Leave them to the Mountains OR Plant them at Home?
Populus tremuloides or Quaking Aspen is not only one of the best-known native trees here in Colorado, but it is said to be the most widely distributed tree of North America. Its narrow, roughly pyramidal form is commonly 25’-30’ tall and 15’-20’ wide, although it can get much larger. The leaves are shiny, dark green above and light gray-green beneath which makes the tree shimmer in the breezes. Also its whitish trunk adds to its attractive architecture. Then in the fall the leaves can turn a rich golden yellow which look glorious on the tree and lovely on the ground. It has a suckering habit which inclines it to clumps and which has put aspen in the running for the world’s largest being.
In spite of these positive qualities, more and more thumbs have been turning down on the use of aspens in our plains landscaping. In my own work as an arborist I have seen many aspens used at lower elevations and some look fabulous and some look terrible. So I decided to ask several excellent horticulturists with experience in these matters about their views and recommendations.
James Feucht, professor emeritus at CSU who for years has been guiding students at the university and many others through the Cooperative Extension program,said that since aspens have not naturalized onto the plains, they are not meant to be here. Natively in Colorado, they grow where there is plenty of moisture, where the soil is somewhat acid and rich in organic matter, and these are conditions that are not common on the plains. Therefore he does not recommend them except in mountain sites of at least 6ooo’.
Robert Cox who is a Jefferson County Extension Agent in horticulture, has done research on aspens and on various aspen clones. These clones are differing varieties that are found in nature which are then propagated for the nursery trade. He said that whereas there were significant differences amongst these clones, some doing better than others at lower elevations, he could not recommend them below 6000’. He said that by far the most common plant coming to his office for pest or disease diagnosis is the aspen. When I asked Mr. Cox where aspens do well and where they do poorly, he replied that they get a lot of problems in our heavy, compacted clay soils and that they thrive in the loose decomposed granite of the mountains where the roots get more air. He said if people want to grow aspens down here, he recommends preparing a very wide soil area, amending it with compost and watering less often so the oxygen isn’t driven out. It is best to plant them on the north and east sides of a building. Also he suggests planting container-grown or field-dug trees since the commonly-sold mountain-dug trees have a root system “like a piece of conduit” and consequently become more stressed and subject to diseases.
Chris Serbousek, one of the owners of Trees, Trees, Trees Inc. agrees. He said that his nursery now only sells aspens that are container-grown from seed and that because they have such a good root system, they grow quickly and experience little or no transplant shock going into the ground. The mountain-dug aspens, he continued, have a root system like a hockey stick and consequently because they are more stressed, they are more likely to get diseases. He recommends planting in well-drained soil or amending the soil with compost in a big hole and/or planting on a mound. Fred Berkelhammer a Boulder area arborist agreed saying that a lot of the problems he runs into with aspens are because the mountain-dug trees are weak to begin with. However he’s not sure is there’s any way to get around the problems aspens are prone to.
Dermod Downs from Denver who has been doing plant propagation, landscaping and consulting especially for finding the right plant for the right place says that generalizations about aspens are difficult because has seen them doing well and doing poorly in a wide variety of conditions. He suspects that genetics are more relevant to aspen success than cultural conditions. He has planted a few aspen clones that are growing vigorously and disease-free but emphasizes this is based on short-term evidence. Dermod also strongly believes that container-grown and field-grown aspens are far superior to mountain-dug stock: “ a container-grown one gallon aspen will outgrow a 10’ collected.” He concluded that whereas aspens have their downfalls, few other trees have their oval columnar and adaptable form. For specific situations and uses, he recommends them.
Larry Watson who is the owner of Plants For Today And Tomorrow, describes himself as a plant-finder. He has long been respected on the subject of Colorado trees. Larry said, of course aspens have problems and get diseases but people want them; and what trees don’t have problems? If we are gong to grow aspens the most important factor is good horticulture: pay attention to the conditions they really need. Like what? Like good air circulation; soils don’t make much difference; watering seldom and deeply is better and keeping the water off the leaves is better. He has grown aspens and doesn’t want them in his yard now. His recommendations for alternatives are: Mt. Ash, Thin-leafed Alder, Tartarian Maple, Hedge Maple (Acer campestre), American Hornbeam, Amelanchier canadensis and Amelanchier laevis.
My own experience with aspens is that I have been growing two aspens in my gravely, low-water North Boulder yard and they are doing very well after 13 years. Of course aspens are short-lived as many of those interviewed mentioned so I should hold my judgment for another 10 years or so, but I won’t. I really love my aspens and believe they do well in the Boulder area where they have good drainage and seem to do poorly in over-irrigated lawns and high water- table areas of clay. I have also seen marked improvement in trees that have been auger-fed using 2” diameter,12” deep holes filled with compost etc. So I think air to the roots is the key. But even though the writer gets the last word, I will bow to the vaster experience of many whom I interviewed and hope that you, as well as I , have appreciated their generous contribution of the fruits of their experience in our Colorado horticulture where, as you know, we can use all the good help we can get.