Most people think of Japanese bonsai in reference to growing trees in containers. But larger ornamental trees can grow in containers as well. An ornamental tree in a container makes a decorative addition to rooftop, balcony and patio container gardens. Gardeners who wish to grow trees in containers must account for factors such as wind and give plants water and nutrients that they would normally get themselves from the soil.
Select a heavy container for growing your tree. Trees are top-heavy plants that normally anchor themselves to the soil with their roots. By choosing heavy containers, you weigh down the tree's root system and compensate for the tree's inability to stabilize itself. Match the container size to the tree's size and repot the tree as it grows. If you place a tree in a container that is too large, the container will hold too much water in the soil and kill the tree through root rot. A thick-sided container such as a concrete container will help trees to withstand temperature extremes and overwinter more efficiently.
Choose a commercial potting mix for growing your container tree. Never fill a container with native soil. The soil contains microbes that can multiply to unmanageable levels and injure a plant when grown in the containers.
Select a tree that has a smaller growing habit. A tree with a large growing habit, such as a live oak, can grow as much as 50 feet tall and wide. Trees such as this can quickly outgrow any container they are put in. A dwarf or smaller ornamental tree such as a dwarf apple or a Japanese maple will maintain a smaller shape and grow well within containers.
Cover the drainage holes of a container with window screen to allow drainage but restrict soil from leaving the container. Fill the container 1/3 full of soil. Place the root ball of the tree into the container and fill in around the sides with potting soil. Do not cover the top of the tree's root ball. Mulch shallowly over the soil with pebbles or sand to help hold in moisture.
Check the tree's soil daily for dryness. Trees in containers dry more quickly than trees in soil. Use a soil moisture probe for depth. Trees have varying water needs, depending on their type. An olive tree might almost completely dry out between watering, while Japanese maples should be watered so that the soil remains as damp as a wrung-out sponge.
Fertilize the tree with a liquid, nitrogen-rich fertilizer diluted with water in spring and summer when the tree produces flowers, foliage and fruit. Decrease fertilizer in fall and winter when the tree goes dormant. Trees should be fertilized according to the directions on the fertilizer packaging.