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March 2002
Plant of the Month
Minnesota Native Oaks
(Quercus spp)
tree
zone: 4
by Tim Power
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Minnesotas Native Oaks
Oaks have made a dramatic resurgence in the nursery marketplace in the last twenty years. Here at Laws Nursery, we used to plant a few gnarly tap-rooted northern red and eastern pin oaks, with poor growth and worse transplant success. Then the major bare-root growers instituted new propagation and cultural techniques, producing vigorous, diffuse-rooted liners and allowing us to grow most of Minnesotas native species. Oaks now represent about 5% of our planting schedule and generate 7% of our sales volume. Bare-root oaks may need to be sweated before planting to ensure good bud break. B&B oaks transplant best in the spring just as the buds swell. Container-grown oaks move anytime, but require careful water and nutrient management during establishment. Be sure to check all trees for stem girdling roots and proper depth to first root at planting time.
Oaks require a little extra work and time to produce, but they are superb shade trees, with strong branch angles and unmatched longevity. Minnesotas oaks are all large trees, inappropriate under power lines, but ideal specimen trees in sites suited to their size. Oaks are particularly sensitive to soil compaction. Many stately oaks have been effectively strangled by construction equipment parking, operating and changing the grade over their roots. Since the damage often doesnt become apparent for several years, homeowners can be mystified by their oaks gradual decline and death, and many contractors dont seem to understand the connection. Oaks are subject to a wide array of leaf and twig galls, which generally cause little more than cosmetic damage in Minnesota. Most other insect problems are a result of stress caused by poor site selection or improper care after planting. Oak anthracnose can cause distorted leaves and reduced photosynthetic capacity, but rarely causes death. Oaks in the red oak group (northern red, northern pin, eastern pin) are far more susceptible to oak wilt than are oaks in the white oak group (white, swamp white, bur). Since this disease is largely transmitted by root-grafting in the wild, planting specimens away from oak woods or from one another may help alleviate this problem. The disease organism can also be transmitted by pruning equipment, so we do not summer-prune oaks before late July.
Swamp white oak (Quercus bicolor), pictured above, is native to riverbottoms in southeastern Minnesota. It thrives throughout zone 4, and transplants and performs well even in upland soils and prairie winds. Leaves are deep green above and silvery white beneath. Fall color is yellowish-brown to red, but the horizontal branch structure and exfoliating bark provide winter interest. Swamp white oak and northern red oak transplant easily, though larger sizes are best transplanted B&B.
Bur oak (Quercus macrocarpa) is the oak of our oak-savanna ecosystem. Native as far northwest as Manitoba, it handles just about any soil type and withstands heavily polluted air. Fall color is yellow-green to brown, but the corky bark, sturdy form and the ability to withstand low humidities and cold winds make this tree ideal in inhospitable sites. Bur oak is tougher to transplant than either swamp white or northern red oak, but not as tough as white oak (Quercus alba), which we have been unable to grow from bare-root liners. Container-grown liners may help us solve this problem. White oak is slow-growing, has a beautiful form and structure and is a wonderful addition to the large landscape if you can get it started.
Northern red oak (Quercus rubra) is native through most of Minnesota, including prairie edges and the Boundary Waters Canoe Area. Its pyramidal form in youth becomes rounded with age as it matures to 50-70 high and 40-80 wide, among the largest of this group. Both northern red oak and northern pin oak (Quercus ellipsoidalis) require well-drained sites, and northern red oak will survive in higher-pH soils than northern pin oak.
Northern pin oak is a relatively recent addition to the nursery marketplace. Native throughout the Great Lakes area and northwest as far as the SE corner of North Dakota, it thrives in light, sandy soils. Though somewhat sensitive to high-pH soils, it does far better in alkaline sites than does eastern pin oak (Quercus palustris), its close relative. Eastern pin oak becomes chlorotic even in neutral soils, and is far better suited to the poorly-drained acid soils in its native range south and east of us. Northern pin oak has a reputation for being harder to transplant than eastern pin oak, but in our limited experience, we have seen no difficulties moving it B&B in the spring. Its rich green leaves turn a fabulous orange to red in the fall and remain on the tree through much of the winter.
Recent years have seen much hybridizing between oak species. Earl Cully has introduced two selections that are particularly intruiging to me. Heritage is a bur x English oak cross and Regal Prince is a swamp white x fastigiate English oak combination. Time will tell if they match the regal splendor of our native species.
Tim Power,
Laws Nursery
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