For several years in the late 1960 and early 1970s, European white birch were trendy. Most lived in ubiquitous groups of three. Where three did not fit, a single multi trunk tree, typically with three trunks, was a popular option. Each multi trunk tree provided as many trunks as a few single trunk trees. For these particular white birch, the elegant white trunks were their most appealing feature.
Multi trunk trees, which are popularly known as ‘multis’, are only structurally different from their counterparts with single trunks. Multi trunk crape myrtle are genetically identical to crape myrtle of the same cultivar, but with single trunks. The only difference is that multi trunk trees branch at ground level, instead of at the top of a single straight trunk. Each needs to be pruned to the desired form.
Multi trunk birch, paperbark and lemon gum exhibit appealing bark. More trunks display more bark than single trunks. Multi trunk strawberry tree, olive and oak exhibit appealingly sculptural form. Cork oak and crape myrtle provide both appealing bark and sculptural form. Silk tree, acacia and deciduous magnolia display their bloom more effectively with lower and broader multi trunk form.
Trees get help to develop into a desired form. European white birch, lemon gum and silk tree are more likely to develop single trunks naturally. Coppicing compels them to regenerate with several trunks. Conversely, olive, crape myrtle and strawberry tree develop a few trunks naturally. Single trunk trees need thinning to remove the superfluous trunks, and staking to straighten a single trunk.
In home gardens, multi trunk trees sometimes evolve from overgrown shrubbery. Pineapple guava may be shrubby for may years before lower growth gets pruned away to reveal sculptural trunks within. English laurel that gets too overgrown for containment pruning might become a delightful multi trunk tree instead. It will be pleased to grow freely from the top if lower growth gets pruned off.
Multi trunk trees are no more natural than trees with single trunks are, but they seem to be.
Tony Tomeo 
This is true. I kind of wish now that I had gotten my crabs and chokecherry and multi-trunk.
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I do not remember ever seeing a multi-trunked crabapple tree in a nursery. Of course crabapple trees are rare here, so there is not much selection anyway.
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There are many hundreds of crabs at the Chicago Botanic Garden and I think all are multi-trunked.
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There are many crabs off the pier in Santa Cruz too!
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Ha!
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I could not pass that one up. Anyway, it only makes sense that flowering crabapple trees should be multi trunked or ‘low bud’, to show off their bloom better. Standard single trunked trees are better for where clearance is important, but are higher up. Mult trunked trees suspend their canopies lower, although they can be pruned up if necessary.
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