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

By Ron Smith, Horticulturist

NDSU Extension Service

Q: It is mid-April here in Michigan. When should I fertilize the asparagus and rhubarb? Is it too early to transplant rhubarb? When is the ideal time to transplant rhubarb? Should I mulch the asparagus and rhubarb during this time of year? What kind of mulch would you recommend? Are pine needles good as a mulch? (e-mail reference)

A: In your area, asparagus should get a light dose of fertilizer after the last cutting. As for rhubarb, do the same after the harvest. It is best to transplant rhubarb before new growth starts. However, many gardeners have transplanted rhubarb successfully after new growth begins. Mulching is not needed or recommended for either crop. As for pine needles, they make excellent landscape mulch, especially where the soil pH needs to be maintained on the acidic side

Q: I have a hibiscus tree in a pot in my office. During the last few months, I have noticed a few long mushrooms growing out of the soil. The last one I pulled out had very small bugs crawling over it. They are so small I was not able to see them without my reading glasses. There does not seem to be a problem with the tree. It seems to be thriving because many new leaves are growing. I am concerned with the bugs and that they may infest other plants. (Langdon, N.D.)

A: These bugs could be fungus gnats or something else that is not a threat to the plant. They feed on the organic matter in the soil, but not the living plant tissue. If these insects bother you too much, then I would suggest repotting the plant using a fresh, basic potting soil mix in which the media is artificial or has been pasteurized. Generally, placing the plant outdoors during the summer months will do away with such trivial pests, but not be an invitation for something more aggressive to move in. That is why the standard recommendation is to repot after summering a plant outdoors.

Q: I have big raspberry patch. For the first eight years, it produced great, but it has gone downhill the last few years. Many of the canes broke off and died. There will be a weak spot in the cane, so above that spot, the plant withers and dies. Also, I've noticed at the base of the canes there is white foam oozing from the stalk. Is this an insect? Are the breaking canes related to the oozing foam? Do you have any suggestions? Also, do you have any other suggestions on how to rejuvenate my old patch? (e-mail reference)

A: It sounds like you have a cane borer problem that will require diligence on your part to control. Raspberry cane borer is one of the most common insects that infest raspberries. The adult form is a half-inch long, narrow-bodied, black beetle with long antennae and an orange band just below the head. The beetle chews two rings about one-half inch apart and 3 to 6 inches below the tip of a young cane and then inserts an egg in between them. The egg hatches and the larva or grub feeds inside the cane. The girdling rings made by the adult causes the cane tip to wilt, blacken and then fall off. Cut off all the wilted tips just below the lowest girdle mark as soon as you notice them. Destroy any wild branches in the area because they act as hosts for this pest. These two practices should bring the insects under control. An insecticide spray just prior to bloom may offer some control of the adults.

Q: Two years ago, I planted a tree line of 10 red maples in my backyard. They are spaced 15 feet apart. My intention is to eventually have the trees growing together to create a barrier. I want to plant evergreen trees behind the maples evenly spaced at 15 feet apart to create an all-year barrier. After it’s complete, there will be the 10 maples and nine evergreens staggered to create an evergreen background with the red maples in the foreground. Would a Green Giant arborvitae work in my situation? (e-mail reference)

A: Green Giant arborvitae would work as a barrier, but keep in mind the mature spread of these trees. The arborvitae will develop a spread of 15-plus feet and the red maples 20 to 25 feet of spread. This will have the branches competing for light, so there will be some die-off that takes place. If this is something you can tolerate, go for it. If not, then spread out the spacing.

Q: I stored a bunch of dahlia tubers last autumn in brown paper bags. Yesterday evening, I checked on the tubers because I want to start planting. Most of the tubers are rock hard. Are they dead or can they be salvaged? (e-mail reference)

A: Rock hard is not good, but hard is better than rotten and mushy. Soak them in water to see what happens. If it results in buds swelling at the "eyes," then you are home free. If not, then they are ready to be dumped.

Q: I live in Virginia. I have a large hedge of peonies that have not started to bloom. The forecast for tonight is freezing temperatures. Should I find a way to cover them? (e-mail reference)

A: They will be OK, but if it will make you feel better, go ahead and cover them for the night. Peonies are one of the most cold-hardy herbaceous perennials in North America.

