Homegrown: Catalpa bunga tree, irrigation silt

I bought a catalpa bungei tree about five years ago and would like to try to control its size. I was told I could trim back all the branches in the spring about one-third of the way.

Does this sound right to you?

Also, I was recently in Alaska and saw many mountain ash trees. I was told that these will grow very well in Colorado. Do you think this is a good tree to plant around the Western Slope? — Joan

Your question causes me to question whether you have a bungei catalpa or not. Bungei catalpa (or dwarf umbrella catalpa) is a slow growing, very small and dense, umbrella-shaped tree that only gets to 20 feet in height or so; and it will probably take 10–30 years to get to that height.

How fast is the tree growing? If it’s more than 6 or 12 inches a year, I’d worry that what you have is a western catalpa instead.

Western catalpa will typically grow 12–24 inches a year, getting to 40 feet or even 50 feet tall in age. It forms an upright, somewhat open growth habit.

Does the tree bloom? Bungei usually doesn’t flower, but western catalpa sets showy ruffled white trumpet-shaped blooms with purple and yellow spots in May that are followed by long pods in fall.

If indeed what you have is a western catalpa, you probably won’t be able to keep it small in the long term. The tree really needs to grow.

If you cut it back each year, the tree will eventually start to die back and die out completely.

While you can’t limit the growth of the tree, you can direct the growth of the tree. If you don’t want it to get too tall, you can do that with proper pruning, but you’ll have to allow the tree to compensate by growing much wider than it normally would.

There are a number of mountain ash in the valley. In fact, I have one in a corner of my backyard.

They are beautiful ornamental trees that grow to 25–30 feet tall with an upright, oval growth habit. They have dark green foliage that turns an attractive apricot to rusty-orange color in fall (it was especially good this year).

Mountain ash set pretty small white flowers in clusters in the spring, but their real show is the orange to red berries that color in late summer and last into early winter.

The robins love eating every berry on my tree. 

Having said that, mountain ash isn’t naturally adapted to this area. They thrive in cool, moist summers and tolerate severe cold in the winter.

You can grow them successfully here, just do a good job preparing the soil before planting. Mix a good amount of decomposed organic matter such as Soil Pep or peat moss into the soil. They like a rich, organic, well-drained soil.

I have mine in a large shrub bed, surrounded by other plants, which helps raise the humidity around the tree.

Don’t water them too much. Though they like regular watering, mountain ash won’t tolerate “wet feet.” Give the soil a chance to dry slightly before watering again.

I have irrigation water, which this last year provided me with an enormous amount of silt that was rich with BIG worms throughout.

I’m wondering for next year: Is this dirt safe to mix up with my vegetable garden? — Tammy

Actually, that “silt” isn’t bad soil to begin with. The texture is usually pretty good and there’s a good amount of organic matter in it.

And having the earthworms is really a bonus. They really help to aerate and enrich the soil.

Use it to your heart’s content.

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Dennis Hill is the nursery manager at Bookcliff Gardens, http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or e-mail .(JavaScript must be enabled to view this email address).

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