August 7, 2003

Hortiscope

Ronald C. Smith, Horticulturist
NDSU Extension Service

Q: One of our cottonwood trees has a scattering of leaves that are turning yellow and falling off. The trunk is developing long cracks, especially in the bark but some are deeper. There doesn't seem to be anything coming from the cracks. I can’t find any insects or eggs on the leaves. Any ideas? (E-mail reference)

A: Have an International Society of Arboriculture certified arborist come out and inspect the tree for soundness. You didn't indicate the age of the tree but I gather it’s up there in years. Cottonwoods are notorious for having problems as they age. If the problem is caught early, it can usually be corrected with minimal expense.

Q: I purchased a double apricot hibiscus about five years ago. Much to my surprise, it bloomed this past winter where I work. Each year I bring it home and put it outside for the summer. With the exception of having repotted it this year, I haven’t changed anything else. Can you explain what happened? (Oakes, N.D.)

A: The only thing I can think of is that this was a grafted plant. Perhaps the grafted scion wood was inadvertently cut out or died, leaving the rootstock to produce the single flower.

Q: My rhubarb plants got in the way of weed and mosquito spraying this spring. I pulled all the stalks and the new ones are coming out nicely. Will it be safe to use the new growth for freezing and canning this fall? (Litchville, N.D.)

A: I wouldn't. Give it a season to get over the trauma and do your harvesting next spring. I am sure you must have some neighbors or a local farmer's market that can provide you with enough stalks for your intended purposes.

Q: I recently moved to a house that has a 20-year-old cotoneaster hedge. I believe the previous owner sporadically trimmed it to the ground. Is August to late to trim? It is more than five feet tall in some places. How do I get rid of the volunteer trees growing in it? (Minot, N.D.)

A: It is too late for a major cutting. You should do it early in the spring before the shrubs leaf out. As for the volunteer trees, cut them back at the base and carefully paint stump remover on them with a paintbrush.

Q: Do you have any suggestions for killing earwigs without chemicals? We are trying to keep an organic garden. I have heard soapy water might work. What concentration of soap to water should I use? (E-mail reference)

A: Purchase some insecticidal soap. It is safe for organic gardening. Simply follow the directions on the container.

Q: Our strawberry plants look healthy, had lots of blooms, but provided very little fruit. Could they be planted too thick? What kind of soil do strawberries need? (E-mail reference, Amour, N.D.)

A: Poor fruit set could be the result of lygus bugs feeding on the blooms, late frost damaging the blooms or too much rain and cool weather during the critical pollinating period. Strawberries do best in well drained soil. Use a sandy loam enriched with peat moss or well rotted compost.

Q: My new hydrangea bushes have black spots on the leaves. One source I read recommended overhead irrigation, another says no. One says apply nitrogen to soil. Can I remove all the effected leaves without killing the bush? Fungicides make me nervous and I have a lot of pets. (E-mail reference)

A: Do not do overhead watering. Contact the local garden center and see if you can find some Schultz products. They have a product called Fungicide 3 that is made with neem oil and approved for organic use.

Q: Can you tell me if the pods that form on the top of my dieffenbachia are harmful to the plant? Should they be cut off? Someone told me the pods are there because the plant is happy in its location. Someone else told me they should be cut off because they take all the plant’s energy. I have three large pods on the top of my plant. (E-mail reference)

A: They are not harmful but some people feel the pods disfigure the plant and remove them. The pods are the end product of sexual reproduction. The plant would probably do better without them unless you want to collect seed.

Q: Can the mineral content of the soil be adjusted to get a different color hydrangea? (E-mail reference)

A: Yes, but only on a particular species of hydrangea, the bigleaf (Hydrangea macrophylla). Aluminum sulfate will make the soil acid and give the flower a blue hue. Adding limestone will make the soil pH more alkaline and turn the flower pink. Have fun trying but don't have high expectations for good coloration.

Q: My neighbor has been replanting a portion of his spruce trees for six years. The trees are planted in a row next to Chinese elm stumps. One side of the stumps he has pine trees that are doing great. The other side is in spruce. One end of the row has died five years in a row and this year looks like it might make six. The year the elms were taken out he sprayed with Banvel, then burned down with Round up. Every year the trees look great until about the last week of July and then within a week they are all dead. One year he attributed it to grasshoppers, one year to drought and so on. This year he has drip irrigation on the trees, has sprayed for hoppers and fungus and is watering but since Saturday the trees have taken a turn for the worse. Could there be any residual in the soil from the Banvel he used six years ago? (Amidon, N.D.)

A: You were smart to ask about chemical residue. It’s the only thing that would cause that sudden a decline in the trees. I would suggest that he get the soil tested for salts. He should do a little bioassay using tomato plants and soil from around the dead trees. They are good indicators of any herbicides present in the soil that could affect dicot plants.

Q: I have two large peace lily plants. They get the same type of water, Schultz fertilizer once a month and are near windows with bright to indirect light. One is doing just fine but the other has brown spots and holes in the center of the leaves. The leaves droop if I under-water but perk up when I water, so I doubt over-watering is the problem. It has flowered and there are new shoots so, other than the strange brown holes, it is doing okay. It appears to be bug free. (E-mail reference)

A: It must be some kind of fungus that has infected the plant. Purchase some Schultz's Fungicide 3 from a local garden center. This is a natural product that controls fungus diseases and some insects.

Q: How do you know when cucumbers are ready to be picked? What size do they need to be? (E-mail reference)

A: It depends on the variety of cucumber. That information is on the seed packet. When in doubt, pick early or smaller, rather than larger. The common slicers get about six to eight inches long.

Q: I have several large, older lilac bushes. They are starting to grow out of control. Can I cut off the outside new growth or do I have to cut the bushes off at the ground? Is it better to cut the old cracked branches off at the ground to aid in new growth? (E-mail reference)

A: Get some good pruning loppers and cut the oldest canes back to the ground. Trim back no more than a third on the rest of the growth, if necessary.

Q: Last summer the city seeded grass on my newly worked boulevard. It came up very well and looks good but I have a lot of white clover, hop clover and pigeon grass. I have been able to control the clover by spot spraying with Weed Be Gone. Is there anything that can be used to spot spray the pigeon grass? (Fargo, N.D.)

A: Don’t worry about the pigeon grass. Your lawn grass will eventually crowd it out. Apply a pre-emergence next spring and fertilize around Memorial Day and you should be free of any pigeon grass or any other annual grasses.

Do you have a gardening or houseplant question? Write to Hortiscope, Box 5051, NDSU Extension Service, Fargo, ND 58105 or e-mail to Ron Smith at ronsmith@ndsuext.nodak.edu. Note to e-mail correspondents: please identify your location (city and state) for most accurate recommendations.

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