Start by choosing containers that suit the style of your garden and that are large enough to house the rootball of your tree. Ensure that the container has plenty of drainage holes. Do not put a small tree in a very large container; instead pot up the plant in stages, finishing with a minimum final container size of about 45cm (18in).
Frost-proof terracotta pots are heavy, providing extra stability to prevent trees blowing down in windy weather. However, the porosity of these pots means the compost dries out quickly and the weight makes them difficult to move around. Lighter-weight plastic is a good choice if you need to move plants around and they are good at retaining moisture. Metal, wood and stone pots are also available.
Most trees will thrive in a loam-based compost, such as John Innes No 3, which is fairly heavy, providing good stability. Before planting, add some pieces of broken terracotta pot to the base to stop compost washing out of the holes and raise up on pot feet to prevent waterlogging under containers. Acid-loving trees such as Amelanchier should be planted in loam-based John Innes Ericaceous compost.
Trees in containers will dry out far more quickly than those growing in the ground and will need regular watering. Several waterings may be needed to ensure the full depth of the compost is moistened. Often the overhanging foliage of plants in containers shields the compost from the rain. Keep the compost moist, but not wet, at all times, even during winter when compost can dry out quickly in wind. In hard water areas, use rainwater for watering ericaceous plants wherever possible, with tap water as a back-up option.
Give trees a boost, by refreshing the top layer of compost annually in spring. Carefully scrape away about 5cm (2in) of compost from the surface and replace with fresh compost – mix in some controlled release fertiliser granules. Alternatively use a liquid feed at regular intervals.
In exposed gardens, plants in containers will need protecting during winter as the roots growing near the edges are vulnerable to freezing temperatures. Wrap the container with bubble wrap or hessian, held securely in place with twine.