Tree · Fagaceae
Bur Oak
Quercus macrocarpa
Plant a bur oak and you hand down a tough, century-spanning shade tree that shrugs off cold, drought, and city soil.
Full sun
Every 7–14 days
USDA 3–8
60–80 ft · 18.3–24.4 m
Overview
The bur oak is one of the toughest trees in North America, a slow but steady giant that can live for centuries and end up 60 to 80 feet (18 to 24 m) tall and just as wide. It belongs to the white oak group and shrugs off conditions that would finish softer trees, from bitter winters down to -40°F (-40°C) to dry uplands and tight city clay.
Its acorns are the largest of any native oak, sometimes 1.5 inches across, capped in a deep fringed cup that looks mossy and gives the tree its other name, mossycup oak. Catkins hang yellow-green and barely noticeable in April, and the acorns ripen in a single season the same fall.
This is a tree you plant for the long haul and for room to spread. Give it full sun and space well clear of houses, foundations, and power lines, and it asks for almost nothing once its roots take hold.
Care
The short version: full sun and plenty of room, soil of almost any kind as long as it drains, steady water for the first few years, and very little fuss after that.
Light
Bur oak wants full sun, meaning six or more hours a day, and that's what gives it a broad, even crown. It will take some light shade when young, but a tree grown in real shade ends up thin and leaning toward the light.
Water
Water a young tree regularly through its first few years while the roots go deep, then step back. An established bur oak is very drought tolerant and handles long dry spells on its own, asking for water only in a severe drought.
Soil
Few trees are less fussy about ground. It does best in moist, well-drained loam but takes clay, dry sites, and even alkaline soil in stride, which is why it holds up so well in tough city spots.
Temperature
This is a cold-hardy tree to the core, comfortable across -40°F to 100°F (-40°C to 38°C) and hardy through USDA zones 3 to 8. Few extremes of heat or cold in its range trouble it.
Humidity
Outdoor humidity is never a concern for a bur oak. It grows across damp bottomlands and dry prairie edges alike, so there's no level to manage.
Feeding
An established bur oak rarely needs feeding and pulls what it needs from a wide root run. If the soil is genuinely poor, a balanced slow-release fertilizer in spring is plenty.
Pruning
Prune only in late winter while the tree is dormant, taking out dead, damaged, or crossing branches. Avoid all cutting from April through October, when sap-feeding beetles can carry oak wilt into a fresh wound.
Repotting
none
Size & spacing
Plan for a tree that reaches 60 to 80 feet (18 to 24 m) tall and just as wide at maturity. Site it well clear of buildings, foundations, and utility lines so its broad crown and spreading roots have room.
When to plant
A bur oak is planted, not sown on a schedule like a vegetable. Set out container or balled-and-burlapped stock in spring, or push fresh acorns into the ground in fall. The catkins come and go unnoticed 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
A healthy bur oak rarely gives trouble, and most of what you'll see is cosmetic. The one problem worth taking seriously is oak wilt, so learn its look and the timing rules that keep it out. Match what you see below before you act.
Crown leaves browning from the edges and dropping fast
This is oak wilt, a fungus that plugs the tree's water channels and kills from the top down. A bur oak belongs to the white oak group, so it usually declines slowly rather than dying in a single season, but drought and root damage can mimic the same look. Never prune or wound an oak between April and October, since the beetles that spread the spores are out then, and call 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
Odd bumps, balls, or growths on leaves and twigs
These are galls, made when gall wasps or mites lay eggs and the tree grows a protective shell around the developing insect. They look strange but almost never hurt a healthy oak, so the usual answer is to leave them alone. Snip off a few heavily galled twigs if they bother you, and keep the tree watered and vigorous rather than reaching for sprays.
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
This is a valued shade tree, but its acorns, buds, and young leaves are not safe to eat. They hold tannins that upset the stomachs of cats and dogs, choke or block the gut when a whole acorn goes down, and cause serious kidney damage in horses and livestock that graze a lot of them. Rake up the fall drop where pets and stock can reach it, and remember that people don't eat raw acorns either.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Mild
A cat that chews leaves or a fallen acorn tends to vomit, lose its appetite, and run loose stools. A whole acorn is also a real choking and gut-blockage risk for a small animal.
Dogs Mild
Dogs are the ones most likely to crunch acorns off the ground, which brings on drooling, an upset stomach, and diarrhea. The bigger danger is a swallowed acorn lodging in the gut, so watch for repeated vomiting or a tender belly.
Horses Severe
Horses are hit hard. A horse that eats 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 wreck the kidneys.
Rabbits Toxic
Don't let rabbits near 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 lethargic.
Livestock Severe
Cattle, sheep, and goats can poison themselves on a heavy load of buds, leaves, or acorns, especially when other forage runs short in fall. The result is 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, but 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 under a tree.
People Caution
People don't 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 mostly grown as the straight species, but it does cross in the wild. Here are the two forms worth knowing.
Quercus macrocarpa (species)
The straight species carries the biggest acorns of any North American oak, up to 1.5 inches across, each sitting in a deep mossy cap that fringes the rim and gives the tree both its names, bur oak and mossycup oak.
Quercus × schuettei
A natural cross between bur oak and swamp white oak that shows up where the two grow side by side, with leaves and acorns that fall partway between the parents.
How to propagate
The simplest way to get a bur oak is to plant an acorn, and the white oak group makes that easy because the seed sprouts fast with little fuss.
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 grows easily from seed if you start with fresh acorns. Gather the ripe nuts as they fall and sow 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 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 right where you want the tree, because the seedling drives down a deep taproot almost at once and resents being moved after that.
Myths
Acorns are safe for pets and livestock because wild animals eat them all the time.
Deer, squirrels, and jays are built to handle the tannins in acorns, buds, and oak leaves, but cats and dogs get an upset stomach from them and horses and livestock can suffer serious kidney damage when they eat a lot. What feeds wildlife can poison your animals, so rake up the drop and keep stock off it.