Tree · Fagaceae
Bur Oak
Quercus macrocarpa
Plant a bur oak and you've started a tough, century-spanning shade tree that laughs off cold, drought, and rough city soil.
Full sun
Every 7–14 days
USDA 3–8
60–80 ft · 18.3–24.4 m
Overview
Want a tree that won't quit on you? The bur oak is about as tough as they come in North America, a slow but steady giant that can hang around for centuries and grow 60 to 80 feet (18 to 24 m) tall and just as wide. It's a white oak at heart, so it brushes off the kind of conditions that would do in a softer tree, whether that's a brutal winter down to -40°F (-40°C), a dry hilltop, or some tight city clay.
Then there are the acorns, the biggest of any native oak, sometimes 1.5 inches across, each one tucked into a deep fringed cup that looks downright mossy. That's where the nickname mossycup oak comes from. The flowers? Easy to miss. Yellow-green catkins show up for a bit in April, and the acorns fill out and ripen that same fall.
This is a tree you plant for the long game, and it likes elbow room. Give it full sun and plenty of space well away from the house, the foundation, and the power lines, and once its roots settle in it'll pretty much take care of itself.
Care
The short version: full sun and lots of room, just about any soil as long as it drains, steady water for the first few years, and barely any fuss after that.
Light
Bur oak really wants full sun, which means six or more hours a day, and that's what gives it that nice broad, even crown. It'll put up with a little shade while it's young, but grow one in real shade and you'll end up with a thin tree that leans toward whatever light it can find.
Water
Give a young tree a regular drink through its first few years while the roots are digging in, then you can ease right off. An established bur oak is very drought tolerant and rides out long dry stretches just fine, so it only wants water from you in a real drought.
Soil
Honestly, few trees are this easygoing about dirt. It's happiest in moist, well-drained loam, but it'll take clay, dry spots, and even alkaline soil without batting an eye, which is exactly why it does so well in those rough city spots.
Temperature
This one's cold-hardy right down to its bones, totally fine across -40°F to 100°F (-40°C to 38°C) and good through USDA zones 3 to 8. Not much in the way of heat or cold in its range is going to bother it.
Humidity
You can stop worrying about humidity outdoors with a bur oak. It grows on soggy bottomlands and dry prairie edges all the same, so there's nothing to manage on that front.
Feeding
An established bur oak rarely needs feeding since it pulls plenty from its wide spread of roots. If your soil's genuinely poor, a balanced slow-release fertilizer in spring is all it takes.
Pruning
Only prune in late winter while the tree's asleep, and just take out the dead, damaged, or crossing branches. Steer clear of any cutting from April through October, because that's when sap-feeding beetles can sneak oak wilt into a fresh wound.
Repotting
none
Size & spacing
Plan for a big one: it'll hit 60 to 80 feet (18 to 24 m) tall and just as wide once it's grown. Give it a spot well clear of buildings, foundations, and utility lines so that wide crown and its spreading roots have room to do their thing.
When to plant
You don't sow a bur oak the way you'd sow a row of beans. Pop a container or balled-and-burlapped tree in the ground in spring, or just push some fresh acorns into the soil in fall. The catkins come and go without much notice in April, and the acorns ripen and drop that same autumn.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: Plant balled-and-burlapped or container stock in spring; sow acorns outdoors in fall
- Harvest: Yellowish-green inconspicuous catkins in April; acorns mature in fall of the same year
Hardiness
USDA hardiness zones: 3–8
Temperature: down to -40°C
Troubleshooting
Here's the good news: a healthy bur oak hardly ever causes a fuss, and most of what you'll spot is just cosmetic. The one thing that's actually worth your attention is oak wilt, so it pays to know what it looks like and the timing rules that keep it away. Match up what you're seeing below before you do anything.
Crown leaves browning at the edges and dropping fast
What you're seeing is oak wilt, a fungus that clogs up the tree's water channels and works its way down from the top. Since a bur oak's a white oak, it usually fades slowly instead of dying off in one season, though drought or root damage can fake the same look. Don't prune or wound an oak between April and October, when the beetles that carry the spores are out and about, and get a certified arborist to confirm it with a lab culture.
