It seems that the fields of landscaping and gardening are full of many timeworn statements that people like to repeat over and over. Some of these green “rules of thumb” are tried and true, while others may not be so true.
One landscape stereotype regarding tree growth rates that is being challenged is the old adage that “oak trees grow slowly”. I have heard and read this many times and I have repeated this mantra myself for years. But a study of urban trees in the Denver Metro area, published by the Colorado State Forest Service, got me to rethink that stereotype.
This tree study recorded the growth rates of 19 commonly planted trees planted in public land in the Denver suburb of Westminster, for 24 years. The authors of the study measured the trunk diameters of the trees in 1992, 2000, 2008, and 2016.
The most eye-opening nugget of information in this report is that the white oak group of trees (bur, swamp white and English oak) were the 3rd fastest growing trees in the study! They grew faster than green ash, lindens or honeylocust trees. They even had the same rate of recorded growth as silver maples, a species often referred to as a “fast grower.”
One important take-away from this study for me is the fact that we should reevaluate what trees seem to be the best for planting in challenging ecosystems such as the urban/suburban areas of the high plains where Denver sits. It is worth quoting the State Forest Service report to emphasize this point:
“Some tree species revealed to be fast or moderate growers in this study have previously been viewed as slow growers, and they are often passed over at planting time. However, equating growth rates with vigor can be misleading, as some of the slower-growing tree types on this list can be the most adaptable to the area (including hawthorn, hackberry and honeylocust). Adding newly discovered fast-growing species to the planting palette and incorporating hardy, slow-growing species will maximize the success of planting projects and promote species diversity.”
To be clear, it is only one type of oak trees (white oaks) that exhibited fast growth in the study sample. Red oak was also in the study and showed slower growth. The authors note this is possibly due to the low pH of high plains soils.
It very well could be that other types of oak trees grow slowly too, but we don’t have data for that. Or, perhaps another study may come along and challenge that, as well. So, as with many “rule-of-thumb” type statements, don’t believe it until you have seen some data to back it up.