Questions on: Dahlia

Ron Smith, Horticulturist, NDSU Extension Service


Q: I planted some very beautiful dahlias, but it is getting cold and I want to save them. What do I need to do to preserve my flowers? Do I have to dig up the bulbs? If so, how do I store them? (e-mail reference)

A: Allow the tops to be nipped by the frost. That shuts everything down. Then dig and clean them by hand, but don’t use water, and dust each one with powdered sulfur for insect and disease protection. Store the dahlias in paper bags in a dark, cool location that will not freeze. Plant them again in the spring when the danger of frost is past.


Q: I have read that I should put mulch around dahlias. This is supposed to help keep the ground moist and protect them from the hot sun. Can I use grass clippings? What are other alternatives? (e-mail reference)

A: Any kind of well-weathered organic mulch will benefit any plant, provided it isn’t overdone. A 2- to 3-inch layer is ideal. If you use more than that, problems such as slugs, soil kept too cool/moist and reduction of air to the roots begin, causing the plants to migrate into the mulch, which defeats the purpose. Grass clippings can be used as long as they have not had any broadleaf herbicides applied during the last three mowings. Keep in mind that grass clippings are “green manure” and too much will cause heating and possible damage to the plant. If you have an area to spread the clippings out for a day or two to dry before spreading them around the plants, that would be ideal.


Q: This spring I purchased a dahlia in a pot. It looked very healthy and happy so I repotted and put it out with my other potted plants. It continued to grow and look very healthy, but it never grew any flowers. Do you have any idea why? Since it is still looking good I thought I would repot it and bring it indoors for the winter. Can this be done? (E-mail reference)

A: Dahlias will not bloom if they don't have enough light, which I am willing to bet is the problem in you case. If it is getting enough light, then you may have given it too much nitrogenous fertilizer. Another possibility is that your season is too short, and it should have been started earlier in the spring. March is what we recommend in North Dakota. There are several things that need to be done with dahlias: allow them to die back naturally in the fall either via light frost or from withholding water; cut the spent stems back to about three or four inches; gently lift the clump of tubers from their planting site and let them dry in the sun for at least a day; gently knock off the dry soil and store them for the winter. Repot the following March after dividing the tubers so that each one has an "eye" or bud.


Q: I seem to have some type of rot problem with my dahlia flowers. As soon as they start budding out, the underside of the flower dries and gets brown. Some of the flowers didn’t bloom out all the way. They are crippled but the bushes seem to be very healthy. The buds seem to be a bit discolored too. Can you tell me what's wrong and how to remedy it? (Columbus, N.D.)

A: There could be a couple of things that are causing this problem. My first suspicion is thrips. They are minute insects that feed on flower buds and cause them to be disfigured and/or leave an aborted flower blooming period. Since you state that the rest of the plant looks healthy, I doubt that it is a fungal disease. When you bring the dahlia bulbs in this fall, dust them with powdered sulfur going into storage. Spray with Orthene, a systemic insecticide, as new growth begins next season. That will eliminate them as they begin feeding.


Q: I planted dahlias this spring. They came up real good but now they look wilted. I planted them in plastic patio pots with potting soil. What could be the problem? (Barnesville, Minn.)

A: The problem could be poor drainage or a vascular rot that has developed.


Q: I live in northern Montana so I think the problem would also apply to your area. I overwintered some dinner plate dahlias in my basement in a container of peat moss. About a month ago they were showing a lot of dryness so some water was added to the container. Several of the roots now have long sprouts. Should these be removed? Should the roots be planted in containers and then kept indoors until such time as they can be planted outdoors? (Choteau, Mont.)

A: Don't remove the sprouts! The corms are doing exactly what you want them to do. All you need to do at this point is to pot them up and place them in as sunny a location as possible and plant outdoors when danger of frost is past.


Q: I planted dwarf dahlias this year that were sold as annuals. When I went to pull them up today I noticed huge tubers. Can I store them over winter as I do perennial dahlias and plant them in the spring? (e-mail)

A: Yes, dig them and store as you would any other dahlia. In North Dakota, the "perennial dahlia" is rare and comes about only because of good protection. We generally treat all dahlias as annuals.


Q: I have some dahlia bulbs that I started in pots to give them a head start, and when they were an inch high I brought them upstairs to give them some light. I was planning to put them outside for short periods of time to harden them up, but before I had a chance the weather turned windy, then rainy. Now they are 2 to 3 inches high. Can I still harden them or can they be trimmed? I also would like to know if I can safely use Preen around my rose bushes and hydrangeas to get rid of the creeping jenny or dharlie that is in them? (Glenfield, N.D., e-mail)

A: Yes, you can still harden them off and even prune them back if you wish. The wind will be the biggest enemy of these (and most plants in North Dakota!), so you may want to stake them or find a protected spot for them in the landscape—if such a place exists.

Yes, Preen can be used safely around roses. Creeping jenny is a tough weed to bring under control. If you have a pure stand of it, use Roundup to kill it off. It may take repeat applications to get rid of it. Where it is mixed with grass, you may have to use a formulation known as Trimec to bring it under control. Fall—between mid-August and mid-September—is the best time to control this and other broadleaf perennial weeds.


Q. I really look forward to reading your column. Last year I had lots of dahlias, both the large cactus-flowered ones and the smaller patio, or border dahlias in my garden. I had problems with the buds not opening, especially in the smaller ones. Could you please tell me what the problem is and what I can do about it, as I really love dahlias in the flower garden. Friends have quit growing them because of this problem. Thank you. (Vergas, Minn.)

A. Your problem sounds like it could be thrips, which one I am not sure. There are as many different types of thrips as there are types of Smiths.

Here are my suggestions for controlling them: (1) If you have stored tubers, dust them in floured sulfur before planting out. (2) Plant dahlias in new location. (3) As flower buds begin developing, spray with Orthene, a contact/systemic insecticide. (4) Never plant dahlias where glads were previously grown.

Thanks for writing and the kind compliment about the column.


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 was given a Christmas Cactus that has probably been in the same pot for 30 years. It just had two blooms on it. Any advice on when and how to repot it would
be helpful.

Also I started my dahlias indoors and they have really taken off. Should I cut them back? Some are 10 to 12 inches tall with lots of leaves. (Sheyenne, N.D., e-mail)

A: Thirty years in the same pot! That has to be a record. Typically, this type of cactus is potted after flowering is completed, which it should be now. It
would also benefit from a summering outdoors in a shady location.

As for the dahlias, yes, cut them back somewhat, and move them out doors during the day when the temperature is 50 F or higher to begin slowing
them down and acclimatizing them.


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