Tip burn

A deficiency of potassium may cause the tips of older leaves to die back.

Day Length

Onions don’t always perform as expected because of their sensitivity to day length. The days must be of a certain length to induce bulb formation and different varieties need different day lengths. If a long day variety is planted where the days are too short, it will grow well enough, but may never produce bulbs. If you plant a short day variety where the days get too long, it will bulb prematurely and the bulbs won’t get very big. 

Short day: 11 to 12 hours

Intermediate day: 12 to 14 hours

Long day: 14 to 16 hours

Very long day: 16 or more hours

Day neutral: A few varieties are day neutral.

Generally short and intermediate day varieties are grown below 40 degrees latitude, long day types above this line.


Onions will sometimes bolt instead of bulbing, or bolt while they are bulbing. This usually happens because the plants were vernalized. For a plant to be vernalized it must be at least ¼˝ in diameter (smaller plants aren’t usually affected) and must be exposed to temperatures below 50˚ F for two weeks. When warmer weather returns, the plant thinks it has gone through a winter and so sets about following its destiny, which is to produce seed.

Bolting doesn’t affect edibility, you can just remove the woody stem core when chopping the bulb. However it causes the plant to devote some of its energy to flowering, so the bulb doesn’t get as big as it could have.


The onion bulb is a food storage organ comprised of layers of specialized leaves. Bulbing occurs when the plant stops producing new leaves and starts to store food in the leaves it already has. This causes their bases to swell and form the bulb. When the bulb is mature, all of the food has been transferred from the rest of the leaves, so they wither, fall over and die.

The most important thing to remember about growing this biennial is that it is day length sensitive. It is programmed to produce bulbs when the appropriate day length arrives, no matter how big or small it is. Your job is to get the plants as big as possible before the onset of bulbing. To do this you must use a variety that is appropriate for the day length of your location. You should also plant them as early as is safe, so they can put on the maximum amount of vegetative growth, before bulbing.

Growing Sets

Growing your own Onion sets is easier than you might imagine and has the advantage in that you can grow whatever variety you want. The other advantage is that the bulbs will grow larger in one season if started from sets than if they are started from seed.

1. Simply scatter the seed on a prepared bed 1/4˝ apart and cover with 1/4˝ to 1/2˝ of sifted soil/compost.

2. Don’t feed the plants and go lightly on the watering. Because they are growing so close together they will crowd and stunt each other.

3. When the tops turn brown, dig and dry the small bulbs for at least 10 days.

4. Store your sets in the fridge or root cellar (below 40˚ F). Sets with a diameter of less than 1˝ are the best, as they are less likely to bolt.