by Mikl Brawner
If you look at old photos of Colorado Front Range cities, you won’t see many trees. And if you look up native trees of Colorado, you will find a lot of mountain-growing conifers and just a few deciduous trees that grow natively along streams. And yet the quality of life for us humans depends on trees.
Trees provide shade which creates cooler conditions; they attract and support birds, bees and other life; they create microclimates that make it easier to grow smaller plants, flowers and fruit; and they create beauty. So early settlers planted trees and created systems to bring water from the mountains to drink, bathe with and to support plants.
This plant culture was not easily gained and in particular, our western conditions are not that easy for trees. At the Eighth Annual Tree Diversity Conference on March 4th, our Front Range conditions for growing trees was described as “harsh”. We have hot summers, cold winters, low rainfall, low humidity, alkaline arid-style soils, late spring and early fall freezes, strong winds and a month shorter growing season than the Midwest and East. It is much easier to grow a tree in Iowa or Pennsylvania.
And now conditions are predicted to be getting worse. At the Tree Conference, speakers were confident in models showing temperatures increasing more rapidly than in the past and even faster for higher elevations because heat rises. On top of that, large numbers of ash trees, cottonwoods and weak trees will be disappearing, increasing the “heat island effect”. Asphalt, metal and concrete in roads, parking lots and buildings absorb the sun’s heat and retain it where there are no trees to provide shade. In addition, trees reduce the compacting effect of rain, and absorb storm water. And with higher density building, there is less room for tree roots. These stressful conditions weaken trees and make them more vulnerable to pests and diseases.
Trees are particularly vulnerable to climate change and very important both to prevent it and to maintain livable conditions for us humans. What can we do to improve our urban forest and prepare it for climate change? One obvious solution is to increase tree diversity because with more varieties it is less likely that large numbers of trees could die because of a pest, disease, drought or other problem. But people naturally want to grow trees that have a history of success, especially where conditions are difficult, like here. That has led to a concentration of a small selection of trees, and monocultures are known to be vulnerable to pests, diseases and changing conditions. We have depended too much on the durable ash trees. Now, especially with Emerald Ash Borer, we need to plant more trees and more kinds of trees. And we need to be asking for trees that are more drought and heat tolerant, more cold and alkaline tolerant. But which trees?
In order to get some good advice about this situation, I interviewed four tree lovers with histories of planting, propagating, studying and observing trees in our Front Range communities.
Scott Skogerboe is the propagator for Fort Collins Wholesale Nursery. Scott has spent a lifetime studying trees, traveling around Colorado and the US, visiting trees, collecting their seeds and growing them. He has been our main resource for learning about the trees and shrubs growing at the Cheyenne Horticultural Research Station. His propagation and promotion efforts have made sure that a lot of those resilient plants are being widely grown in Colorado.
Scott thinks we should rely more on native trees that provide for native wildlife, insects and birds. He says in the past, more focus was on northern selections because that brought cold tolerance, and because of late spring and early fall freezes, cold tolerance is still important. However with a warming climate, he says we should be looking south to Texas, Oklahoma and southern Nebraska for seed sources and superior selections.
His specific recommendations are the hardy Arizona Cypress, the Caddo Maples from Oklahoma (like the John Pair selection) and the hybrid chokecherry “Sucker Punch” which is so hardy, doesn’t sucker and supports so much wildlife. Scott also thinks we should be growing more oaks, because they are so durable and because, as Doug Tallamy says, they provide the most ecosystem services. He is growing a Gambel Oak selection called ‘Gila Monster’ from western New Mexico that doesn’t sucker, has a single trunk, is larger growing, cold tolerant to -30F and is very drought tolerant.
Scott also likes the very drought tolerant and strong Hackberry, which some people shy away from because of the bumps on the leaves caused by an insect. But he says that problem does not harm the tree and the larva inside those Nipple Galls are a bird favorite. He also likes our native Boxelder which is drought tolerant and supports a lot of birds. He especially likes the ‘Sensation’ Boxelder that is a male selection that does not get Boxelder Bugs and has a good fall color. He has been growing some northern selections of our native Big Tooth Maple that have redder fall color.
At the Cheyenne Station, Scott saw how well the Ohio Buckeye has done since 1974, growing to 25’ with little to no irrigation. He likes Kentucky Coffee Tree because of its handsome form and drought tolerance. There is a male selection called ‘Espresso’ that does not make the pods. Scott says that Catalpa is well-adapted to Colorado, and he is a big fan of hawthorns, especially Russian Hawthorn that is so drought-tolerant, has beautiful flowers and berries for wildlife.
Tim Buchanan served as city forester for Fort Collins for 41 years. During that time he greatly expanded the biodiversity of their urban forest. He studied, collected, grew, trialed and directed the planting of many new or unusual varieties. Now many of those trees are quite large and have proven their worth. And he knows them all, where they are growing and how they are doing. Now that he is retired, he has reduced his seed-grown pets to around a hundred.
Fort Collins is particularly challenging. It’s not that far from Wyoming, is colder than Denver and Boulder and soils there have a pH around 8, which is quite alkaline. He says the very popular Autumn Blaze Maple doesn’t do well there in their alkaline conditions, and neither does Silver Maple, or Norway Maple. A lot of Sugar Maples have not done well though he thinks selections from more northern and western areas where it is colder and drier, might do better. He likes Caddo Maple from Oklahoma and Big Tooth Maple, and says Acer nigrum, Black Maple, has better heat and alkaline tolerance.
