The tree watering bags sold at gardening centers deliver the slow, steady water trees need, especially when they're young. Commercial watering bags are built with special materials that result in a flexible bag that has the rigidity of a bucket. Because these materials aren't easily available to the thrifty do-it-yourselfer, it's best to pair a plastic bag with a garden container. The container ensures the watering system stays in place near the tree, while the bag prevents too much water from escaping from the container's existing drainage holes.
Place an opened large, heavy-duty plastic bag into a large garden container that holds 10 to 20 gallons. Smooth the bag against the sides and bottom of the container to form a liner.
Turn the bucket over. Use a knife or nail to poke two or three 1/8-inch holes into the bag, each centered in one of the container's existing drainage holes.
Set the container right side up again. Make sure the bag's holes and the container's holes line up.
Set the bucket about 12 inches from the base of the trunk, and use a hose to fill it with water.
Check the rate of flow during the first week or two. The bucket should drain every few days, needing a refill about twice a week in dry weather. If the bucket drains in one day, replace the plastic bag and poke fewer holes in the bottom of the bag. If it drains too slowly, poke another hole or two in the bottom of the bag.