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Hortiscope: Don't use nails if plants have an iron deficiency

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.

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?

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?

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.

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.

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