Northern Red Oak is problematic because of high alkalinity, but Texas Red Oak, Quercus buckleyi, does well and has nice red fall color. Tim says Kentucky Coffee Tree is trouble-free and ‘Espresso’ is a good selection. The conifers he prefers are Blue Spruce, Douglas Fir, Englemann Spruce, Concolor Fir and Swiss Stone Pine, especially ‘Chalet’.
Tim has collaborated with Scott Skogerboe over the years trialing a hardy Northern Pecan from seeds Scott collected and grew and Tim planted, which have become “very nice trees.” He thinks Catalpa is a good, solid tree that could be planted more. And as long as Redbud comes from a northern seed source, it is a good small tree with great flowers. In flowering crab apples, he favors Red Baron, Thunderchild, Spring Snow and Radiant. For elm trees that have little or no scale insect, he recommends Accolade and Choice City, and he likes drought tolerant and tough Hackberry and American Linden which needs less water than most Lindens and is great for the bees.
Panayoti Kelaidis, Senior Curator and Director of Outreach at Denver Botanic Gardens, is known to most of us as the Rock Garden Guru. Few people know that he has always had a passion for trees, and has been involved since its inception in the Annual Tree Diversity Conference. Like Scott and Tim, Panayoti knows a lot of trees personally, checking up on them and their success. He worked with landscape designer and tree enthusiast, Al Rollinger and others to update Al’s 50 year tree survey of Denver’s unusual trees. Trees that did best since first recorded in 1968 were Bur Oak, Kentucky Coffee Tree, Chinkapin Oak, Texas Red Oak and Yellow Buckeye. (See the full report searching for “Rollinger Tree Collection.”)
Panayoti said that we Make trees grow here. By building houses that help create microclimates and protection, by adding compost to the soils, and by watering, in time we cultivate soil and environment more supportive of trees. I learned from him that the US has two basic soil types: a midwestern/eastern type called Pedalfer soil that forms in wetter, moister climates and is dark brown or black and is very fertile and more acidic, like what is under the hardwood forests; and our western Pedocal soil that forms in arid and semi-arid conditions and is rich in calcium carbonate, low in organic matter and more alkaline. This explains why some eastern/midwestern trees that are perfectly hardy don’t thrive here, and also why some trees thrive in old neighborhoods where people have for years been composting and watering, but languish or die in new neighborhoods.
Panayoti thinks that the practice of cloning trees (grafting from a single variety) that produces individuals with identical genetics, is sad and a disaster in the making. He says we’re not thinking about what is good for nature because we are so focused on convenience, uniformity and neatness. We need to experiment more, let nature make more fruit and eat and preserve that fruit. Trees are noble; there’s a reason people honor trees. It is humbling that they are so big and strong and can live beyond our lifetimes.
Sonia John is curator of the Regis University Arboretum and has worked on the DU Arboretum. It was her idea to start the Annual Tree Diversity Conference in 2014, and since then she has helped present the conference. She is growing over 100 small trees in her yard, grown from seed, liners and bigger starts, which are then planted in the Regis Arboretum.
She is very knowledgeable about trees and interested in unusual varieties. She likes American Smoketree, Cotinus obovatus which is a beautiful, tough small tree. And she likes the Yellowhorn, Xanthocerus, because of its drought tolerance and terrific flowers. Other favorites are Soapberry, Hickory and Northern Pecan. She loves oaks, especially some Bur Oak hybrids with Gambel Oak, called bur-gambel, like ‘Westward Ho’ and ‘Jack Mze’. And she likes Black Jack Oak, Lacy Oak, Netleaf Oak and one called ‘Azul de Salinas’. She thinks oaks are particularly smart for Colorado because they are late to leaf out, and don’t seem to be hurt by our freezes. And because oaks interbreed so easily, she believes it could be possible to purposely breed them for more heat, cold and drought tolerance.
Sonia likes Catalpa, Kentucky Coffee Tree and Hackberry and says Nipple Gall is not a big deal. And if trees do need more water than some other plants, most of them need less than a bluegrass lawn, and in terms of Climate Change, they are worth it.
She encourages people to visit trees in Ft. Collins at The Gardens on Spring Creek, the City Park Arboretum and the CSU Arboretum and in Denver at Regis and DU Arboreta and Denver Botanic Gardens.
Further development in resilience might be possible using seed and natural hybrids collected from sources in drier and more southern climates, selecting trees for more disease and pest resistance, and using root-pruning pots that prevent girdling roots. Inoculating tree roots with mycorrhizae when propagating and planting can help with establishing and with stresses. Some trees, like oaks, really need specific mycorrhizae so culturing the fungi taken from the soils of thriving native communities could be beneficial. Adding 20%-30% compost when planting helps to hold moisture and feed the soil life.
I have been an arborist for 35 years and it is my opinion that it is healthier for trees to be planted outside the lawn area. The irrigation systems designed for lawns too often deprive trees of oxygen by frequent and excessive watering, and most trees prefer deep, infrequent watering. Also the dense root systems of turf grasses do not allow trees the deep watering that rain storms provide trees in a forest.
It would help if we humans could be more tolerant of the irregularities that come from seed propagation in order to benefit from increased genetic diversity. Also allowing insects to damage 10% of the leaves of a tree before applying controls, allows for caterpillars which become butterflies and moths that are beautiful and important food for birds. And it is now know that this predation stimulates the strengthening of the trees’ immune system.
And lastly, more diversity and resilience will only be achieved if gardeners are willing to try new trees, maybe even a few risky varieties so that if they are successful, others will be more interested and willing to try them.
Resources: Search for: Front Range Tree Recommendation List (2010) and bouldercolorado.gov/best-trees-boulder (see Approved List and the list of pest and disease prone trees that are not recommended)