Answer: Ivy (and all trailing vines) prefer to grow up and out rather than down, but it may cascade once it gets thick and heavy. If you can, plant it at the base of your rock fence so it can climb up and along rather than down and along. I think you'll be happier with the results. I know that many people encourage ivy to climb the trunks of trees, and it can look quite nice, but the tree's health will eventually suffer. Ivy gets heavy when it's wet and if there's a windstorm, limbs and branches can be broken. Ivy also tends to hold moisture against the bark of a tree; too much moisture can cause decay. It's also a great hiding place for rodents. I'd keep the ivy trimmed so it doesn't climb up the catalpa tree.
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