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Ron Smith answers reader's questions about the world of plants and gardening.

By Ronald C. Smith, Horticulturist

NDSU Extension Service

Q: A nursery told me that I shouldn't place landscape fabric and rock around bushes. What is your opinion? Also, we have a Bergeson ash tree that has green leaves falling off and there are black spots on the leaves. Any idea what is going on? Thank you. (e-mail reference)

A: Total, absolute, 100 percent agreement with the nursery! A thesis could be written on why one shouldn't do this, but, for now, take my word for it. The ash tree problem very likely is ash anthracnose, which is common during wet, cool springs. Considerable leaf drop occurs, especially from the lower areas of the canopy. Though this causes concern when leaves litter the ground in late spring, damage to overall plant health is, generally, not severe and plants typically releaf. As leaves mature, they tend to become more resistant to infection. Fungicide applications, if warranted, should be made at bud break, with several repeat applications early in the season. When fungicide is required, use a labeled material containing thiophanate-methyl, chlorothalonil or mancozeb.

Q: I have a question about spraying dandelions in our campgrounds. What is the best time to spray and what is the best chemical to use? Is it possible for us to still spray in mid-June and, if so, what would you recommend that we use? (e-mail reference)

A: Everybody wants to spray dandelions in the spring while they easily can be spotted. Some die, but many simply shrug off the dose of herbicide and are around the following year to send out millions of seeds. Give the dandelions a spraying whenever you can in June and again after the Labor Day weekend. Those are the most effective times to get a more complete kill. There are plenty of products on the market. Weed-B-Gone is one of them. It contains a combination of broadleaf herbicide products, which makes it a fairly potent herbicide for controlling more than just dandelions.

Q: I want to plant a hedge of common or Chinese lilacs. How far apart should I plant them? There is landscape fabric and rock where I want to plant. Do I need to remove the rock and fabric? Also, this spot is next to a hay field. How far from the field should I plant so the plants won't grow into the irrigation pipe? Is there a preference between common or Chinese lilacs for a hedge? (e-mail reference)

A: For a more sophisticated-looking hedge, go for the Chinese lilac. It is a hybrid of the common and Persian lilac. It gets anywhere from 8 to 15 feet tall and as wide. You have to remove the fabric and stone before planting. As for the distance away from the irrigation pipe, my guess would be about 20 feet. Spacing is up to you, depending on how quickly you want a solid hedge.

Q: Can you tell me the difference between wave and trailing petunias? Which would you recommend for flower boxes? Thanks very much. (e-mail reference)

A: I prefer the wave petunias because they simply are overwhelming in energy and flower show. Any of the new petunias on the market these days are fantastic, so I'm sure you would be happy with either one.

Q: I found your very informative site on Google and thought maybe you could help us. Three years ago we found worms on a couple of low-growing evergreens. They were about an inch to 1 1/2 inches long. They are green, with black heads. They hung in a cluster of 10 to 40 and bobbed and weaved in unison (I assume in imitation of a blowing breeze). In just days they decimated a couple of adjacent trees/bushes. We wound up pruning the branches and drowning the worms. The worms went away for three years, but now they are back on a young pine tree. In just a few days, they ate half the needles. In a crunch, we used the only thing we had, which was Terro ant killer spray. It worked and they died on contact. We checked the original bush/tree they were on and they were back in force there, too. Any idea what they are? (Burnsville, Minn.)

A: They are sawfly larvae, which are ravenous feeders. Spraying with Sevin or whatever you have will do them in. They come and go, but you should monitor your evergreens to catch them in their initial stages before too much damage is done. There could be a second generation later in the summer, so keep a sharp eye out for them.

Q: Two years ago I planted 25 arborvitae trees. This year they are starting to turn brown. They are greener toward the outside of the tree. I used Miracle-Gro arborvitae tree stakes about two weeks ago. I was hoping to see improvement, but nothing so far. Any ideas? Everything else in my yard is doing well. The trees are getting about two hours of soaker hose watering every other day. They are shaded in the morning and have full light for most of the day. (Long Island, N.Y.)

A: You are watering too much, so back off on that. Don't expect to see any significant results from the fertilizer spikes. You may see some improvement through the summer after you allow the trees to dry down. A good soaking no more than once a week should be more than enough.

Q: I have a pair of walnut trees that are more than 17 years old. How does a walnut tree set fruit? These trees never have produced walnuts. (e-mail reference)

A: I assume you have black walnuts. You are close to the fruit-bearing stage of the trees, which is about 20 years. You might consider these fruitless years to be a blessing because every squirrel in the county will be doing the harvesting for you and then planting them wherever they wish once the trees start bearing walnuts!

Q: I've had three spider plants for about a week. After about three days, I saw that the leaves had droplets of water. Can you please tell me what this is? (Cambridgeshire, England)

A: These are droplets of guttation being exuded from the leaf pores because of root pressure. It is nothing to worry about because, in most cases, the plant will quit doing this in due time.

Q: My husband has a dwarf orange tree, but it never has bloomed. He waters it almost every day. The tree is pretty and has plenty of leaves, but he is concerned about the blooming. The tree was purchased in Florida and we have had the tree for two years. Thank you for your assistance. (e-mail reference)

A: The orange tree will bloom when it matures enough to do so. This could take up to five years. In the meantime, enjoy the fact that you have a healthy, beautiful tree to enjoy. It will surprise you someday with beautiful, fragrant flowers.

Q: I planted several wave petunias a couple of weeks ago. Now something has eaten all of the flowers. We are thinking it's rabbits. Do you have any suggestions or ideas? Do you think that it might be something else? If so, what would you recommend? Thanks. (e-mail reference)

A: Spray what is left of the plants with Liquid Fence or Plantskydd to repel whatever thinks of your planting as a free meal. Rabbits are good suspects because they seem to be everywhere in the suburbs these days.

