Tree · Fagaceae
Bur Oak
Quercus macrocarpa
A bur oak anchors a property with the quiet authority of a heritage tree, the kind of shade a place grows up around.
Full sun
Every 7–14 days
USDA 3–8
60–80 ft · 18.3–24.4 m
Overview
Few trees set the tone of a landscape the way a bur oak does. It is one of the most enduring trees in North America, a slow and stately presence that settles in for centuries and eventually fills a property at 60 to 80 feet (18 to 24 m) tall and just as wide. A member of the white oak group, it carries on through the conditions that defeat more delicate trees, from deep winter cold of -40°F (-40°C) to dry ground and the hard clay of a city lot.
Part of its appeal is the way it marks the seasons without ever demanding the spotlight. Yellow-green catkins drift through the canopy almost unnoticed in April, and by fall it drops the largest acorns of any native oak, occasionally 1.5 inches across, each one cupped in a deep fringed cap that looks softly mossy and lends the tree its second name, mossycup oak.
This is a tree to plant for the long arc of a home, the one that shades a porch, frames a view, and outlasts the people who set it. Give it full sun and open ground well away from the house, the foundation, and the lines overhead, and it becomes the calm, permanent backdrop a yard arranges itself around.
Care
The short version: a full-sun spot with room to stretch, soil of almost any character so long as it drains, attentive water for the first few years, and an easy, hands-off relationship after that.
Light
A bur oak comes into its own in full sun, six or more hours a day, which is what shapes the broad, even crown that gives a yard its sense of scale. A young tree tolerates a little shade, but one raised in real shade grows thin and leans toward the light, never settling into the form a property wants from it.
Water
A young tree appreciates steady water through its first few years while it establishes, the brief season of attention it asks before it stands on its own. After that an established bur oak is very drought tolerant, carrying a landscape through long dry spells and wanting a drink only when a drought turns severe.
Soil
Almost no tree is so unconcerned with the ground beneath it. It does best in moist, well-drained loam yet takes clay, dry sites, and even alkaline soil in stride, which is precisely why it suits the difficult corners of a city lot.
Temperature
This is a tree that holds a landscape through any weather, at ease across -40°F to 100°F (-40°C to 38°C) and hardy through USDA zones 3 to 8. Little in the way of heat or cold within its range disturbs the steady presence it brings.
Humidity
Humidity simply never enters the picture for a bur oak outdoors. It thrives along damp bottomlands and dry prairie edges with equal ease, so there is no level to manage in any setting.
Feeding
An established bur oak rarely needs feeding, drawing what it requires from a generous spread of roots beneath the lawn. Only on genuinely poor soil does a balanced slow-release fertilizer in spring earn its place.
Pruning
Reserve pruning for late winter while the tree is dormant, lifting out the dead, damaged, or crossing branches that spoil its line. Avoid all cutting from April through October, when sap-feeding beetles can carry oak wilt into a fresh wound.
Repotting
none
Size & spacing
Plan around a tree that will reach 60 to 80 feet (18 to 24 m) tall and just as wide at maturity, a true centerpiece for a generous yard. Site it well clear of buildings, foundations, and utility lines so its broad crown and spreading roots have the room a tree of this stature deserves.
When to plant
A bur oak enters a landscape by planting, not by a sowing schedule. Set a container or balled-and-burlapped tree in spring, or work a few fresh acorns into the ground in fall and let time do the rest. The catkins pass quietly through April, and the acorns ripen and fall that same autumn, marking the turn of the year.
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
A settled bur oak asks for very little, and most of what shows up in the canopy is purely cosmetic. The single concern worth keeping in mind is oak wilt, so it helps to recognize it and to honor the timing that holds it at bay. Read the entries below against what the tree is showing before stepping in.
