image by Josep Altarriba
Onions will grow indoors if taken care of properly. Because they need a lot of light, you may have to use a grow light. If you love onions, indoor container gardening will provide you with fresh onions year round.
Purchase a container that is several feet wide or long, and about 1 foot deep. If the container does not have drain holes, cut or drill holes in the bottom of the container. Fashion a catch for the water that drains out of the holes; place a tarp over a long cardboard box, or use a plastic tray.
Mix soil and compost together--one part compost to five parts soil. Fill the container to about 10 inches.
Plant onion sets at least 3 inches apart, and deep enough so the very top of the set is just under the soil. If you start with sets, you will have onions this year. However, if you start with seeds, you will not have onions until the second growing cycle.
Water the onions between 2 to 3 inches a week. If the top of the soil feels dry, water the onions more.
Give the onions at least six to seven hours of light each day. If you do not have a window that provides that much light, use a fluorescent or shop light over the container to provide additional light.
Harvest the onions when a quarter to half of the stalks have fallen over and the skin on the top of the onion is like thin paper. They may not all be ready at the same time, so inspect each onion before pulling it out of the soil.