American Red Maple - Acer rubrum

General Information: Also called Swamp Maple, A. rubrum in nature is a tree, 60 to 75 feet tall. Young trees are often pyramidal or elliptical and are fast growers with strong wood. Older trees develop ascending branches, resulting in an ovoid or rounded crown. There is ample evidence to suggest that trees native to a specific area are more likely to be hardy there. Tolerates ozone and is somewhat tolerant of sulfer dioxide, making it a good tree for urban areas.

Leaves are opposite, 2 to 4 inches, quite variable in shape, 3-5 lobes, dullish green, with reddish leafstalk. They turn brilliant red, orange and yellow in fall.

Family: Aceraceae

Lighting: Full sun to part shade.

Watering: A. rubrum prefers wet to moist soils.Water copiously during the warm months, especially if the soil does not hold water. In hot climates keeping the tree in a shallow tray of water may be necessary.

Feeding: Fertilize weekly, beginning in early spring; bi-weekly in fall until leaves begin to turn.

Pruning and wiring: A. rubrum can be styled as informal upright, slanting, clump, group and forest. I have never grown A. rubrum as a cascade or root-over-rock, since neither represents its "natural" growing habitat.

The bark of A. rubrum (as other maples) is tender and scars easily. On the rare occasion that I do wire, I use unstripped aluminum wire. Since the natural growth habit of A. rubrum is to have ascending branches, forming a generally oval shape with a rounded top, wiring to pull down branches is not necessary.

Wires must be watched carefully. Even rubber-coated aluminum wire will scar before a summer has passed.

Pruning is the primary means of shaping A. rubrum. Pruning and leaf trimming can be used to increase the ramification of twigs, to induce branches to grow where none grow now, and to create valuable open spaces.

Trim new growth by allowing one or two nodes to grow, then pinching back to one. The more frequent and more severe the pinching back, the shorter the distance between the leaves and new branch nodes.

Major pruning should be undertaken with care. A branch, once removed, is difficult to replace. Although you might be able to induce sprouting at the site of the scar, the resulting branch will be thin and awkward looking. In the case of major branch removal, A. rubrum will repair pruning scars quite well if the cut is made flush with the trunk.

A. rubrum will endure leaf stripping (complete removal of leaves), but leave the leaf stem attached (it will fall off when new leaves appear). You also may cut leaves in half. I have seen no benefit from partial leaf stripping (stripping leaves from selected areas of the tree in an attempt to develop better twig and branch structure).

Propagation: Seed germination Up to 60%.They also propagate easily from cuttings.

Repotting: Small specimens may be bare rooted for transplant. Larger specimens must be balled and burlapped in order to be moved.

Potting and root pruning should be done in spring, as leaf buds turn a brighter red. This tree, when mature, is quite hardy and will suffer severe root pruning to no disadvantage that I can see. (However, I would not strip leaves in the same year as the severe root pruning.) Since this is a tree of usually wet to damp soils, the tap root is not well developed.

Root pruning probably should be done every other year on a mature maple, but the amount of annual root growth will vary, depending on conditions, the pot, fertilization schedule, watering regime, and the individual tree. Younger trees and seedlings-in- training may be root pruned annually.

Pests and diseases: Pests: Leaf stalk borer and petiole-borer cause the same type of injury. Both insects bore into the leaf stalk just below the leaf blade. The leaf stalk shrivels, turns black, and the leaf blade falls off. The leaf drop may appear heavy but serious injury to a healthy tree is rare. Gall mites stimulate the formation of growths or galls on the leaves. The galls are small but can be so numerous that individual leaves curl up. The most common gall is bladder gall mite found on silver maple. The galls are round and at first green but later turn red, then black, then dry up. Galls of other shapes are seen less frequently on other types of maples. Galls are not serious, so chemical controls are not needed. Crimson erineum mite is usually found on silver maple and causes the formation of red fuzzy patches on the lower leaf surfaces. The problem is not serious so control measures are not suggested. Aphids infest maples, usually Norway Maple, and may be numerous at times. High populations can cause leaf drop. Another sign of heavy aphid infestation is honey dew on lower leaves and objects beneath the tree. Aphids are controlled by spraying or they may be left alone. If not sprayed, predatory insects will bring the aphid population under control. Scales are an occasional problem on maples. Perhaps the most common is cottony maple scale. The insect forms a cottony mass on the lower sides of branches. Scales are usually controlled with horticultural oil sprays. Scales may also be controlled with well-timed sprays to kill the crawlers. If borers become a problem it is an indication the tree is not growing well. Controlling borers involves keeping trees healthy. Chemical controls of existing infestations are more difficult. Proper control involves identification of the borer infesting the tree then applying insecticides at the proper time. Twig borers can cause die-back of the terminal 8 to 12 inches of small-diameter branches. This is usually not serious and does not require control measures, but it can be a problem on young trees in the nursery.

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