If you only plant one thing, plant an oak.

Part I

If you only plant one thing, plant an oak.

8 min read

If you do nothing else after reading this, do this: when you next have a choice about what to plant in your backyard, plant an oak.

Not because it's beautiful, though it is. Not because it lives a long time, though it does — an oak planted in your lifetime can outlive your great-grandchildren. Plant an oak because there is no other single decision you can make about a piece of ground that does as much ecological work, for as little effort, as putting an oak in it.

The case is almost embarrassingly simple. Oaks host more species of caterpillar — the most important insect food on the planet, the thing that nine out of ten songbirds depend on to feed their young — than any other genus of tree in North America. More than four hundred species of caterpillar can complete their life cycle on an oak. The next-most-useful native tree, willow, hosts about half that. Most of the trees you can buy at a garden center, especially the popular ornamentals like Bradford pears and Japanese maples, host fewer than ten. Many host zero.

This is the central insight of the entomologist Doug Tallamy, who has spent four decades counting these things. He calls oak a keystone species — a single organism that holds up an entire local food web. Take it out and the web frays. Put one in and the web rebuilds itself, often within a single growing season.

What "feeds" actually means.

When we say an oak "feeds" four hundred species, here's the literal version of what that means. A female moth lays her eggs on an oak leaf in spring. The eggs hatch into caterpillars. The caterpillars eat the leaves. A chickadee parent, who needs to feed her chicks somewhere between six and nine thousand caterpillars in the two weeks they're in the nest, hunts those caterpillars from the oak. The chicks fledge. They become adults. Some of them get eaten by hawks. Some of them lay eggs themselves the next spring.

Multiply that small story by hundreds of bird species, by thousands of moth species, by the bats that eat the moths and the foxes that eat the bats. That's the food web. A single mature oak is feeding a small city of beings, most of which we never see, almost all of which depend on it.

And here is the part that's hard to believe but is true: this works at any scale. A young oak, ten years old, eight feet tall, in a small suburban backyard, is already feeding hundreds of species. A sapling planted next to a sidewalk in a strip of dirt three feet wide will, within five years, be hosting caterpillars and feeding migrating birds. The benefits do not require a hundred-year-old specimen tree on a country estate. They start the year after you plant the thing.

This is the argument we want to make in this Issue, in four short parts. One: the simple case for oak, which is what you just read. Two: why oak specifically, and what it does that other trees don't. Three: what to do if you can't plant an oak — if your patch is too small, too shaded, or made of concrete. And four: a small how-to for putting your first one in the ground, written for someone who has never planted a tree before.

Read it in order if you can. Each part is short. The whole Issue is about thirty minutes. By the end, you'll know what to do in your backyard this season, and why.

Part II

What an oak hosts that nothing else does.

7 min read

The reasonable question, after reading Part I, is: why oak? Why this one tree and not another? Trees are trees. They all have leaves. Birds eat insects on all of them. What makes oak so different?

The honest answer is that oaks have been here for a long time — longer than almost anything else in the eastern North American landscape — and the insects of this continent have spent tens of millions of years figuring out how to eat them. Most plants, when chewed, fight back. They produce defensive chemicals: tannins, alkaloids, terpenoids. These are bitter. They make leaves hard to digest. They are, ecologically speaking, the leaf's way of saying do not eat me.

An insect that wants to eat a particular plant has to evolve specialized enzymes to handle that plant's specific defenses. This is a slow process. It takes thousands of generations. And the result is what ecologists call specialism: most caterpillars can only eat the leaves of one or two or three closely related plants. A monarch caterpillar can only eat milkweed. A spicebush swallowtail can only eat sassafras and spicebush. A luna moth can only eat a small handful of native trees, and oak is one of them.

When you plant a Japanese maple in North America, you are planting a tree that the local insects have had no time to evolve enzymes for. They cannot eat its leaves. The tree might be beautiful, but ecologically it is a desert. A nice-looking, well-watered, completely empty desert.

The four-hundred number, in detail.

Tallamy and his colleagues counted, region by region, how many species of caterpillar can complete their full life cycle on each genus of native tree in the United States. Oak (the genus Quercus) wins almost everywhere they looked. In the mid-Atlantic, where this site is being written from, the count is 477 species of butterfly and moth that can host on oak. The next most useful native tree, black cherry, hosts 456 species. Then willow, around 450. Then a steep drop-off.

Most ornamental trees, even native-looking ones, host between zero and twenty. The Bradford pear, planted by the millions across American suburbs in the 1980s and 90s, hosts essentially nothing. A street planted with Bradford pears can be lined with what looks like trees, and have no insects on them, and therefore no birds. The block looks green. It is biologically vacant.

An oak, by contrast, is a fully populated city of small lives. It is the difference between a glass-front office building and a coral reef.

Part III

If you don't have room, what else.

