Keep the area weed free and water in dry periods. Feeding. Harvest your spring onions after 8 weeks. Growing Onions. Regrow? Once you are done with them (any of the above four), simply place the root end in a jar of water & it will begin to regrow within just a few days. "Being a complete novice, I found all the step-by-step information easily to follow from start to finish. FYI: The same growing method can be followed for many types of alliums: spring onions (like I used), leeks and ramps, to name a few. This image may not be used by other entities without the express written consent of wikiHow, Inc.