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Ron Smith answers questions about the world of plants, trees and gardens.

By Ron Smith, Horticulturist

NDSU Extension Service

Q: Two years ago, I planted eight quaking aspens on my property line for privacy. The trees are tall and appear to be healthy. Can I cut off the tops of the trees? The purpose is to slow the upward growth and get them to fill out a little more. What I don't want is to end up with two headers on the top. If I'm going to do it, I need to do it this year because they'll be too tall for me to get at next year. I'm thinking of late September or October when the leaves turn. (e-mail reference)

A: Go for it at the time you mentioned. I used to have aspens in my backyard. They grew as fast as Jack's beanstalk! I was out there pruning them every year to keep them in bounds, but finally gave up and had them taken out. When you do prune the trees, do not leave any stubs. In other words, don't hat-rack the tree or you will have some real problems.

Q: I have a few dozen poplars that are of various ages. They are all losing their lower branches. This started two years ago. The bark splits lengthwise at and just above the intersection with the trunk of the tree. This split is green inside, but soon opens up wider. Eventually, the bark comes off all the way around the branch. That leaves the next layer exposed, which then turns black. It does not look like cankers, but rather starts as fine splits on the bark. It has not affected the aspen trees. I was trying to cut off the sick branches on the smaller poplars to get ahead of the problem, but they are running out of branches. (e-mail reference)

A: This is a new one for me, especially when you say it is hitting all of the poplar plantings no matter how old they are. All I can suggest is to get in touch with an International Society of Arboriculture certified arborist to identify the cause of the problem and see if any corrective action can be taken. Sorry I can't offer anything better, but I just don't know what could be causing the problem.

Q: We have two large Schubert chokecherries in our backyard. We love the trees and they are providing us with a lush canopy for our patio. We have had instances of black fungus, which we cut out as we can. From your Web site, I see that we may need to spray in the spring. The problem I have with that is that one of the trees starts to shed its leaves very early. The leaves seem to be much more curled than the other tree. However, both were planted at the same time, but the one with the leaf problem was a bigger tree to start with. We have had both trees since 1990. The curly leaves are very noticeable this year and we are sweeping up bucketsful of leaves every day. It would be a shame to lose this tree, even though it is somewhat of a nuisance in some ways. (e-mail reference)

A: Continue with the sanitation right through the fall. Pick up every leaf! Next spring, spray the tree with lime-sulfur while it is dormant. Once the leaves open all the way, spray the tree with a fungicide, such as a Bordeaux mixture or Funginex. Repeat the spraying in seven to 10 days. Both are broad-spectrum fungicides that should protect the tree from whatever it is that is causing this premature leaf drop and disfigurement.

Q: Yours is the most informative site on the Web. Thank you for the excellent information. You confirmed the black knot disease on my Schubert chokecherry tree. An arborist took off the infected central branches, so it looked a bit strange for two years. The new stalks have shot up taller than the rest and its shape has been restored. However, the bottom part of the tree is too thick, so it needs to be pruned. There are many branches heading south and overlapping, so these will be pruned first. It's also branching out and interfering with the sidewalk. Do I have to wait until March to do it? (e-mail reference)

A: I remember an old forestry professor telling us that one can prune anytime the pruning shears are sharp. Of course, he was being a little facetious. What he meant was go ahead and do it if something needed pruning. He later modified his statement by saying “always prune for a reason and not just the season." In a nutshell, it sounds like you have some good reasons to do some selective pruning right now.

Q: We have a very old tree that we heavily pruned last year. It was the tree’s first pruning in decades. The tree had a bad aphid infestation this year and last. We finally got around to spraying the tree with Malathion a week ago. We then had three days of 80-degree temperatures and three of 90 degrees. Now the leaves are dropping off the tree. Could the spray have caused the problem? The aphids are gone, but so are the leaves! I do not see any insect damage to the leaves. (e-mail reference)

A: Malathion is an oil-based insecticide. If used during high-temperature periods, it can cause the defoliation you described. This is more of a defensive move by the tree rather than a dying statement. You also got rid of the aphids. The tree will leaf out normally next spring and should be free of aphids. If not or another insect begins ravaging your tree, use Bayer Advanced Tree and Shrub Insecticide to control the problem. It is poured around the base of the tree and taken in systemically. It will kill the insects as they feed. You also will not have any spray or other environmental contamination.

Q: I've had some African violets under fluorescent lamps, but they won't bloom. I let the plants’ surface dry before bottom watering. I have the lamps on for about 12 hours a day. The lamps are about 7 to 10 inches above the leaves. I fertilize with a diluted form of Peters African Violet 12-36-14 formula. When I bought the plants, they were in full bloom. They lost the blooms within two to three weeks and have not bloomed since. The leaves are OK (no brown spots or curling) and I don't see anything unusual. New leaves are growing. Is it too much fertilizer or water? Should I wait until the leaves start to droop before watering? I am using a planting mix, which drains quickly and the surface dries in a few days. (e-mail reference)

A: You are too impatient. The plants will bloom when they are ready to do so. Sometimes it takes a full year to get them to bloom again. Blooming is an energy-draining event in a plant's life. If the bloom is particularly heavy, it may take considerably more time to get them to reflower. Try a little benign neglect. For example, don't be so generous with your fertilization. Allow the plant to starve a little. Many times this will bring the plant around to getting into a faster reproduction cycle.

