Today's dahlias are available in every colour in the rainbow and provide blooms that will last most of the summer. Gardeners with limited space can grow them in pots, urns, tubs or any other container that will meet the plants' space requirements. Container gardening also offers the advantage of portability. Plants like dahlias that require lots of sunlight can be moved to take full advantage of available daylight. Unlike dahlias that must be lifted from the garden each fall before the ground freezes, potted varieties can be easily stored in their containers during the colder months.
Select the container in which to grow your dahlia. It should measure at least 12 inches across the top from side to side. Place several pieces of broken clay pot or several small stones over the hole to prevent water from draining too quickly.
Combine potting soil with garden soil using a ratio of 2:1. This means using 2 parts garden soil to 1 part potting soil. Make enough mixture to fill the pot to about 4 inches from the top, which will leave room to add more soil as the plant grows.
Add a handful of bone meal to the soil in the pot and mix it in well with the garden trowel. Dig a hole in the centre of the soil to a depth of 6 inches. Set the soil on a sheet of newspaper to be added to the pot later.
Place the dahlia tuber horizontally in the bottom of the hole. Dahlia tubers develop several eyes, or areas of new growth, along their sides. Locate them and try to position the tuber in the hole with the eyes facing up as much as possible, as new plant shoots will develop from them.
Fill the hole above the tuber with some of the reserved soil. Tamp it down lightly and set the container in a sunny spot for the summer. Water the soil evenly but do not saturate it, and water regularly whenever the soil seems dry.
The 12-inch container size will do for average-sized dahlias. Check the plant's maximum height on the package and adjust the size of the pot to compensate. For instance, if the plant will grow to 3 feet, then choose a larger container to accommodate its root growth. As the plant grows, add more soil mix to give it stability. If the plant seems top-heavy, push a bamboo garden stake into the soil about 3 inches from the base of the plant and secure the plant to it with garden twine.