Sometimes, while gardening, you find yourself thinking about lots of things, or more correctly, the mind wanders and wonders.

Thinking always leads me to the biggest garden mistakes I've ever made. There are small mishaps. There are small boo-boos. There are bigger boo-boos. And then there are the big mistakes.

We all make them. That's because while gardeners want to say, "Hey, we're good landscapers," we're not. We are like kids in a candy store. We want it all. Developing a landscape plan is a nice blueprint, but it's so easy to modify. And truthfully, the designer puts in what he wants at that time. What about all the plants we discover down the road?

But what separates true green thumbers from others is that we are willing to admit our errors, and also admit we will probably make many more. That's the fun of gardening. As a group, we share our mistakes, not with a measure of humility, but with pride. It's almost a can-you-top-this approach.

So, here are some things you should not do, based on experience.

Do NOT plant hostas under a birch tree. Birch trees are shallow rooted, and love water, especially river birch, which should have been a clue in itself. The birch roots are bigger and woodier than hostas. Hostas will suffer. You could throw in silver maples and sweet gums. Some people have been successful, but more have not.

Ivy would have been a better bet. Or vinca. Or pachysandra.

Do NOT plant trees that grow more than 3 feet in one year, even if they are river birches and essentially free. Lots of growth means good shade. It also means limbs that break and snap in wind, ice and snowstorms, if you happen to be in an area that gets those. In other words, you might be okay in the deserts of Arizona.

Do NOT accept plants from strangers.

Do NOT accept plants from friends. This is especially true if they say "Would you like some of this plant that is doing well in my yard?" What they really mean is, "Would you like some of this plant that is taking over my flower bed because it's not quite a weed, but it's pretty darn close."

Of course, if friendship is important, you can always accept the plant and then give it to someone else quickly. It is not lying to say, "Oh, the plant just didn't do well in my garden." Avoid any offered replacements.

Purple and orange don't really go together. It looks good on some collegiate uniforms, but not that great in the garden. Keep them separated by a building or two city blocks.

Plants grow. They grow tall and they grow wide, usually the former first. Blue spruce planted at the corner of the house, or at the edge of the sidewalk seem to grow overnight into huge monsters. Okay, it doesn't happen over night.

Which leads me to the best advice I can give: Find out how big the plant will grow when it's mature. We can all envision a cat or baby getting big. Our minds seem to go blank when confronted with a plant. Don't fall into that trap.

Do NOT assume your yard expands at the same rate as the universe, or your waistline. It does not. It probably never will in your lifetime unless you start taking over your neighbor's property, or the surrounding farm field. Plants like to be cramped and crowded into a tiny space as much as you do. We all need our personal space.

Consider what plants you put under your windows, especially if you keep them open. Some plants really do stink when they flower. The spring Fritillaria and Korean Spice Viburnum are two examples.

Other plants, when they start to rot, smell. And if that's not the case, plants attract all sorts of insects and other creatures that are small enough to get through the screen's mesh.

Finally, bend at the knees and not at the waist. A lifting belt is a must for moving rocks and lumber. That is, unless you have a chiropractor or masseuse in the family.

David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension, P.O. Box 8199, Springfield, IL 62791. Telephone: (217) 782-6515. E-mail: drobson@uiuc.edu

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