A single small tree in a tiny garden can become both a focal point and a problem solver. If lighted carefully it becomes a jewel in the nocturnal view from windows and doors. The problem is finding naturally small trees that don't produce aggressive root systems. Because planting areas are so limited, a well behaved root system is vital to protecting masonry and utilities.

Dogwood

Cornus florida Rutgers Hybrids

Fauriei Crape Myrtle

Lagerstroemia indica Fauriei hybrids

There is little difference between a large shrub and a small tree. It's all based on how the plant was raised and pruned. Prune a lilac to a single trunk and it becomes a perfectly sized flowering tree for a small garden. If you're lucky enough to have an existing shrub that could be pruned into a single trunk tree rather than replacing it, by all means give it a try.

Monrovia Nursery has made its reputation on growing patio trees, which are shrubs grown with a single trunk topped with a beautiful ball of foliage. These are sold in 5 gallon nursery containers for use in ground or in containers. Monrovia offers many types of shrubs in patio tree forms such as large gardenias, hibiscus, citrus and fruitless olive. These are exceptional in pairs flanking an entry or fountain. Over time the patio tree will grow as large as you let it, or keep it small with gentle shaping.

Related:

Small Yard Landscapes
Accent Trees
Olive Trees
Grouping Trees
Planting Beds