Fix:
- Avoid wounding or pruning oaks April through October
- Contact a certified arborist or extension diagnostic lab for confirmation
- Sever root grafts between adjacent oaks where feasible
- Remove and properly dispose of severely infected trees to limit spread
Weird bumps, balls, or growths on leaves and twigs
Those odd things are galls, and they form when gall wasps or mites lay eggs and the tree grows a little protective shell around the insect inside. They look strange, sure, but they almost never hurt a healthy oak, so the usual move is to just leave them be. Snip off a few heavily galled twigs if they really bug you, and keep the tree watered and happy instead of reaching for a spray.
Fix:
- Tolerate galls in most cases as they rarely harm tree health
- Prune out and dispose of heavily galled twigs if desired
- Maintain tree vigor with proper watering
Toxicity
It's a wonderful shade tree, but here's the catch: the acorns, buds, and young leaves aren't safe to eat. They're loaded with tannins that'll upset a cat's or dog's stomach, choke or block the gut if a whole acorn goes down, and do real kidney damage to horses and livestock that graze on a lot of them. Rake up the fall drop wherever pets and stock can get to it, and no, people shouldn't be munching raw acorns either.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
If a cat chews on a leaf or a fallen acorn, expect some vomiting, a lost appetite, and loose stools. And a whole acorn? That's a real choking and gut-blockage risk for a little animal, so it's worth clearing them up.
Dogs Mild
Dogs are the ones most likely to crunch acorns right off the ground, and that brings on drooling, an upset stomach, and diarrhea. The bigger worry is a swallowed acorn getting stuck in the gut, so keep an eye out for repeated vomiting or a sore, tender belly.
Horses Severe
Horses really get hit hard with this one. A horse that eats a lot of acorns, buds, or young leaves can show colic, constipation that turns into bloody diarrhea, a dull mood, and dark urine, and those tannins can wreck the kidneys.
Rabbits Toxic
Just keep rabbits away from oak, plain and simple. The tannins in leaves, buds, and acorns are way too harsh for their gut, and even a small nibble can leave one off its food and dragging around.
Livestock Severe
Cattle, sheep, and goats can poison themselves on a big load of buds, leaves, or acorns, especially once other forage gets scarce in fall. You're looking at kidney damage, weakness, and in bad cases death, so fence your stock away from any dropping oaks.
Birds Caution
Plenty of wild birds shell and eat acorns no problem, but those same tannins can bother pet birds and poultry, and a whole acorn's just too big and hard for most of them to handle safely. Don't offer any to caged birds, and keep an eye on backyard flocks hanging out under a tree.
People Caution
People don't eat raw acorns, and a mouthful is bitter and rough on the stomach thanks to the tannins. Traditional acorn foods get leached for hours first, so treat anything straight off the tree as not for snacking, and keep little kids from popping them in their mouths.
Rake up and dispose of fallen acorns and leaves where pets and livestock can access them.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Most folks plant bur oak as the plain old species, but it does cross with its neighbors out in the wild. Here are the two forms worth knowing about.
Quercus macrocarpa (species)
The straight species grows the biggest acorns of any North American oak, up to 1.5 inches across, and each one sits in a deep mossy cap that fringes the rim, which is exactly where both names come from, bur oak and mossycup oak.
Quercus × schuettei
When bur oak and swamp white oak happen to grow next to each other, they sometimes cross into this natural hybrid, with leaves and acorns that land somewhere between the two parents.
How to propagate
Want the easiest way to get a bur oak going? Plant an acorn. The white oak group makes it simple, because the seed sprouts fast and doesn't ask for much.
SeedModerate
Collect ripe acorns in fall and sow promptly outdoors or stratify before spring sowing · Acorns germinate within a few weeks; seedlings rapidly develop a deep taproot, making later transplanting difficult
Growing a bur oak from seed is genuinely easy as long as your acorns are fresh. Scoop up the ripe nuts as they drop and sow them right into the ground that autumn, since white oak acorns barely go dormant and sprout within a few weeks. Got acorns from way up north? They do better after 30 to 60 days of cold, moist storage at 32°F to 41°F (0°C to 5°C) before a spring sowing. Plant the acorn exactly where you want the tree, because the seedling shoots down a deep taproot almost right away and won't take kindly to being moved later.
Myths
Acorns must be safe for pets and livestock since wild animals eat them all the time.
Deer, squirrels, and jays are built to deal with the tannins in acorns, buds, and oak leaves, but cats and dogs just get a stomachache out of them, and horses and livestock can end up with serious kidney damage when they eat a bunch. What's fine for wildlife can still poison your animals, so rake up the drop and keep your stock off it.