming iris bloom? They are only one year old but are nice and big. I was told that they need more fertilizer. (Tyndall, S.D.)

A: Not likely, as I have iris that I have never fertilized and they bloom profusely! More likely it could be insufficient direct sunlight or they are planted too deeply. I’d suggest waiting one more year to see if they bloom for you in 2001. If not, then move them to a sunnier location.


Back to Flowers Menu
Back to the Hortiscope Table of Contents