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Monday - September 20, 2010

From: Portland, OR
Region: Northwest
Topic: Trees
Title: Trees for traffic buffer in Portland OR
Answered by: Barbara Medford


Hi, saw the question about small space plants. On this topic, our street in Portland OR is looking for a fast growing, 20-30 ft tree that can go in a 12" wide parking strip along our road (we have major traffic problems on our street and are trying to slow people). I was thinking of the velvet mesquite because it can take tough conditions and has a taproot. Will it do ok in our zone?


Okay, we're a little confused. Did you mean a twelve FOOT space, or just 12" as your question says? No 20-30 foot tall tree is going to be able to survive in a 12-inch wide "planter." I understand why, if it is really that narrow, you are asking for a tree with a taproot. Many trees have taproots, usually when they are small, so they can get down and get some moisture to grow with. However, they all develop lateral roots as they grow, both to extend out for more nutrients and to balance the tree. Generally speaking, a tree's roots will extend 2 to 3 times larger than the "drip line" or outer edges of the tree above ground. Twelve inches wouldn't even be big enough to dig a hole for the root ball to plant such a tree, and if a taproot is all it has when you plant it and the taproot is broken in transit, that tree will almost surely die anyway. And certainly the lateral roots cannot survive under the impermeable hard surface of the roadway.

So, let's talk a little bit about Prosopis velutina (Velvet mesquite). For openers, this is a desert tree, which grows mostly in Arizona and down into Mexico, although there are small populations, some of them introduced by humans, in New Mexico, Texas and California. The USDA Hardiness Zones for this tree are 8b to 10. Portland is in Zone 8b so, in that respect, it would be okay in Portland. There the okayness ends. You note we said it is a desert tree, and one thing Portland is not is desert. See this University of Arizona webpage on Prosopis velutina. From our Native Plant Database page on this plant:


Native Distribution: Extreme SW. New Mexico west to central Arizona and NW. Mexico; at 500-5500 (152-1676 m).
Native Habitat: Along washes and valleys and on slopes and mesas in desert, desert grassland, and occasionally with oaks."

So, unless we misunderstood your question, I think we have exhausted that subject. We also are a little confused about your calling this a "parking strip." If you meant it was twelve feet wide, supposedly some cars could park on it, but that, in turn, would make unfeasible planting trees in the same space, not just because the cars need room, but because cars on the space would compact the soil and, again, make it inhospitable to tree roots.

Since we don't seem to be doing you much good, let us refer you to our Recommended Species section. Click on Oregon on the map, and you will get a list of plants native to Oregon that are considered beneficial and are usually commercially available. Go to the right-hand side of the page and make some selections. If you have the space for and still want trees, click on that under General Appearance. You could also try shrubs or even grasses. Farther down on that sidebar, you can select which Light Requirement your space (sun, part shade or shade) has, soil moisture and so forth.  Narrow Your Search, and you will get a list of whichever type of plant you are considering that fits your specifications. 

Beyond that, we would suggest you contact your city's Street Department, for possible help or suggestions on your problem. You might also contact the Oregon State University Extension Service for plant suggestions appropriate to your area.

From our Native Plant Image Gallery:

Prosopis velutina



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