Common names: Bur Oak, Burr Oak, Savanna Oak, Overcup Oak, Prairie Oak, Mossy-cup Oak, Mossy-Overcup Oak, Blue Oak
Burr oak is a relatively slow-growing (less than a foot a year) large, deciduous tree with a very wide, open crown. Its many crooked branches and deeply trenched bark give it a lot of character that some see as unattractive, but we disagree, but make up your own mind!
Its massive trunk anchors heavy, mostly horizontal limbs with deeply ridged bark. Leaves can be an impressive 9 inches. Its over-sized acorns (the largest of all Texas native oaks) are quite distinguishable with very deep fringed cups. The acorn cup is where this oak gets its name because it resembles the spiny bur of a chestnut.
Burr Oak Native Range
The Burr oak is just as durable as its craggy appearance looks growing in a native range from Canada in Saskatchewan east to New Brunswick across the northeast and Atlantic board states southwest to Texas and north up through Montana. Its adaptability to varying light, soil, moisture, and temperature conditions make it capable of growing on prairies, open wood, stream edges, sandy ridges, and cold rocky mountains.
General Characteristics of Burr Oak
- Tree Size: large tree, generally wider than tall
- Exposure: sun to shade
- Fruit Characteristics: winged samara in the fall
- Height: can reach 100 feet though 36-72 feet is more likely growing shorter in more northerly climates
- Width: can reach 100 feet wide, but usually about 80% of its height
- Plant Character: deciduous
- Heat Tolerance: very high
- Water Requirements: medium
- Soil Requirements: slightly acidic preference but adaptable to alkaline; very versatile growing well in sand, clay, caliche, limestone-based, or sandy, clay, or medium loams; adaptable to dry or wet conditions
- USDA Hardiness Zones: 3-8
Leaf Shape & Habit
Often over-sized leaves are 6–12″ long leathery leaves with 5–9 lobes separated about halfway down by a pair of particularly deep sinuses. Leaf characteristics:
- Leaf Complexity: Simple
- Leaf Shape: Elliptic , Ovate
- Leaf Venation: Pinnate
- Leaf Margin: Lobed
- Hairs Present: Yes
- Leaf Base: Cuneate , Rounded
- Leaf Color and Sheen: Bright glossy green being brown/copper or gold/yellow in the fall
Bark, Limbs & Canopy
The Burr oak’s bark is ashy gray to brown color in a scaly in deeply vertical ridges.
Its massive trunk anchors weighty horizontal limbs that are generally low and spreading. The resulting canopy is often quite wide though may also be rounded depending on how wide the horizontal limbs grow before branching into small limbs.
Flowers, Fruits, & Seeds
Flowers in late spring with the emergence of leaves after winter dormant period. Yellow-green to brown tiny barely noticeable flowers form in thin, hanging catkin clusters. Wind-pollinated. Rarely self-pollinated.
Acorns are round in shape from 1-1½” long. The acorn cap is fringed rather like a bur (its namesake) and surrounds ½ or more of the top half of the acorn.
Habitat & Conservation
Its ample branching and dense canopy attracts songbirds, ground birds, and many types of mammals such as squirrel and rodents. The burr oak acorns are favorites of many types of birds and mammals including rabbits, squirrel, rodents and deer.
Its bark creates havens for insects that in turn attract insectivorous birds. In fact it is supports a variety of moths and butterflies where they lay their brood.
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