Quercus macrocarpa - Bur Oak

If you have space for just one shade tree, consider an oak! Botanists, foresters, and ecologists tend to rate oaks among the most ecologically important trees in North America - with around 90 species found across 85 percent of the nation's counties. They provide more food and shelter for wildlife than any other group of trees, making them "keystone species." Here in the “Keystone State”, where forests and woodlands are the default, nowhere is this more demonstrably true.

The environmental benefits of oaks don't stop there. Due to their size, habit, and longevity, they’re remarkably effective in managing erosion, buffering the worst effects of wind and storm runoff, and storing carbon. Of course, oaks are fantastic trees for use in gardening and landscaping which is our primary focus here.

Picturesque and attractive in habit and form, oaks make great focal points and specimens. They naturally and effectively anchor our manmade landscapes to the land. When sited properly, they buffer our homes and buildings from harsh winter winds and shade them from brutal summer heat. Most oaks are adaptable and extraordinarily resilient. Some are tolerant of extremes. Here at the nursery, it's rare for them to need warranty replacements.

In Pennsylvania and the Mid-Atlantic region, there are 20 or so native oak species. In nurseries, including ours, you’ll usually find even more, as well as various hybrids and cultivars. Because of this, there's an oak for almost any outdoor space.

Bur oak (Quercus macrocarpa) is, undoubtedly, one of the hardiest oaks in the eastern North America. It's most predominant in the Midwest but its native range also extends up and down portions of the US west of the Appalachian Mountains, including southwest and southcentral PA. With the exception of wet soils, Bur oak will grow just about anywhere. It grows well in acidic soil but also thrives in alkaline soils, often found growing on lime-based sites in nature. It's also considered the most drought tolerant oak in the eastern half of the US as well as the coldest tolerant, being the most northernly growing species in North America. Tradeoffs include slower growth and being mildly less forgiving with mishandling during transplant. On containerized trees, loosen the roots gently but be careful to not remove too much. With B&B trees, expect very little top growth the first few years as the tree puts more energy in regaining a robust root system. 

While not ornate in appearance, Bur oak is undeniably among the most dignified of trees. Its large leaves are thick and vibrant green. In youth, bark is a smooth gray but soon becomes deeply ridged and furrowed with a cork-like appearance. Fall color is a modest yellow to yellow brown. Oaks are usually one of the last groups of trees to change colors in fall, usually 2-3 weeks after maples do, prolonging autumn color.

Most oaks practice "marcescence" which means young trees don't drop their leaves till new leaves emerge in spring. On older trees, leaves on the bottom of the crown hold till spring while leaves higher up drop in fall. One theory that explains this phenomenon is it helps protect buds from herbivory. Regardless, this phenomenon adds winter interest to the tree by way of texture, color, and the sound of rustling leaves.

Oaks don't produce copious amounts of acorns annually. Instead, they choose to produce large quantities once every 2-5 years (provided they are pollinated by other oaks). Years of heavier acorn production are called "mast years." Oaks usually don't produce significant acorns until they're a couple decades old. While some see acorn production as a nuisance, we encourage you to think of it as providing a valuable resource to the animals and ecology of your yard and neighborhood. Bur oak acorns are the biggest of any species in North America making it very beneficial for wildlife (and easier to rake up than small acorns). It's certainly wise, however, to carefully select a good site for any oak or other large tree. Maybe the middle of the lawn is a wiser spot than above the swimming pool, for example.

For the layman, oaks are generally organized into three categories: red oaks, white oaks, and live oaks. This helps with identification as well as care. Live oaks are typically evergreen that are found further south. All native oaks to Pennsylvania are in the red oak and white oak groups. White oak is, you guessed it, in the white oak group. Species in this group are identified with rounder or blunted leaf margins and indentations. They drop their acorns in fall and germinate soon thereafter. Species in the white oak group are also more resistant to introduced pathogens, such as oak wilt and bacterial leaf scorch. More on this momentarily.

Going back to wildlife, the reason oaks are so well utilized is that while oak tannins are bitter, they aren't highly toxic or lethal. This is evidenced by the 500+ species of caterpillars of butterflies and moths that are supported by oaks. (Remember, if we want pollinators and healthy bird populations, we need caterpillars!). And of course, the acorns are rich in nutrients for mammals and birds. Acorns can be soaked and ground into flour to use in baking as well, with Bur oaks and other white oak group members having sweeter acorns than most others.

The wood and branching structure of Bur oak is very strong, especially in context of its slower growth. Unlike rapidly growing trees with brittle wood, such as willow and poplar, this isn't a tree that's prone to rotting, splitting, and falling on homes during windstorms. That said, we advise planting large trees the radius of a mature crown away from structures. As this species typically matures at 50-70' wide in most landscapes, we suggest planting this tree no less than 25-35 feet from your house. Most oaks are long-lived in the wild, with Bur oaks often living for 300 years or more. With the right conditions and care, it's possible your White oak could achieve that lifespan too.

While nevertheless hardy, many oaks in the eastern US have fallen prey to two introduced pathogens in recent decades: oak wilt and bacterial leaf scorch. To reduce the risk of introducing these pathogens into your tree, only prune (if you must at all!) during the colder months (November through March). Pruning during the warmer months leaves open wounds that run the chance of attracting insects that might transmit these pathogens into the tree’s vascular system. The good news is that species in the white oak group are the most resistant to the worst effects of these diseases. 

If deer are a problem for you, we recommend you provide your tree with protection, such as a metal cage, until the tree is large enough to not be severely damaged. Once the tree is tall enough that the cage can be removed, protect the trunk with a trunk protector to prevent damage from buck rub from September through December. Tree guards are usually no longer necessary when the diameter of the trunk at chest height is 5".

Written by Plumline staff and not AI.