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Hortiscope

Ron Smith answers questions about flowers, trees, gardens and shrubs.

By Ron Smith, Horticulturist NDSU Extension Service

Q: I have a 12-year-old river birch that keeps dropping small limbs, but it looks healthy. I seem to remember hearing something about possibly some type of worm that may be causing this to happen. I don't see any signs of where a squirrel may be doing the damage. The limbs that are falling off are blunt at the end, so they look like they have been cut with a sharp object. (Lexington, N.C.)

A: Some twig drop is normal with birch trees. My tree does it all the time. However, your tree could be getting bedeviled by a twig girdler based on your description. To be on the safe side, when spring starts to arrive in your part of the country, I would suggest getting some Bayer Advanced Tree and Shrub Insect Control. Follow the directions when applying the product. It controls the problem for the entire season. If the limb drop is caused by insect activity, then the density should decline as the spring and summer progress. As I said, this is somewhat normal with this species as the tree gets older. It is like humans losing hair!

Q: I read that it would be beneficial to stick rusty nails into the soil of potted plants to provide iron. Another source says this does not help the plant. Any idea if this works or not? (Concrete, N.D.)

A: If the plant needs an iron boost, the best way of doing this is to apply some chelated fertilizer. If the iron is there but not available because of a high soil pH, then get the soil acidified with a sulfate-based fertilizer.

Q: I have a shamrock plant that I was given, but I know very little about how to care for it. I think I have a problem with the plant. The long stems of the plant have a tendency to crack and/ or break about halfway up the stem. I placed some soft sponges under the stem around the lip of the pot. It works but doesn’t add to the plant. Can this plant be repotted without breaking the long stems? Does it need much watering? Does it flower and where are the flowers attached? How can I start new plants? Is this plant from a bulb? Does it need sunlight or would fluorescent light be OK? (Crookston, Minn.)

A: Your shamrock is the glorification of a common weed known as yellow wood sorrel that thrives in greenhouses and shady, damp locations around residences. It is a bulb that produces the plant. The plant thrives best in damp soil, which leads to the occasional buildup of salts. While unsightly on the soil, the salts do not seem to discourage or inhibit the plant’s growth to any extent. However, it is a good idea to leach the salts from the soil. A shamrock should have bright to moderate light throughout the year but not necessarily direct sunlight. It will grow using fluorescent light. The plant should receive a regular monthly fertilization using material high in phosphorus. This is the middle number on a bag of fertilizer. Sometimes in the market, liquid forms of phosphorus are available. If not, purchase the material for houseplants with the highest phosphorus number you can find. With proper care and repotting, you can enjoy the delicate beauty of this plant for many years.

Q: I just read Hortiscope and am wondering about thuja green giants. I was told that these trees would not survive in western North Dakota. (Killdeer, N.D.)

A: You were told correctly. If the temperature drops below minus 25 degrees, they usually are wiped out. If you have a well-protected microclimate, that would make a difference. However, I wouldn't bet the ranch on it. Generally, with snow cover, thuja plants will hang around for a few years, but they discolor badly during the winter. Unfortunately, there seems to be a lot of confusion as to just what constitutes a green giant arborvitae. If it is sold locally, called a thuja green giant and there is evidence it has been established for many years, then go for it. I certainly would not purchase any through the mail.

Q: I have a dwarf orange tree. The tree has a green orange on it that has been that way for seven months. Why is it taking so long for it to turn color? Is it bad to leave it on the tree for so long? (e-mail reference)

A: Oranges should not be judged by rind color for ripeness. Oranges can stay green as they go from immature to mature and then to overmature. Commercial growers employ the use of ethylene gas to ripen the fruit. The gas gives the rind the orange color we all associate with ripeness. The only way to know in your case is to pick the fruit and taste it. Or, if you are in an experimental mood, pick the orange and place it with ripening bananas. If the ethylene they generate doesn't do the trick in coloring this orange, then likely nothing will!

Q: I have two maple trees, but only one leafed out last year. They both had buds. I suspect the one froze out. Will my tree leaf out this spring? I also have a big perennial garden with two arbors. Both of them are covered with honeysuckle vines that are 10 years old. Last year, both vines were covered with aphids. I didn't bother to spray them because the vines are very large and thick. I didn't think that I could get them all. Will aphids kill my vines or will they be OK? Should I cut them back all the way to the ground and let them start over? (Jamestown, N.D.)

A: The failure of a tree to leaf out in the spring could be due to a number of factors, such as root rot, verticillium wilt, borers girdling the tree’s cambial tissue or freeze damage. If no leaves emerged in 2009, then I am certain there will be none this year because the tree is dead. Mother Nature has designed trees that are adapted to our area to have more buds in reserve that would not be vulnerable to freezing out. This means there will be a late flush of leaves to get the plant through the summer. Now let’s look at your honeysuckle vine. Spraying into the canopy is not something needed any longer with plant-feeding insects. There are systemic insecticides that can be applied around the base, such as Bayer Advanced Tree and Shrub Insect Control, that will kill them as they feed. You can cut them back early this spring before new growth begins. This is a good idea because the new growth generated by the energy stored in the crown would pick up the systemic insecticide and offer protection to the plant through the growing season. It would be more effective on the new succulent growth where the aphids would be feeding.

Q: Our tulips have sprouted even though the temperatures are in the 10-degree range and we have snow. Should we cover them, dig them up or what? The ground is frozen. (Vermont)

A: They should be OK, so leave them alone. Anything you would do to them would cause more harm. Tulips are winter weather hardy. The biggest threats to the plant are field mice and rabbits, which would be attracted to any covering you place over the plants.

Q: For the second year in a row, my cucumber vines died midsummer. The problem then spread to my squash and pumpkin vines. I have been told this is caused by something in my soil. I also saw on the Internet that cucumbers and squash should not be planted next to each other for more than a year. Do you know the reason for this? If I plant my cucumbers on the opposite end of the garden, will that help fix the problem next year? (e-mail reference)

A: Crop rotation every year fools diseases and plant-destructive insects into not settling in and raising a family. Continuous cropping with the same species or family of vegetables will encourage the entrenchment of these characters, so the gardener will be fighting a losing battle. In a nutshell, the more you can plant away from or use different crops (cucumbers followed by beans, for example), the lower the incidence of disease and insect problems. Also, look for disease-resistant cultivars when making seed purchases.

To contact Ron Smith for answers to your questions, write to Ron Smith, NDSU Department of Plant Sciences, Dept. 7670, Box 6050, Fargo, ND 58108-6050 or e-mail ronald.smith@ndsu.edu.


NDSU Agriculture Communication

Source:Ron Smith, (701) 231-8161, ronald.smith@ndsu.edu
Editor:Rich Mattern, (701) 231-6136, richard.mattern@ndsu.edu
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