Questions on: Crepe Myrtle

Ron Smith, Horticulturist, NDSU Extension Service

Q: I am writing to inquire about crape myrtles. Do you know of a variety that would possibly make it through our hard winters? We traveled to Arkansas last summer and noticed the beautiful flowers on the trees. One lady at a local retailer said there were some that might make it through our winters. Any information on crape myrtles would be greatly appreciated. (West Fargo, N.D.)

A: Dream on! Sorry, but they wouldn’t even come close to surviving our winters, even as a herbaceous plant. Below zero temperatures take them out. With our frost line down to more than 30 inches this winter, we’re going to find other plants that have made it through past winters also dying because of the cold weather that hit us in February.

Q: I have four crepe myrtles in a large bed on the north side of my house. Every year, starting with the crepe myrtles, a black, soot-looking substance starts forming on the trunks, then covers the leaves and somewhat spreads to the larger leaf shrubs in the bed. You even can see it in places on the ground. It almost looks like something that has been on fire. Is this a fungus? Do you know how it can be treated? (e-mail reference)

A: That black, sooty stuff is aptly described as ďsooty mold.Ē Sooty mold is a charcoal black fungus that appears as a black coating on the surface of leaves, fruits, twigs and the branches of many deciduous and evergreen shrubs and trees. This fungus is not pathogenic to plants, but obtains its nourishment from insect honeydew. Honeydew is a sweet, clear, sticky substance secreted by insects, such as aphids, mealybugs, scales and whiteflies. The honeydew drops from the insects to the leaves and twigs. Wind-blown sooty mold spores (seeds) that stick to the honeydew then have a suitable medium for growth. When spores germinate, they send out black fungus strands (mycelial threads) that bring about the discoloration. A heavy coat of black mold may build up on twigs during more than a growing season. To control or prevent sooty mold, control the insects causing the honeydew deposits. The insects have to be in large numbers somewhere, on the infected plants or tree branches overhead. They will not disappear without some intervention on your part. I suggest you contact a pest control specialist in your community to come in and get after whatever insect is causing the problem. Be sure the specialist is licensed.

Q: Can I root a crepe myrtle tree from a stem or cutting? (e-mail reference)

A: Yes you can. Take cuttings from the current seasonís growth and dip it in a rooting hormone powder. Stick the cuttings in a sand/peat mix (50/50) and keep moist. The cuttings should root in four to five weeks or sooner.

Q: I have had a white crepe myrtle tree five feet or so from the end of our house for approximately 14 years. Last year I cut it down because I noticed we were getting roots growing toward our house. Even though I cut it down, the thing will not die. This year the roots are worse. We hear creeking during the day, probably at others times also. Iím afraid the tree is doing damage to our foundation. Iím almost sure itís damaging our concrete slab. Please suggest something I can use to kill the roots. I have tried to dig it out but the roots are everywhere. I believe I will have to dig more out and replace all of the dirt (about a dump truck load) because the dirt has small roots in it that can reproduce. Can you suggest something chemical I can use? I will try almost anything. (E-mail reference)

A: I find it hard to believe that crepe myrtle can be a threat to the foundation of your house but it is too late to be concerned with that now. The best thing you can do to kill the roots is to spray the shoots that come up with a broadleaf herbicide. In other words, treat it like an unwanted broadleaf weed and don't bother digging everything up. The roots that remain will eventually rot and become an organic source of nutrients for your lawn and other landscape plants.

Q: Can a crepe Myrtle be moved and transplanted? If so, when do you do it? (E-mail reference)

A: It can be transplanted as a balled and burlapped plant when dormant. The best time is either early spring or fall.

Q: I understand that a crepe myrtle blooms on new wood. If I deadhead my crepe myrtle will it bloom again in one season? (E-mail reference)

A: Be careful how much you prune! "Deadheading" is not proper pruning of anything that is a shrub or tree specimen. Cut back nothing larger than the diameter of your thumb. It will bloom on subsequent growth. Try to get the pruning done ASAP to avoid winter damage.

Q: I read the question and answer about the crepe myrtle that changed colors. I have a friend whose white crepe myrtles have gradually changed to a lilac color. They began changing last year, and the plant had white blooms as well as the light purple ones. This year the plant has all purple blooms. I was wondering if you have discovered new anything about this. (E-mail reference)

A: No! I'm hoping somebody out there can tell me! I can make all kinds of guesses -- chimera, sport mutations, etc., but I certainly don't know for sure, and none of my references give me any information on concerning that characteristic. Sorry.

Q: I have crape myrtles in a bed. They are about four or five years old and have been deep pink or red from the beginning. This summer they were very slow blooming and have changed to white. I also have a bush which was probably 60 feet from the bed of six crape myrtles and it too has changed colors. Could you tell me why this might have happened? (E-mail reference)

A: There is a first time for everything, and I guess this is mine for the crape myrtle shrub turning color! You must have different plants or they have different colors grafted on the same plant.

The only other possibility is that the crape myrtle is sensitive to soil pH changes, which again I have never heard of. Sorry I canít be much help!

Q. I planted two crepe myrtle bushes that I got down south this year. I would like to know if they have a chance of growing in North Dakota. I planted them in a very large planter, using potting soil mixed with peat moss. After the leaves fall off, I plan to put the pot in our basement to lay dormant over the winter. I bought some Jobes spikes, systemic insecticide with 14-8-4 fertilizer for flowering shrubs, but I have not used this yet. Do you have any hints, suggestions, or words of hope for me in trying to grow these bushes? (Westhope, ND)

A. Where there is life, there is always hope, as the saying goes. However, crepe myrtle bushes are indigenous to the south and will stand temperatures down to about 0F to 10F. Your moving them inside will definitely save them from the ravages of winter, but you need to keep them in as cool and dark a place as possible so they won't begin coming out of dormancy too early. Let a couple of good North Dakota frosts nip them this year and the leaves fall off before moving them inside. Then simply water them once a month with cold water (helps to keep them dormant) until spring weather appears favorable for planting.

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