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Hortiscope: Horses should be kept fenced away from trees

Q: A few days ago, I finished reading the woody plants section in the master gardener handbook. The recommendation was (after digging the hole for a tree or shrub) to backfill the hole with native, not amended, soil. Why is native soil preferred compared with amended soil?

A: Native soil is better because it is the permanent soil the tree or shrub will be in. Amended soil alters the physical characteristics of everything around it, most notably the movement and retention of water and the air balance in the root zone.

Native soil is the undisturbed soil of the local ecosystem. Any soil in the constructed landscape is altered somewhat, even though it is referred to as being native. Altered (designer soil) would be acceptable if done extensively enough.

When the alteration is confined to the planting hole, it has the potential to be ineffective. Alterations can be in the form of extra sand and organic matter mixed for uniformity. This stratifies the soil profile, which causes problems, such as a very porous, well-drained soil in heavy, hard or compacted clay. This creates a perched water table in the planting hole.

The homeowner typically is told to water his or her new planting two to four times a week, which in many cases results in anaerobic conditions in the root zone. This stresses the plant and kicks pathogens into becoming active.

The plants either die or look stressed, so the homeowner adds even more water. This is something we all learned the hard way by having to replace a lot of plants. If the plant is going to make it, using native soil will do the job. If it isn't, using amended soil will not save it. If the soil is really bad, then modify the entire planting site down to a depth and width to allow the roots to grow and mature enough to support the tree without holding water in the immediate root area.

Q: I have a quick question for you about tree spacing. We would like to use Amur maples as a hedge in front of our house. I understand they can get tall, but can be trimmed back and kept to the size of a bush or tall hedge. I have seen some around town that are used as hedges and have a nice fall color.

I read that the recommended spacing is 6 to 10 feet between trees. I would assume that is because you are planting it in a tree row and intend to grow it to full size. If I am planting the Amur maples as a hedge and want them close enough together so they fill in to make a dense hedge with no gaps, how far apart should I plant them? Thanks for your help.

A: If you plant Amur maples 6 feet apart, they will make a nice dense hedge. However, you will have to keep them pruned to do so. If you want to accomplish your objective faster, then spacing the trees every 4 feet would be better. When purchasing the Amur maple, go for the hedge or multi-stemmed form. There are other plants that also would make an attractive hedge with good fall color, but are not as much work, such as the American cranberry bush viburnum.

Q: I've just been looking at your Web site and am wondering if you could help me. We have a rental property that has a number of weeping willows along the creek bank. The trees look stunning. However, we've just realized that all of the lower leaves and branches have been eaten by the horses our tenants have.

I am devastated by this. Can you tell me if the leaves and branches will grow back to their former glory or are the trees permanently damaged? Any help you can give me would be greatly appreciated.

A: Horses do a good job of chewing things up, as many folks know! Whether they will return to their original glory depends on just how much damage the animals have done. Generally, willows are vigorous growers. Unless they receive a direct hit from an incoming missile, willows will survive and come back in pretty good shape.

Horses can do other damage as well, such as compacting the soil under the canopy and scraping off the bark on the trunk. I would suggest temporary fencing to keep the horses from getting to the trees.

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