Q: A few years ago, I noticed some very lush, green grass on my front yard with mushrooms growing in it. Now those spots have died and seem to be expanding. What can I do about it? (e-mail reference)

A: I would rake those spots to rough up the soil surface, overseed and cover lightly with mulch.

Q: My wife and I planted a lilac on Mother's Day four year ago. There were plenty of flowers the first year, but less and less each year since. There was only one flower this year. I was told to add lime to the dirt around the plant and add 10-10-10 fertilizer. I did this, but I still do not have any new flowers. I have two questions. Some of the leaves on the plant seem to be getting small holes in them. How do I prevent this from spreading? Also, how do I get it to flower again? (e-mail reference)

A: The lack of flowering could be from overfertilization, improper pruning or too much shade. Flower buds for the next season are formed during the previous summer. In other words, the flower buds will form or not for 2008 sometime this summer, depending on whatever pruning is done or anything else that might prevent bud formation. As to the holes in the leaves, you can spray with Malathion to prevent any future eating, but I doubt that the action is worth it. Usually, when the damage is observed, the insect pest already is gone.

Q: We moved into a house about a year ago that had been landscaped about a year before that. In the front of the house, about a foot away, is a blue spruce. The spruce seems to be growing nicely. Do I need to be concerned about the roots of this tree affecting the foundation of my house? Do I need to be concerned that its proximity to the house may affect its ability to grow? Thank you for your advice. (e-mail reference)

A: The roots won't be a problem, but the distance from the house will. These trees have the potential to grow to 60 feet or more with a spread of 15 to 20 feet.

Q: I have four, beautiful, endless summer hydrangea bushes in front of my house. They're new plants and are full of blooms. I'd like to cut some blooms for fresh flowers in my home, but I don't want to damage the plant. Is it OK to cut stems off for a bouquet? How far down should I cut them? Also, I've heard that the endless summer variety will bloom all summer, so I am curious how to best facilitate this continuous blooming. Will cutting blossoms help? Thanks for your advice! (Omaha, Neb.)

A: Yes to both of your questions. Keep the plant hydrated with regular watering, but don't keep the soil soggy. Fertilizing the plant once a month with something, such as Miracle-Gro, also will help. You can cut back far enough to make a nice bouquet as long as you leave some foliage behind on the stem.

Q: Why do my peach trees have blossoms, but don't produce fruit? Is it a pollination thing? (e-mail reference)

A: It could be the lack of bee activity, windy weather when the pollen is ripe, rain or cold weather. Take your pick!

Q: I've had a corn plant for about two weeks. I've noticed that the leaves are getting brown spots and the tips of some of the leaves also are a bit brown. There's some yellowing of the leaves as well. I have a north-facing window that receives low light. The plant is in a 10-inch diameter pot. I know these plants can be overwatered easily. When I repotted, I noticed the soil was almost muddy around the roots. I removed some of it and added fresh, dry soil to the new pot and gave it a mild watering. What can I do so my corn plant doesn't die? Any suggestions or advice would be greatly appreciated. Thanks! (e-mail reference)

A: All I can do is give you some guidelines to what you might be doing wrong. If you correct them, perhaps the plant will recover. The container should be free-draining, not sealed on the bottom. The water should be room temperature before giving it to the plant. The soil should be allowed to dry down somewhat before being watered again. Usually, these are tough plants. We've had one in our house for more than 10 years. It is potbound, attacked occasionally by our cats and watered or fertilized when the spirit hits us. We also summer the plant outdoors where it is subject to the elements, such as wind, rain and fluctuating temperatures. It sounds like the pot size is about right for the plant's size, so all I can guess is that it has been overwatered.

Q: The cotoneaster on the north end of my garden is overrun with grass, while the one at the other end has hardly any. Both are along the side of our lawn. Is there any safe way to get the grass out from under the one besieged with grass? I've tried hand pulling, but it's too overwhelming. Thanks! (e-mail reference)

A: Look for a grass killer that has sethoxydim as the active ingredient to selectively take out the grass without hurting the cotoneaster.

Q: I am hoping you could share some of your knowledge about Manchurian elms. This spring I noticed that many of the elms had a heavy seed set with minimal leafing out. Is this a characteristic of the elms? Is this a sign that the tree is dealing with other issues? We have many Manchurian elm plantations in our park system that were established in the early '60s. They are high maintenance because of all the pruning and spraying to control cankerworms. What is the expected longevity of Manchurian elms under irrigated and nonirrigated conditions? We spray with Btk (Dipel and Foray 48B) as a control for cankerworm. Considering the high public use in these areas, is this the most effective, efficient and safe control measure for cankerworms? Thank you for your time and anticipated response. (e-mail reference)

A: Elms can live for more than 100 years in their natural environment. Generally, cut that length of time in half or more if the trees are in a landscaped environment, depending on the conditions. Manchurian elms have no log of longevity that I can find in any of my references, but the fact that the trees are showing heavy seed set and little vegetative growth is a very ominous sign of the end being near. If you can, you might try to collect a specimen branch showing exactly what you described to me and send it to the land-grant university diagnostic lab in your state for analysis. The materials you are using are about the best and safest on the market. You might check with the people taking care of the turfgrass areas in the park to find out what is being used for weed control. If the product contains Trimec, that could be a contributing cause for the decline. If they never have core aerated the turfgrass, it might be good to have that done because the trees would benefit as well.


NDSU Agriculture Communication

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