Aspen trees are all the rage in Colorado’s high country now, as leaves change to autumnal yellow.
But aspen don’t always fare well in city environments such as Denver, where hot temperatures and heavy clay soils prove detrimental to the tree’s well being. Plus, aspen can’t filter air pollution effectively. Injury by hail, pruning, and other elements render aspen susceptible to fungus; and other diseases readily set in.
Around metro Denver, Quaking Aspen is the most typical species. Big Tooth Aspen can be found, but is rarely commercially available. Aspen don’t make ideal street trees, but can make nice accent trees in yards.
For those who want urban aspen, note the following considerations:
* First, make sure you’re selecting and planting a tree with a well developed root system. A lot of aspen are collected from the mountains, and sometimes the trees don’t have much of a root system, which means they don’t have much of a chance of surivival.
Ask questions about where the aspen came from and when. If the trees are in containers, look at the bottom of the container to see if you can see roots. Sometimes, the aspen are collected when they’re much younger, but if they’ve been growing in the nursery for a year or two, they would have a more developed root system.
* Another key factor in successfully growing aspen is finding the correct site. Don’t plant aspen on the south side of a house or anywhere the tree will get a lot of reflective heat from the street, sidewalk or buildings. Aspens are prone to sun scald.
* Amend the soil. Aspen typically grow in decomposed granite. The key to successful growing is well drained soil. Denver soils tend to be heavy clays, but they can be amended with organics.
* Water slowly and deeply. Aspen growing in their native habitat get a lot of moisture. Aspen in the city need a good watering regime. Water every couple of days after the soil has dried out–about every three days during summer. Like all plants, aspen need water, but also oxygen. If the site is constantly wet, the trees won’t get the oxygen they need.
* Due to the big, woody roots that connect trees, aspen tend to produce sucker growth, especially when placed close to lawn areas. Aspen can fill in, so keep them in isolated beds filled with other plants or heavily mulched and you’ll have less of a problem. But this is important: Do not mulch aspen with cedar.
* Aspen are the shortest living tree species in Colorado. People growing aspen in the city should view them as 20-30 year plant. Some live longer, but not many.
* Be aware that aspen are prone to twig gall disease and leaf spots, both fairly difficult to control.
All that said, I enjoy the volunteer aspen in my secret garden. A sucker from my neighbor’s aspen, it adds verticality and dappled light to a shade garden. And the sound of the leaves in the wind triggers happy mountain memories of Colorado’s high country even while I’m smack dab in the middle of Denver.
••• “Cultivate your corner of the world. You grow your garden; your garden grows you.” •••
Colleen Smith’s debut novel Glass Halo, set in Denver, was a finalist for the Santa Fe Literary Prize and was praised in the latest issue of The Bloomsbury Review. The novel is available online and through your favorite bookstore.
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