Red Maple (Acer rubrum) Bonsai

Species name:  Acer rubrum
Common names:  Red maple, American red maple, Swamp maple
Description:  Small to large tree, upright and oval-shaped.   3-5-lobed opposite leaves from 2-5 inches in length with equally long petioles, highly variable even on the same tree.  Bark is light grey in young trees and becomes dark grey and rough on old trees. New and autumn leaves are red, as are new twigs, dormant buds, leaf petioles, flowers, seeds, and actively growing roots.  The root system in moist soils tends to be quite shallow.  One of the most colorful spring and autumn North American trees -  it is related to the sugar maple (Acer saccharum).  The red maple is a fast-growing tree that may live from 100-150 years. 
Range: Eastern Canada and eastern U.S. from New England to Florida, west to Minnesota and Texas.  3 described varieties, but they are not accepted by all botanists due to the extreme variation in leaf shape.  12 or more cultivars have been recognized, often selected for autumn color in horticulture.  In Florida, the red maple is found primarily in moist wetlands, often in swampy areas, but sometimes in drier uplands.
Why use this species for bonsai?  Red maple has rapid growth, attractive leaves, and good fall color.  It is locally adapted to the part of its range where it occurs.  The North American Collection of the National Bonsai Foundation contains a red maple group planting designed and donated by the late Vaughn Banting, showing the bonsai potential of this species.  I usually get good fall color, even in the warm winters we are now having.  It is a strong tree that forgives a lot of mistakes (except underwatering).  Due to the relatively large leaves, it is probably best suited to taller bonsai.
Propagation:  Easy, from freshly fallen seed (no stratification needed), cuttings, or air layering (at least up to 3/4-inch diameter).  Seedlings are easily collected and transplanted in the spring.  Try to get seeds, or even better, cuttings, from a small-leaved tree. 
Training:  In general, follow standard bonsai rules on design; e.g., avoid bar branches, reverse taper, etc.  Training can be in the ground, in growing boxes, or other containers.  From early on, efforts should be made to keep the root system shallow, so that it will gradually become plate-like, for good nebari.  Don't grow roots in a deep container unless you want to develop a root-over-rock style.  You should decide on a style and size early on.  In this area, red maple has a very short period of leaflessness.  Although not as fine-twigged as Japanese or trident maple, the leafless shape can be attractive. It's important that the appearance is also good when in leaf.  Forest or group plants are most easily grown separately for a few years before planting together. If you want small bonsai from this species, hard pruning from the early stages will be necessary to keep the modes short and to produce trunk taper.  This will prevent having to chop off the trunk at the top later on, resulting in poor taper.  If you want to grow a red maple from cutting or seed, you might want to try 2 or three at once- some respond to training better than others.
Refinement:  Takes place after the tree already has good roots with nebari development, good basic branching, and a well-developed trunks.  It involves improving ramification, keeping nodes short, and replacing branches, especially at the top, when they become too thick.  The tree cannot be kept continually in a refined state; it must be allowed enough growth to maintain health.         
Red maple is a top-dominant fast grower; neglect can quickly thicken the top branches of the tree and spoil its shape.  It is also possible to lose lower branches.  Red maple is easy to thread graft using long twigs on the same tree to improve its design and replace lost or damaged branches.  These grafts can be done through the trunk and even through branches, as needed.   Winter is a good time for this, and for starting air layers.
Top dominance can be controlled by cutting strong top growth back to one node, or by pinching off the second set of leaves before they enlarge.  This may restrict node length to as little as a millimeter or two.  Areas lower on the tree need to be allowed to grow to several nodes before cutting to keep their strength.  Some twigs and weak branches may need to grow out long for thickening.  Over time, the top of the tree will likely need twig replacement and rewiring.
Leaf cutting can be done once the plant is in the refinement stage.  Late spring, after the leaves have hardened, is a good time.  I do this only once per season on older plants, and not necessarily every year.  The second set of leaves are smaller and seem to have better autumn color.  There is no point in leaf-cutting plants that are in early development.
Fertilization:  Since red maple is a fast grower, I don't fertilize until the first leaves harden in the spring.  I use fish oil (5-1-1) with humic acid and a chelated iron/trace element liquid.  For fertilizer cakes, I use a combination of bone meal, blood meal, cottonseed meal, kelp meal, coffee grounds, and worm castings.  Red maple is not fussy in this regard - just don't use excessive nitrogen.  In the fall, 0-10-10 is recommended and will cause no harm.  I fertilize about every 10 days during the growing season. Lessen fertilizing during the hottest months if growth slows or stops.
