What kind of trees can be planted in a large planter? I am interested in slow-growing trees.
There are so many shrubs and trees that can be grown in containers it is difficult to select just a few. We recommend consulting a specialized reference book and choosing those trees that meet your landscaping plans, are suitable for your climate zone, and the location in which they will be planted (garden, patio, terrace, etc.). If you are concerned about size then remember that all trees will grow more slowly when grown in pots (under restricted conditions) than they would in the ground.
An important factor in growing trees is containers is to choose a pot that is large enough. As a rough guide the pot should be about three times as wide as the root ball. As the tree grows you will need to transplant it to a larger pot or prune the roots. Pots should have holes for drainage.Soil mixes suitable for containers are available commercially. They usually contain peat moss, vermiculite, perlite and fertilizer. This web-site contains good advice on how to plant trees in containers - https://www.arborday.org/trees/planting/containerized.cfm. Remember that trees grown in containers need more watering than trees grown in the ground.
The following plants are worthy of consideration:
Maples grow well in containers as they have shallow roots systems. Amur maple (Acer tataricum subsp. ginnala) has lovely fall color. A host of Japanese maples make beautiful container plants, e.g. Acer palmatum 'Dissectum'.
For spring flowers, consider apples, crab apples or red bud (Cercis canadensis). Crape myrtle (Lagerstroemia indica) is a good choice for summer flowers.
Conifers make excellent container grown plants. You can refer to the NYBG guide Conifers as Container Plants for some great ideas. Mugo pine (Pinus mugo) is particularly suitable as it can be kept small by pinching out the new green shoots (candles) in early summer. Hollies are also good. The Chinese holly cultivar (Ilex cornuta 'Burfordii Nana') is a dwarf variety particularly suitable for containers.
Numerous books on container gardening are available, some of which have sections on trees, e.g.
Container Gardening. The American Horticultural Society, Mount Vernon, VA (1981). pp. 66-75.
S. Connolly (Ed.) Gardening for Containers & Window Boxes. Chartwell Books, Edison, NJ (1995). pp. 35-43.