6 min read

An oak is a big tree. We are not pretending otherwise. A mature white oak can reach eighty feet tall and eighty feet wide. If your patch is a balcony in the city, a sixteen-by-twenty backyard behind a townhouse, or a strip of dirt next to a parking space, an oak is not the right answer.

The good news is that the keystone-species idea generalizes. If you can't plant the most useful tree, plant the most useful thing that fits your space. Here, in rough order, are some smaller alternatives that punch above their weight.

If you have a small backyard, six to twenty feet of space.

• Serviceberry (Amelanchier). Small native tree, twenty feet tall at most, white flowers in spring, edible berries in summer that birds love. Hosts about a hundred caterpillar species.
• Native viburnum. There are dozens of native viburnums; pick one that grows in your region. Berries feed birds in winter when nothing else is fruiting.
• American hazelnut. A shrub that grows about ten feet high. Hosts about a hundred and fifty caterpillar species. Produces hazelnuts.

If you have only a few square feet, or a balcony.

• Goldenrod (Solidago). Easy to grow, late-blooming, feeds the most native bees of any plant in the late summer. Innocent of all hay-fever charges; that's ragweed.
• Native asters. Same window as goldenrod, same incredible insect benefits. Beautiful in pots.
• Common milkweed. The only host plant for monarch caterpillars. Without it, no monarchs. Grows happily in a large container.
• Native sunflowers. Birds eat the seeds in the fall. Pollinators work the flowers all summer.

If you have only a window box.

Even here, you have options. A window box of native flowering plants — coreopsis, black-eyed Susan, native salvia — will be visited by bees within days of flowering. Pollinators find these plants from a surprising distance. You will not feed a chickadee from a window box, but you will feed a small native bee. That counts.

The point of this Issue is not to be doctrinaire about oak specifically. It is that plants are not interchangeable. A native oak feeds four hundred species. A native serviceberry feeds a hundred. A non-native ornamental feeds nothing. The choice you make matters more than you've been taught it does. Pick the most useful thing you have room for, and you've already done the work.

Part IV

A small how-to: planting your first one.

9 min read

If you've never planted a tree before, this part is for you. There is more mystique around tree-planting than there needs to be. The actual process is straightforward. You can do it on a Saturday morning. You will probably do it imperfectly, and the tree will probably live anyway, because trees have been figuring out how to live for two hundred million years and they are good at it.

What to buy.

You want a young tree, two to five feet tall. Counterintuitively, smaller saplings often establish faster than larger ones — their root systems haven't been disturbed as much by transplant, and they don't suffer the years-long shock that big nursery trees do. A four-foot oak you plant this spring will, within a decade, be larger than a fifteen-foot oak you spent four times as much money on.

Buy from a native-plant nursery if you can find one. A regular garden center is fine if you can't, but ask: is this species native to my region? If they don't know, buy somewhere else. Audubon's native plant database by zip code is a good place to start.

When to plant.

Spring or fall. Not summer. Not winter. The tree wants cool weather and its leaves wanting to be off — or just barely on — while the roots establish. Late September to early November is ideal in the mid-Atlantic; March through April for spring planting.

Where to plant.

Sun matters. An oak wants at least six hours of direct sunlight a day. Look at the spot you're considering at noon and at three p.m. on a sunny day. If it's shaded both times, that's not the right spot for this particular tree.

Don't plant within fifteen feet of your house's foundation, or directly under a power line, or somewhere a future patio will need to go. Trees are slow to plant and very slow to move.

How to plant.

Dig a hole twice as wide as the tree's root ball but no deeper than the root ball itself. The top of the root ball should sit slightly above the surrounding soil — never below. Settle the tree in. Backfill with the soil you dug out (not fancy potting mix; the tree wants to learn the soil it'll be living in). Water deeply, until the soil around the tree is soaked. Add a two-inch layer of mulch in a wide ring around the trunk, but not touching the trunk itself.

That's it. Stake it if it's in a windy spot. Water it deeply once a week for the first summer, especially in dry weather. By the second year it'll be on its own.

What happens next.

The first year, almost nothing visible. The tree is putting all its energy into roots. You'll see a few new leaves, slow growth, maybe a few inches.

The second year, the tree's first real growth. A foot or two of new height. Maybe its first acorns or flowers, depending on species.

The third year, the tree starts to host. Caterpillars find the leaves. Birds find the caterpillars. A few species of small beetle, a leafhopper or two, a wandering moth. You probably won't notice.

The fifth year, the tree is twelve feet tall. A pair of chickadees nests in the lower branches. A red-bellied woodpecker investigates the trunk. Late-summer afternoons, the tree is loud with insects.

The tenth year, the tree is the most ecologically active object on your block. It is doing the work that no other single thing you could do in your backyard could do. You planted it on a Saturday morning ten years ago, and it has been quietly building a small city of beings in the air above your lawn ever since.

That is the whole point of this Issue. Plant the oak. The oak does the rest.