Q: I have a huge, old eucalyptus in my backyard. It is at least 80 years old. Could you give me some tips on trimming it? It has been trimmed by the previous owner but needs to be trimmed now. I also wonder if it would be better to have it really lowered, possibly to just an interesting trunk, more of a tree sculpture than the actual tree. I do have liability concerns. Jane

A: In a situation like this where you have a very large and relatively old eucalyptus tree in your yard, the liability concerns are realistic. Anytime you have a scenario where a large and somewhat geriatric tree is close to or growing over a home or other area of constant human activity, the concern for physical safety and monetary liability increases along with the size, age and location of the tree.

If you want to control the overall size and shape of a tree indefinitely, it is imperative to trim the tree at least once a year. This technique of regimented and timely trimming is to some extent how bonsai trees are created and maintained. Practitioners of that art will continually trim off the end buds of a tree that would normally mature to more than 60 feet tall. Using this technique, and with regular attention, they can train and maintain a mature tree at only 2 feet in height for its entire life.

I have two alder trees in my front yard that I want to maintain no taller than 45 feet, so every year I have my crew prune them to keep them that way. Using this technique, all of the pruning cuts are small and the trees always look good. At the same time, the size of the trees is within my control.

When a tree gets old and large, however, and you want to make it into a smaller tree, it can become a real problem.

Dramatically reducing the size of a big tree involves making large and radical cuts on both the top and the side scaffold branches. This procedure is called topping, and it rarely results in a good-looking or healthy tree.

When tree branches grow in a normal way they are continually engulfed by the ever-expanding diameter of the tree trunk. It is by this method that the branches become strong and well-attached to the main stem of the tree.

After large cuts are made on some mature trees, however— eucalyptus included—new shoots generally appear in clusters just below the large cuts. These new shoots, called watersprouts, are very weakly connected because they are attached directly to the outer portion of the large remaining stubs of wood.

Even as the watersprouts increase in size and the expanding bark begins to envelope them, they are never attached as strongly as a natural branch is. As the watersprouts get older and larger they become a liability problem because, unless they are constantly reduced in size and kept under control, they can easily break off.

This is just one reason why it is a bad idea to have a large tree drastically trimmed in order to lower it. Another is the aesthetics; the tree will never look normal again.

If the tree is just too big for your yard and you have genuine safety concerns for not leaving it tall, you can radically reduce it in size if you want to, but you probably won’t be happy with the results.

If you just want to be left with a trunk because it would be something interesting to look at, I suggest removing the tree and building a gazebo where the tree used to be.

It will end up being a safer structure and a better looking one, too.

David D. Mortimer is a cer tified arborist with more than 30 years’ experience in the tree care industry. E-mail questions to dmortimer@theacorn.com