Q: My question is about newly planted hackberry seedlings. We have approximately 600 feet of them and another 900 feet on the way. They appear to be doing well. I was a little nervous after their first winter. I thought they had died, but it appeared they were just waiting for it to warm up. The trees look to be taking off, but they are drooping on top. It looks like they are getting too big too fast. Is any pruning necessary to help them grow upright or should I let nature run its course? (e-mail reference)

A: At this stage of development, allow nature to run its course. The tree is building its caliper, crown and root system through the carbohydrates the leaves are manufacturing during the summer. Allow the tree to do its thing for now. When they are about three years old, go in and do some corrective pruning. The trees will be more established and you will be happier with their success.

Q: We found beetles that had burrowed holes into the base of our elm tree and left a pile of sawdust behind. After searching the Web, we think it may be stag beetles (lucanus capreolus). Is there anything we need to do? Will these beetles kill the tree? Should we cut it down? Any information would be greatly appreciated. (e-mail reference)

A: Unless the elm is already dead and decaying, I doubt what you saw was a stag beetle. Stag beetles are the cleanup laborers of the insect world. They feed on decaying or dead wood. If it is stag beetles you saw milling around the sawdust, then they probably were being curious as to what is going on. If the tree is worth saving, then I would suggest getting some Bayer Advanced Tree and Shrub Insecticide and follow the directions in using it to control this borer activity. The insecticide is taken up in the vascular tissues of the plant and distributed throughout, which kills any feeding insects. An application now still would be potent all next summer.

Q: My wife and I were talking the other day about what plants bees had to visit during spring, summer and fall. We have a bunch of blooming plants in the spring and summer, but not that many in the fall. Can you suggest what type of annual we can put out that blooms in the fall? My wife has some mums, but there has to be other types of plants that bloom in the fall of the year. Also, what do bees do when it's cold other than keeping the hive warm? Do they need to get outside and fly around? Is this what an entrance guard is for? Any information on both matters will be appreciated. (e-mail reference)

A: Let me make it clear that my wife and I are beginners at beekeeping, so we are learning as we go. In the winter, bees will swarm together and generate heat in the center of the hive. They feed on some of the honey they made during the summer. It is the excess that we humans take and use for our pleasure. Some species of bees are hardier than others. Some species will survive our winters with minimal protection. Some species will die if not moved to a warmer climate. Some of the more advanced and larger beekeepers will lease their hives to Texas, Arizona and California growers of citrus and nut crops. Almonds are almost 100 percent dependent on bee activity for pollination. Without the bees, the almond growers would be out of business and we would be missing that healthful nut in our diet. As for fall-blooming flowers, it all depends on where you live. Generally, bees forage on weeds in flower and will go some distance from the hive in search of flowers. My wife and I have a wildflower planting that has a succession of bloom. Any annuals, such as morning glories and sweetpeas, will provide a source of food for the bees. Mint, borage and echinacea are common in my plantings. The bees work them every day.

Q: I just read the information on birch trees you have provided other people. You are very knowledgeable. Like you, birch trees are my favorite, so I've always wanted them. We have landscaped our property (new home) and had a three-clump whitespire birch planted in mid-June into a plant bed. The river birch was recommended, but I chose the whitespire and all its potential problems. It seems to have adapted well and has increased in height significantly. It is now early August and I'm wondering what to do for the tree on an annual basis. Should I give it a yearly application of Bayer Advanced Tree and Shrub Insect Control in September or October (lasts up to 12 months) to ward off those spring leafminer and borer insects? I've read so much on their destruction once they've attacked a clump. Do you not treat trees as a precaution before an infestation? Are there plants that you can plant at the base of a tree to ward off these insects? I've planted marigolds. Will geraniums or any other plants help? Do I feed them in the spring or fall? If so, with what fertilizer number? I had the soil tested and found out the soil is suitable for birch trees. I'm years away from pruning, but is it better to prune in the spring before the leaves appear or in the fall after the leaves drop? (e-mail reference)

A: Generally, I don't recommend preventative chemical treatment without there being a good reason to do so. The more you can keep the root system around a birch tree cool and somewhat moist, but not soggy, it will tend to be resistant to any marauding, destructive insects. What you are doing is going in the right direction. I would select a more perennial herbaceous plant that would cover the roots, but not be overtly competitive. I protected my young tree some 23 years ago with strawberry plants. The strawberry plants provided protection for four to five years before dying out. The plants were replaced with more permanent perennials. Birches need little pruning until they get some age to them. The time to prune is early summer. As you know, birch trees will bleed (excessive sap flow) if pruned before the leaves fully elongate. It doesn't hurt the tree, just shakes up the owner and looks pretty bad. At the age and stage that my cutleaf weeping birch is at, I pay to have a competent, certified arborist prune it in May or June. The birch is right next to my home, so it provides shade, screening and a slight overhang to the roof. I asked the young man who does the pruning to reduce the wind resistance somewhat so that I can minimize storm damage. He has done a commendable job every year.

