Planting onions is one of my favorite April vegetable garden projects. This year I am planting Walla Walla onions, our regionally famous sweet onion. In late August we will harvest the bulbs and enjoy them raw on hamburgers, sandwiches and in salads.
Onions are planted from seed, sets or small onion bulbs or – in the case of the Walla Wallas – as small plants. They are a biennial plant which means they will grow a bulb the first year and if left in the ground over winter, they will produce a seed stalk the second year. We generally harvest them at the end of the first year for the bulb. The pungent aroma that brings tears to your eyes is caused by the release of sulfur compounds when they are cut.
Onions form their bulbs in response to the length of daylight. Here in the northern part of the country, long-day onions begin setting bulbs after we start getting more than 14 hours of daylight. This is why it is critical to get onions planted as early as possible in the spring so the plants have a chance to grow before they begin setting bulbs.
Not all onions will keep through the winter. Summer onions like the Walla Walla sweet onion and other mild, Spanish-types will keep for a couple of months while the winter or storage onions will keep well into the winter when stored in a cool, dry place. Use up the short keepers first and then enjoy the long storage ones through the winter.
Onion sets are much easier to grow than seed. Onion seed is very fine and must be started indoors early in the winter or in the garden the previous fall. Onion sets can be purchased in the spring and should have a base no larger than a dime. Sets larger than this will bolt or flower instead of setting a bulb. Walla Walla sweet onions are usually purchased in the spring as bareroot bunch of 25 to 50 plants.
Onions grow best in full sun in an evenly moist, sandy loam soil rich in organic matter. Onion sets and Walla Walla starts should be planted an inch deep about four inches apart in rows 12 to 15 inches apart. Onions from seed started indoors in late February can be planted in April when they are about the size of a pencil lead. It will take about two weeks for the green shoots to appear and start growing.
Once the plants are about six inches tall, apply a 10-10-10 fertilizer every three weeks until the tops start falling over. Keep the soil evenly moist until the tops begin to yellow and die then discontinue watering so the bulbs can harden off before harvest.
When most of the tops have fallen over, gently dig them and let them dry in the sun for a couple of days to harden the skins before storing them in cardboard boxes or mesh bags in a cool, dry place for the winter.