Crown leaves browning at the margins and dropping fast
This is oak wilt, a fungus that clogs the tree's water channels and moves down from the top of the canopy. Because a bur oak belongs to the white oak group, it tends to decline slowly over seasons rather than vanish in one, though drought and root damage can wear the same look. Leave the tree unpruned and unwounded between April and October, when the beetles that spread the spores are active, and bring in a certified arborist to confirm it through 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
Unusual bumps, balls, or growths on leaves and twigs
Those curiosities are galls, formed when gall wasps or mites lay eggs and the tree builds a protective shell around the insect within. They look unusual but rarely trouble a healthy oak, so the graceful response is to let them stay. Trim away a few heavily galled twigs if they distract from the canopy, and keep the tree watered and vigorous rather than 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
For all its presence in a landscape, the bur oak comes with one caveat worth respecting: its acorns, buds, and young leaves are not safe to eat. They hold tannins that turn a cat's or dog's stomach, choke or block the gut when a whole acorn is swallowed, and inflict serious kidney damage on horses and livestock that graze on a lot of them. Keep the autumn drop cleared from spots pets and stock frequent, and remember that raw acorns are not for people either.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that chews a leaf or a fallen acorn tends to vomit, turn away from food, and run loose stools. A whole acorn is also a genuine choking and gut-blockage risk for so small an animal, which is reason enough to keep them cleared.
Dogs Mild
Dogs are the likeliest to crunch acorns up off the ground, which brings on drooling, an upset stomach, and diarrhea. The larger worry is a swallowed acorn lodging in the gut, so watch for repeated vomiting or a tender belly.
Horses Severe
Horses are affected hardest of all. A horse that takes in a lot of acorns, buds, or young leaves can show colic, constipation that turns to bloody diarrhea, a dull mood, and dark urine, and the tannins can do real harm to the kidneys.
Rabbits Toxic
Rabbits are best kept well away from oak. The tannins in leaves, buds, and acorns are too harsh for their gut, and even a small nibble can leave one off its food and listless.
Livestock Severe
Cattle, sheep, and goats can poison themselves on a heavy load of buds, leaves, or acorns, especially once other forage runs short in fall. The result can be kidney damage, weakness, and in bad cases death, so fence stock away from dropping oaks.
Birds Caution
Many wild birds shell and eat acorns with no trouble, yet the same tannins can bother pet birds and poultry, and a whole acorn is far too big and hard for most to handle safely. Offer none to caged birds, and keep watch over backyard flocks ranging beneath a tree.
People Caution
People do not eat raw acorns, and a mouthful is bitter and rough on the stomach because of the tannins. Traditional acorn foods are leached for hours first, so treat anything off the tree as not for snacking and keep young children from putting 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
Bur oak is planted almost always as the straight species, though it does cross out in the wild. The two forms below are the ones worth recognizing on a property.
Quercus macrocarpa (species)
The straight species bears the largest acorns of any North American oak, reaching 1.5 inches across, each cradled in a deep mossy cap that fringes the rim and gives the tree both of its names, bur oak and mossycup oak.
Quercus × schuettei
Where bur oak and swamp white oak share a setting, they occasionally meet in this natural cross, whose leaves and acorns settle gracefully between the two parents.
How to propagate
The most satisfying way to bring a bur oak into a landscape is to plant an acorn and watch it begin, and the white oak group obliges, since the seed sprouts quickly with little persuasion.
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
Bur oak takes readily from seed when the acorns are fresh, which makes raising one from scratch a quiet pleasure. Gather the ripe nuts as they fall and set them straight into the ground that autumn, since white oak acorns carry almost no dormancy and sprout within a few weeks. Acorns from far northern trees settle in better after 30 to 60 days of cold, moist storage at 32°F to 41°F (0°C to 5°C) ahead of a spring sowing. Plant the acorn exactly where the tree is meant to stand, because the seedling sends down a deep taproot almost at once and resists being moved after that.
Myths
Acorns must be safe for pets and livestock since wild animals eat them all the time.
Deer, squirrels, and jays are built to handle the tannins in acorns, buds, and oak leaves, while cats and dogs come away with an upset stomach and horses and livestock can suffer serious kidney damage after eating a lot. What sustains the wildlife under a tree can still poison the animals in your care, so clear the drop and keep stock away from it.