Watering:  Red maple needs moist soil.  It is better to overwater than the opposite.  Lack of sufficient water rapidly results in a sequence of  leaf scorching, leaf death, twig and branch death, and tree death. This can happen in one hot afternoon.  In our long season of hot weather, I water up to 4 times daily.  This is with a timer system; if I'm at home, I water most often by hand as needed.  If you are not using a timer, you must not forget!   In hot afternoons,  maples in full sun like a light shower over them and the benches to cool things down.  Spraying a tree in the full sun will not burn the leaves, despite the myth.  If your hose is lying in the sun, however, run out the hot water before spraying or watering. Afternoon shade is a good location for maples in our area.  Placing pots in a shallow water container during hot weather, as is done for wisteria, might be good insurance againt drying, but I have not tried this.  I grow thick moss on my potted red maples; they like to grow roots in surface moss and this helps protect shallow pots from drying out.  The smooth bark is not harmed by moss and it is easy to  remove if necessary.
Pruning:  Hard pruning is best done in the fall.  In this area, the rise of sap begins early in the winter.  Red maple has strong bark growth to repair injuries.  Even if the center of the tree rots, it will continue growing with rapid  bark formation over the remaning sapwood. 
Prune close with a slight indentation for good bark rollover.  Prune triple branch forks down to the best two twigs.  Triple forks are unsightly and will overthicken the branch junction. During development, cut off any growth with excessively long nodes, even if it means cutting off much of the tree.  Rub off unwanted buds throughout the growing season.  Seal all cuts, even small ones.  
Wiring:  It may be possible to grow upright red maples without wiring, but I think if is easier and faster to use wiring at all stages of development and for fine work on refined trees.  Wiring, combined with pruning, is the best way to develop trunk and branch movement.  Wiring is best done in the fall or winter.  Maple bark is delicate and wiring must be removed early, about the time the leaves have hardened in the spring. Leaf cutting is a second opportunity to rewire.  Don't wire on any tighter than necessary because branch scarring takes several years to disappear.  Aluminum wire is fine.  Branches can also be pulled down by tie-down wires, but protect the attachment points with rubber.  Small diameter auto engine hose works well.  Branch tips should be allowed to point upwards, even on downward sloping branches.
Repotting:  Repot when the leaf buds swell, before they begin to open.  Here, buds open early, from February into March.  Watch carefully after the native red maples in the area bloom - this can be as early as December.  When repotting, the root system must be pruned as carefully as the top, to keep roots lateral and on the same plane.  Seedlings and saplings will tolerate removal of most of the root system, so be sure to select and arrange the best roots when the tree is still young.  Root improvement on old trees should be done gradually.  Young trees should be repotted every year; developed trees can go for 2-3 years.  I've not tried mycorrhizal innoculation for red maple; repotting would be the time to do this.
Soil:  Some have grown red maples in a heavily organic medium (see Jim Lewis' reference), because of its need for moist soil.  I do the opposite, growing potted trees in 50-75% grit (Turface and sand) with the other 25-50% consisting of sieved pine bark.  I train root-over-rock plantings in pure grit- they are in 5 gallon buckets for root development.  These mixtures require frequent watering and fertilizing, but give good drainage during rainy periods.  This also allows for longer intervals between repotting.
Pests and Disease:  Although red maple can be attacked by a variety of insects and diseases, I've experienced few such issues.  There is may be occasional tip shoot borer damage that kills the very tip of growing shoots, causing the terminal leaves to die, but this hasn't been a serious problem for me.  Snip off the terminal leaves and shoot and the remaining shoot will continue growing.  Borer attacks of all kinds may be prevented through the use of a systemic insecticide, but I haven't used this much.  There are fungal diseases that cause leaf spotting, but I've never had to treat for them.
Pot selection:  Maples look best in shallow pots, but this can be stressful in our climate.  One could use a shallower pot in a year when the tree is to be displayed, then a deeper one at other times.  I like unglazed rustic pots, but it's up to you to decide what you like.  
References:
https://www.evergreengardenworks.com/redmaple.htm
Jim Lewis' red maple information sheet is a good reference for this species.  He did some things differently than I, and is one of the few with interest in and experience with this species.  Formerly in Tallahassee, he moved to North Carolina; his Florida red maples apparently have not done well there.
Kurz, Herman and Robert K. Godfrey. 1980.  University Presses of Florida.  311p.  Trees of Northern Florida.  Gives a brief description and illustration of the leaf variation and  ecology of red maple in Florida.  

Prepared by Michael M. Bentzien, June 2019