Q: My wife and I are wondering what ratio of N-P-K will acidify our soil. I'm thinking a high nitrogen content, such as 12-12-24, will do it. Am I correct in assuming this? I have looked on the Web, but have found nothing to help answer our problem. (e-mail reference)

A: Working generous amounts of sphagnum peat moss into the soil will help in lowering the pH level. Acidifying fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea and ammonium nitrate. Read the label on the fertilizer bag to determine if it is an acidifying fertilizer. Also, any fertilizer that is labeled for fertilizing azaleas or blueberries will work. It is not so much the NPK ratio as it is the source of these nutrients, such as nitrogen in ammonium form.

Q: I would like to know if I could spray dormant oil on my fruit trees at this time of year. I have a peach tree that is blooming, but last year we had worms inside the fruit, so most of the fruit fell from the tree. My apple tree did the same thing. It bloomed nicely, but dropped all of its fruit. I sprayed with lime sulfur in late February. I don't know if I have to apply the dormant oil while the trees bloom or do it after blooming. (e-mail reference)

A: It would not be safe to apply dormant oil on the tree while it is blooming. Wait until the petals drop and then spray with a product that contains acetamiprid. It is found in Ortho MAX Flower, Fruit and Vegetable insect killer. Follow the instructions on the label.

Q: We are planning to do some landscaping this spring, but we want to keep several birch trees. However, when we slope the land to run away from the house, the base of the trees will be covered with about 6 inches of topsoil. I was told this would kill the tree. Is this true? Is there anything we can do to save these trees? (e-mail reference)

A: Build a retaining wall around the trees that is under the spread or drip line. This is where the feeder roots are that need a balance of air/water to survive. Six inches may not seem like much and the trees might survive, depending on the species, but why take a chance? The trees would go into a gradual decline in quality and vigor during the following three to five years before dying. Any good landscape contractor could assist you with the retaining wall.

Q: The brochure I have says to prune apple trees in February, but I haven't been able to do that because of all the snowbanks in my yard. Is it too late to prune it now? (Ashley, N.D.)

A: Prune away! Pruning in February is for idealists, those farther south and those who have cabin fever. Early pruning usually is carried out in apple orchards so they have the time to get it done before new growth begins in the spring.

Q: I have two brothers who live across the river in Mandan. Both complain of squirrels eating the bark off their trees. One brother has Siberian elms and the other has some kind of linden that the squirrels attack. I've never had this problem or heard of anybody in Bismarck who has the problem. What can the brothers do to prevent this problem? (Bismarck, N.D.)

A: I suggest using a Have-A-Heart live trap. Place some nuts or peanut butter on a piece of bread in the trap. The squirrels should go right after the bait. Once trapped, it is your choice as to what you want to do with them. You can move them to a wilder area of the region where their damage will not be so intense or come up with a different solution. If you move them somewhere else, make it at least seven miles and to an area where they will not be a nuisance to anyone else. Just be careful that you do not get bitten or scratched. Once caged, they are not going to be happy campers, so be careful where you place your hands and wear heavy gloves.

Q: If I cut willows back to the ground and grind down the stump, will it keep the suckers at bay or spread them farther out? (e-mail reference)

A: Grinding out the stump takes care of the energy that would have gone into the tree's new growth for the season. Sucker growth from the root system can be expected. Suckers will grow even if the stump is removed.

Q: I’ve had a violet plant for almost a year, but it never has bloomed. I know it's not blooming due to a lack of light because we have overhead lights in my office. Other people right beside me have violets that bloom. Could there be another reason my violet is not blooming? (e-mail reference)

A: Violets will bloom when they are good and ready to do so. It could be you have been too nice to it, it is in a pot that is too large or it hasn't become pot-bound. Try a little benign neglect to see if that will bring it around. Let it dry down completely before watering, don't fertilize and have your overhead lights changed. If they are more than a year old, they might not be putting out enough energy for the plant to make carbohydrate energy to flower.

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

NDSU Agriculture Communication

Source:Ron Smith, (701) 231-8161,
Editor:Rich Mattern, (701) 231-6136,
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