With over 600 hundred species of oaks (genus Quercus), nearly 100 of them native to the U.S., it would seem a difficult group of trees to sort out, much less identify individual species. However, only a few dozen species will be encountered in our area, even including common imports from Europe and Asia. Identification is made easier by recognizing that all of our oaks are in one of two large groups, known generally as red oaks or white oaks, which are easily discriminated by their leaf tips: Red oak leaves have hair-like bristle tips and white oak leaves lack bristle tips and tend to be more rounded. Other identifying features for red oaks include two years for acorns to mature (one year for white oaks), hairy inner acorn shell (hairless for white oaks), reddish tinged wood (whitish and stronger among white oaks), and usually darker bark. Oaks within these two large groups frequently hybridize, often making firm identification difficult.
The oak was declared the National Tree of the United States in 2004. All oaks have acorns, all have simple, alternate leaves and most are deciduous. Maryland’s most famous tree, the Wye Oak, lived nearly half a millennium and was believed to be the largest white oak (Quercus alba) in the country when it was felled by a thunderstorm in 2002. The white oak is the state tree of Maryland, the inspiration of Gaithersburg’s logo, and appears to be the model for the acorn and a bunch of leaves on the Town website. White oaks are magnets for caterpillars, which are needed to support populations of birds. Thus for many of us, our mental image of an oak is Quercus alba. Here is help in identifying Quercus alba from other types of white oaks: Leaves are twice as long as wide, wider at top than at base, vase shaped, have 5-9 rounded lobes and are dark-green above, pale beneath. At its best, leaf fall color is of red wine. Acorns are ¾-1” long, about 1/3rd enclosed in a light chestnut brown bowl-shaped cap. However, trees may take 20 years old before they start to produce acorns. Bark is ashy gray, scaly, and arranged in vertical blocks.
A recent online article titled “Why Are So Many Oak Trees Dying This Year?” noted that mature trees in both the red and white oak groups are dying throughout the mid-Atlantic. While not fully understood, much of the blame has been placed on environmental (abiotic) culprits, most prominently last year’s record setting rainfall, which may have drowned tree roots, and the recent drought. In the last few years we have lost numerous majestic white oaks in both Town and in the woods, many well over 100 years old. Once in decline, it is difficult to prevent eventual death. One bit of good news is that populations of gypsy moths, a major predator of oaks, have declined recently. Because white oaks are not commonly sold, it is important to encourage the growth of seedlings, which can be transplanted in early spring during their first couple of years.
Plants in Washington Grove, from F&B member and Master Gardener Jay Everhart