ITG non-native trees

Colorado blue spruce is a non-native, commonly planted landscape tree that suffers from many issues in Illinois.

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Spring is a great time for planting new trees. There is ample rainfall to support them and the warming temperatures and mild weather motivate many of us to get out and plant. However, there are some commonly sold trees that fall onto my “do not plant” list. Many make my list for their invasive habit, but some are on there for insect or disease issues or just generally poor performance as urban trees.

Colorado blue spruce (Picea pungens) tops my list of trees not to plant. As its name implies, this tree is native to the Rocky Mountains of Colorado and several surrounding Western states. It is adapted to the cool, dry climate of the Rockies, as well as the sandier and very well drained soils. When you plant this tree in Illinois, with our humid climate with poorly drained soils, it simply cannot thrive.

I will be the first to argue that the beautiful blue-green foliage of blue spruce is something to admire in any landscape. And many times, these trees look wonderful at planting time and for a short period thereafter. However, unless this tree is planted in near optimal conditions for Illinois, the stresses of our climate and soils add up over time to reduce its heath and vigor. As these trees mature, it is a sure bet they will suffer from a variety of common disease issues (For more information on spruce diseases, see this Plant Clinic Report: go.illinois.edu/SpruceProblems).

While there are treatments available to slow the symptoms, there really is not a cure, other than a change in climate and soils. In my experience, treatments are costly and most often ineffective as a long-term solution, making this species a relatively short-lived landscape plant when compared to others.

Callery pear (Pyrus calleryana) is widely planted for its ornamental value, with over 20 cultivars available at my last count. Bradford is one of the most common cultivars, leading to the more frequently known common name of Bradford pear. The plant certainly has ornamental appeal, with nearly a month or more of flower display in the spring, beautiful dark green foliage and a nice pyramidal habit.

Being a native of East Asia, Callery pear has relatively few disease or insect issues here in the U.S. However, it is highly susceptible to storm damage. Its compact and pyramidal habit can be attributed to a propensity for narrow branch angles and weak branch attachment. In the words of a veteran arborist I worked with in the past, the ultimate fate for many of these trees is to “peel apart like a banana” in a wind or ice storm, leaving an irregular and unattractive canopy.

On a more serious note, this species has shown invasive character over the past few decades. While most cultivars are sterile, cross pollination between the many commonly planted varieties has created fertile fruits. In open, sunny and unmowed areas, the fruits, being spread by birds, pop up all over the place. From fence rows to highway right-of-ways to edges of our yards to local natural areas, you can see Callery pear on nearly any drive around central Illinois when it is in flower. It is one of the only species to have white, abundant flowers during its bloom time, making it stick out like a sore thumb.

Callery pear is another plant that I would argue has great ornamental value, like the blue spruce. However, its vulnerability to storm damage can limit its longevity in the landscape, and when you combine that with the threat it poses as an invasive species, it’s just a tree we all need to stop planting.

In the past several decades, it has been widely overplanted. I recall one college professor in the late 1990s predicting it may be the next monoculture, simply due to overplanting. That was before we really understood its invasive character.

I always hate to pick on certain tree species, because as I have noted, there is beauty to found in all of them. However, in my career, I have seen these two species disappoint homeowners too many times to recommend planting either. Please join me next week as I explore a more positive spin on tree planting by looking at some of my favorite and most recommended tree species.

Ryan Pankau is a horticulture educator with the UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his ‘Garden Scoop’ blog at go.illinois.edu/GardenScoopBlog.