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.