I’m headed to the Seattle Tilth Early Spring Edible Plant Sale this Saturday, and ‘Walla Walla’ sweet onions and ‘Red Long of Tropea’ onions are on my list. I prefer to grow bulbing onions from seedlings because they bulb up more reliably than “sets” which are just small whole onions. Most gardening books recommend using a stick (sometimes called a dibble) to make an individual hole for each onion seedling. You then drop the onion into the hole and backfill soil around it. I find this incredibly tedious and instead plant my onions in a trench, which streamlines the planting process. Here’s how to do it:
Create a 3- to 4-inch deep V-shaped trench (also called a furrow) using a hoe or a trowel. Lay out the onions side by side against one bank of the furrow. I plant the seedlings only an inch or two apart. This spacing allows me to gradually thin out the onions and eat them at different stages. I thin out every other plant when they reach the scallion stage and again when the bulbs begin to form. I leave the remaining onions in the ground to grow into mature bulbs.
To plant the onions, simply draw the soil from one edge of the furrow over the seedlings roots, burying them and about the bottom 1/3 of the seedling. The seedlings will still be cockeyed at this stage.
Straighten out the seedlings by pushing the soil from the other side of the furrow in towards the middle. You want about two-thirds of each seedling to be above the soil line. This is not brain surgery or a precise science. If a seedling is buried too deeply, simply pull it up. If another needs to be buried deeper just hill a bit more soil around it.
Planting onions in a furrow also makes it easy to water them. Just put a bubbler attachment on the end of the hose and set it in the furrow. You can also water by hand by filling up the furrow repeatedly with water. Or, you can run a soaker hose or drip tape down the length of the furrow. The onions will push themselves up out of the soil as they grow (see top photo). Don’t hill soil up around the bulbs, as this prevents them from forming their papery protective outer layer. Onions, shallots, and leeks can all be planted this way.