You can get a bigger container for a much better price than a regular pot, allowing you to grow more plants than otherwise.
You just have to make sure there are enough drainage holes drilled. Placing the tub on bricks or some other sort of platform will further aid draining.
Onions are another vegetable that can do well in a tub (or any other container.) You need a container that is at least 10″ deep. You would also want to get one that is at least a few feet wide – that way you can harvest lots of lovely onions.
For onions, you can plant either seeds (which takes the longest), baby onion bulbs (referred to as “sets”) or transplants.
Seeds should be started inside, especially in areas with shorter growing seasons, and planted 1/2″ deep. Find out when your last frost date is, and plant your seeds 2-3 months before that.
Sets should be planted 2″ deep and at least 4″ apart to make sure you get bigger onions. You can start planting these after temperatures are around 50F.
Transplants are your best choice if you want large onions. They should be planted 4″ apart at least.
No matter what method you use to start your onion plants, make sure that your onion plants get as many hours of sunlight a day as possible – at least 10-12 hours for “short-day”(Southern) varieties and 14-16 for “long-day” (Northern) varieties. Most catalogs will note which kind each variety is, or you can check with your local extension office for suggestions that will work in your area.
They also need to get at least 1-3″ of water a week. Be sure to check and see if water is needed – if the top of the soil is dry, it is time to water.
You will know it is time to harvest when most of the leaves have fallen over and begun to turn yellow (however, if any have flowered, dig them out right away and use as they will not store well). Pull them out early in the day and let them dry for most of the day. They are then ready for storage. The length of time that they are able to be stored depends on the variety – sweeter varieties do not store as well.