Q: I am new to growing raspberry plants. In June, I planted 27 different kinds that I ordered from a nursery. All the plants were two years old. The plants are on a raised bed because we have clay dirt. My garden is also on a raised bed. The raspberry patch is on the other side of the yard, so it is about five acres away from the garden. I try to water every few days for 10 to 15 minutes per plant. All my plants seem to be growing like weeds. However, I wish I would have put the summer plants together in one place. Same thing for the fall plants, but no one told me to do that. The himbo top raspberries are getting berries on them now and so are the heritages. Someone told me that I should let them alone right now because they are bearing fruit. Any tips on what I need to do right now or this fall would be very helpful. (e-mail reference)

A: The person who told you to keep the summer and fall berries separated was correct, although this is not considered a major sin in gardening. Go ahead and harvest the crop as it is coming in. You are a lucky lady. This fall or early next spring, but before new growth begins, cut the fall-bearing canes back to the ground. The summer-bearing berries probably did not bear fruit for you this year because I'm sure you would have mentioned it if they did. Leave those alone for now, but cut them back to about breast height next spring. Cut out the skinny, puny canes and leave only the most robust ones. You can do one of two things with these canes. You can loosely tie them together with cotton string (which is the easiest alternative) or drive supporting poles into the ends of the rows and run a twine or plastic-coated wire along either side of the plants. These practices are to keep the canes somewhat upright for harvesting the berries. A heavy crop would have them bending down to the ground. Once these canes are done bearing fruit for the year, cut them back to the ground. Do that right away (ideal, but seldom followed). You also can do it this fall before the snow flies or early next spring before new growth emerges. Other than a complete course in growing raspberries, these are the essentials needed to get you off to a good start. Good luck!

Q: I read with interest your answer to the man who had trouble with Canadian thistle in his strawberry patch. Roundup is a great chemical on grass and a few other weeds, but it usually will just burn down Canadian thistle for the season. The thistle will return the next year. Any product that contains dicamba (which is in Banvil or Trimec) is better at killing Canadian thistle. A few years ago, I read in a farm magazine about using vinegar. Just put it in a spray bottle and spray it on. I have had good luck with it. Other people who I have told or also saw the article are very happy with the results. If it is hot and dry and the Canadian thistle doesn't seem to be actively growing, you may need to hit it twice about a week apart. I also have used vinegar on Russian thistle, but without any luck. There does not seem to be any lawn damage when I have used vinegar on Canadian thistle. I use the white vinegar you buy from the store. Use it straight; do not dilute. Any chance there are trials at NDSU using vinegar on any other weeds? You have an excellent column! Keep it up! (Parshall, N.D.)

A: Thanks for your comments about using vinegar for weed control. For a long time, white vinegar has had the reputation as being a herbicide, fungicide and soil acidifier. Is it possible that one product can be effective at all three roles? That is the question Jeff Gillman, University of Minnesota horticulturist, asked as he launched studies to find out what the truth is. Sticking with just the story of using vinegar as a herbicide, Gillman used concentrations of white vinegar ranging from 20 percent to 100 percent. He found that concentrations of less than 100 percent did not give satisfactory control. At 100 percent concentration, he found that the weeds that were sprayed turned brown, but resprouted shortly thereafter. This makes sense because vinegar is a contact herbicide and is not translocated. However, as you stated, he read that vinegar completely kills weeds. In addition to spraying the plants in question, he poured some around the roots of the plants. This method completely killed the upper part of the plant and the root system. This explains the successful effect of vinegar and enhances the old saying that the poison is in the dose. Roundup is a systemic herbicide. However, systemic action is most effective when the plants are in the assimilative mode in the fall and least effective when it is applied during the surge phase of growth that occurs during the spring. Again, another saying that holds true is that timing is everything. If you are satisfied with vinegar doing the job, great and keep it up! Thank you for the nice comments about the column. I try my best!

Q: We have a concord vine that produced a lot of grapes last year. This year, I cannot find a blossom or any grapes. The vines look healthy. What could be the cause of the problem? (e-mail reference)

A: Without knowing where you are from, here are some generic reasons why no fruits this year. A spring frost at the time of flowering may have killed the flower buds. Windy or rainy conditions may have kept the pollinators from being active. Another possibility is that the vine was exhausted from over producing last year.

Q: My large cottonwood is dropping leaves at this time. We have had a drought this summer. Should I water the tree or is the tree just displaying a defensive mechanism? If watering is OK, what is a good schedule? (Minneapolis, Minn.)

A: This is nature's way of saying protect yourself by getting rid of leaves earlier than normal. A tree this size and species will not suffer to any extent. The tree probably has been around longer than you have and has seen many a drought, but survived quite nicely. Save your water!

NDSU Agriculture Communication

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