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oak A strong, hard, heavy, wood.

oak moth A lepidopteran insect that is pale brown or tan. Their larvae are caterpillars that eat foliage and are about an inch long. When full sized, the larvae have bulbous brown heads and olive green bodies with black and olive or yellow stripes. They mostly attack oak trees, and are only a significant problem when populations are heavy enough to defoliate trees. They will usually only have populations of this size for 2 to 3 years and then diminish in number. They are most active in climate zones 7 to 9 and 14 to 24.

oak root fungus The Armillaria fungus causes this disease, breaking down root tissue, and girdling plants. It not only affects oak, but about 700 other species of plants. Infected plants' leaves turn yellow, wilt, and die. In fall or winter, clumps of mushrooms may appear at the plant crown or on soil near the plant. Planting resistant plants and avoiding sites with poor drainage will help prevent this disease.

Oa layer or Oa horizon The humus layer within the topsoil. It is highly decomposed organic residue. Here the decomposable components of the soil have been affected by physical and chemical processes until decomposition is essentially complete. At this stage nitrogen can be supplied from this horizon for plant growth instead of robbed for decomposition of materials.

obcordate A leaf that is inversely heart-shaped with the notch being at the apex (heart-shaped with small end basal). Compare with cordate.

obelisk

obelisk A tall, four-sided shaft tapering to a pyramid at the top.

oblanceolate In botanical terms, leaves with the broad end near the tip and tapering or narrowing to the base from the tip.

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