Two gardeners can plant healthy onion seedlings, water carefully and end up with very different bulbs. One detail on the seed packet may explain more than another bag of fertiliser: whether the variety is short-day, intermediate-day or long-day.
These names describe the daylight response that helps trigger bulbing. They do not mean that one onion likes shade or that another needs an unusually long growing season. Latitude and planting season affect the daylight a crop encounters, so a variety that performs well in one region may be a poor match elsewhere.
Read the variety description before the planting calendar
The University of Minnesota’s onion guide recommends long-day types for its northern growing conditions. It explains that an unsuitable short-day type can begin bulbing too early there, before enough leaf growth has developed. That is regional advice, not an instruction to plant long-day onions everywhere.
A productive search therefore starts with your local agricultural extension service or a seed supplier that identifies regional suitability. Look for the actual variety name and day-length class. A label such as “large”, “sweet” or “red” does not provide the same information.
A simple way to compare two varieties
For a small trial, choose two locally suitable varieties rather than deliberately setting up an unsuitable one to fail. Give each a labelled section in the same bed and keep a short record:
- Variety name, day-length class and seed or transplant source.
- Planting date and approximate seedling size.
- The date you first notice clear bulb swelling.
- Harvest size and any losses, recorded separately.
Keep spacing and watering comparable so that the notes answer a useful question. If one section was crowded or shaded, write that down instead of attributing the difference entirely to genetics. A photograph from the same position each week can show the transition from leafy growth to bulb development surprisingly clearly.
Day length is one part of onion growing, alongside temperature, plant establishment and ordinary crop care. The practical benefit of checking it early is that you can rule out a basic variety mismatch before changing everything else. Save the packet with your harvest notes; “the onions from last year” is much less useful than a name you can look up and grow again.