Onions are a fairly easy vegetable to grow, being that they are a cool-season crop and can be successfully grown throughout most of North America. There are many different varieties of onion, thus ideal planting times and the duration from planting to harvest tends to vary a little bit. In general, the overall length of the day is what affects the growth time of onions. Different onions do better with different lengths of day, and will require different amounts of sunlight for bulbing to occur. Generally, the different varieties of onions are split into two different groups: long day onions, which are grown in northern latitudes, and short day onions, which are grown in southern latitudes. As a general rule, because of the different conditions, long day onions tend not to do well in the South, and short day onions tend not to do well in the North.
Under ideal conditions, with plenty of sun in a properly prepared bed with well-draining soil of a neutral pH (from 6 to 7.5), along with an adequate amount of water and nutrients, a general outline of growing times can be estimated. Usually, for maincrop onions, seeds should be sown as early as possible (as early as the soil will allow), which tends to be from late February to early April. Once planted, with proper care, germination will take effect in about 21 days. Over the course of the growing season, assuming normal weather conditions, the general harvest time for maincrop onions is somewhere between August and September. If you’re growing Japanese Varieties, then the harvest time will fall from June to July. And if you’re cultivating Spring Onion varieties, harvest them between March and October. You can tell your crop is fully mature when the leaves turn a yellow color and fall over. Once this happens, wait a few weeks and then harvest.