There have been more than a few people I know that, when starting a garden, opt for the bag of onion sets one finds at most garden centers or big box stores. Later in the season the tops die down, a usual sign that your onions are ready for harvest. When the onions are dug the gardener gets the disappointment of having small onion bulbs or even no bulbs at all. What happened? The gardener did all the right things, used compost, watered well, planted the onion set per the instructions on the package, so why no onions? There may be a variety of reasons behind this, but what the gardener may have not taken into account is the difference between “short day” and “long day” onions.
Onions are one of the first crops that are set out by gardeners as they are pretty cold tolerant. Onion sets and plants are the most popular way of growing onions by home gardeners as the plants are already started, thereby cutting out the work of starting the seeds, however, seeds would obviously be the way to go for those wishing to save heirloom varieties. Seeds are also available from most stores and seed catalogs and should be started in flats before setting out. Onion sets and plants arrive in the stores usually in early spring and you may be tempted to grab what catches your eye, although if you live in the northern part of the country you should be on the outlook for long day onions.
Onions form their bulbs in conjunction with the length of the day. In the summer as the days get longer, onions start to store the energy of the sun in their bulbs. Long day onions need about 14 or so hours of daylight to bulb. This happens normally in early June. Short day onions need about 10 hours of daylight. You would think that the short day onions would then do better in northern areas, but that is not the case. Once an onion starts to bulb, top growth slows. Since the day length in the north is already 10 hours a few weeks into the growing season, the plant has not grown large enough to glean enough energy from the sun needed to form a full bulb. The result is small bulbs at the time the plant goes dormant.
Short day onions, grown in the south, are planted during the cooler months when the day is shorter. As the day lengthens in the southern latitudes, the onion bulbs out. This is normally during a different time of the year than it would be happening in the north. Unfortunately, many stores in the northern part of the country stock onion sets and plants started in the south. Many times these are actually short day onions and will not do so well for the northern gardener.
There are also varieties that are day neutral. Day neutral onions form bulbs regardless of daylight hours and produce well in most of the country. A good seed catalog or garden center will label different varieties of onions with the appropriate day length label. Note that some may refer to the latitude range that the onion variety does best in.
One more thing about onions and day length varieties: many people are aware of this distinction but still get confused as to which variety is grown in which parts of the country. They assume that since the southern parts of the country are generally warmer, that means that the days are longer. This may be true in a way during the winter months, nevertheless not true in the summer. The further north you are in the summer, the longer the day is. For example, on June 21st, the day length north of the Arctic Circle is 24 hours!