What to Know About Oak Trees

Creating New Oak Trees

Oak trees have particular features that make them wonderful additions to Nashville yards and wooded properties: they’re big, hardy, valuable for many kinds of wildlife, and can live a long time. In The Nature of Oaks, his fascinating book about “our most essential native trees,” Douglas Tallamy writes, “Oaks support more forms of life and more fascinating interactions than any other tree genus in North America.”

In choosing an oak for their yard, many homeowners carefully consider various oak species to find the best size, growth rate and habit for their space. But did you know that in the wild, oak trees can naturally cross-breed to form new varieties? Let’s take a closer look at how oak species hybridize and why hybridization is a useful process for tree growers to harness.

Identifying Oak Trees

There are over 400 species of oak, many types being native to the Southeastern U.S. In Middle Tennessee and elsewhere, oak trees must compete with surrounding vegetation to survive and grow into the massive trees we know and love. Nature’s strategy is to jettison lots of seeds with many combinations of the parents’ genetic material in the hope that favorable conditions will allow those with strong characteristics to survive to maturity.

Oaks of the same species will often produce saplings, but sometimes one kind of oak’s pollen will be brought to the flowers of another species of oak, resulting in a hybrid oak variety. The tree usually has dominant features from one of the parents, but sometimes it expresses features of both. This can result in many different types of oak trees – one might grow faster or produce more acorns than others of its dominant background. Oak varieties can also be chosen for urban hardiness to filter air or manage rainwater. New oak tree varieties can be intentionally propagated, with many of the most successful ones being sold in nurseries.

How Do Oak Trees Hybridize?

Michael Davie, NTCC board member and certified master arborist at Bartlett Tree Experts, notes, “It’s very common for oaks to hybridize, especially among the red oaks, but it’s not uncommon in white oaks either.” Red and white are the major groups of oaks, into which species are divided. White oaks include swamp oak and bur oak, while red oaks include black, pin, and shingle. No single species is more or less likely to hybridize than another, as Davie explains: “Many of the species pollinate at the same time and the pollen grains are able to fertilize closely allied species. Often the offspring are infertile, or the progeny are one of the parent species.”

When a forester, nurseryman, or gardener notices a hybrid offspring in nature with favorable features, they can take cuttings from the tree’s branches. This hybrid tree becomes the original parent of a new variety that, if it reproduced naturally, wouldn’t pass on its unique set of characteristics.

To ensure that the tree can be reproduced, cuttings from the original tree are grafted onto compatible rootstock. The grafted branch will heal and fuse to the roots, growing into a clone of the original hybrid tree. More advanced systems of hybridizing involve pollinating one species type with another, but the outcomes are less exact than grafting.

Why Would We Want Hybrid Oaks?

“People are constantly hybridizing and then propagating clones of those with cuttings or grafts,” Davie tells us, “because those trees will be genetically identical to the original tree. There’s a hybrid of the fastigiate (upright branching, narrow profile) English oak (Quercus robur 'Fastigiata') and swamp white oak (Q. bicolor) that’s being sold as Kindred Spirit, and I think it’s been crossed with white oak (Q. alba). The hybrids often have ‘hybrid vigor’ and are robust growers.” 

In addition to producing fast-growing oak trees, hybrids may be chosen for characteristics that allow them to grow where other varieties may not be able to. The Kindred Spirit Oak’s tight-growing branches mean it can be planted in spots where many other oaks may be too wide to fit.

An oak variety may also be chosen for a particular autumnal leaf coloration, for its heavy acorn productivity to attract deer, or low acorn productivity to keep a yard clean. Some types of oak trees change their colors or drop their acorns earlier than others, while most do later in the season. Choosing to plant a mix of varieties spreads out the unique characteristics of each to maximize overall effects throughout the season, while planting many specimens of a preferred variety will keep things uniform.

Deciding on a variety of oak can become quite the strategic choice, and if you’re not sure which tree is right for your property, get in touch with us or with a local certified arborist to talk it out. If you’re in the market for an oak tree, browse our selection of oaks available through the Nashville Tree Conservation Corps’ Tree Sale (open through the end of February), and be sure to sign up for our newsletter to receive regular tree facts and updates on local tree news!