How to Grow Series
These instructions for growing onions are intended for cooler growing zones from 3-6. A different technique may be required for warmer climates.
Onions are pretty easy to grow and a great crop for beginner gardeners as well as more advanced gardeners. Generally hardy from zones 3-9, onions are considered cool season crops, meaning they perform better in cooler climates or during the cool season. Onions are available by seed and by onion sets. Onion sets are more desirable as they are easier to grow and are not usually damaged by frost, so this article will focus on how to grow onions from sets.
In mid to late spring, when the soil is ready to work and not too wet, it is time to plant onion sets in the garden. Depending on where you are in the country, this could be as early as late March. In general, plant when temperatures are not likely to be below 20 degrees F. Choose a spot that gets full sun and is well drained. Onions do not grow well in clay soils, and the soil needs to be evenly moist, yet well drained, so adding compost or aged manure to your soil in the fall, prior to planting, and again at planting time, is ideal.
Onions are heavy feeders, meaning they need a lot of nitrogen rich fertilizer, not only at planting time, but throughout the early part of the growing season. Dig a shallow trench that is about 3” deep and mix in some high nitrogen fertilizer and compost in the bottom of the trench. I like to use a granular, slow release fertilizer at this point. Fill the trench with the remaining soil so that the rows are slightly raised over the rest of the garden and then press in the onion sets so that the bulb is about an inch below the soil surface. Onion sets may still have the green stalks attached, if so, keep the green stalks above the soil. Planting your onion sets slightly raised from the rest of the garden will ensure they drain well and will not rot.
Space the onions about 4 or 5 inches apart with rows at least 12 inches apart (or according to the directions on the package of onions you purchased). You can plant the bulbs closer together if you want to harvest small green onions over the course of the growing season. In this case, plant 2” apart and harvest every other onion as needed. Just make sure that you actually harvest them so that the remaining onions have room to grow larger. To help conserve moisture and keep weeds under control, mulch the rows with untreated grass clippings or compost. Onions do not like competition from other plants, so make sure the onion beds are kept weed free.
Onions will need to be fertilized every few weeks as they are heavy feeders. Stop fertilizing when the onion bulbs begin to push out of the soil on their own. This is about 4-6 weeks before harvest. Keep the bulbs exposed to the sun, and move away any mulch that might still be touching the exposed bulb. The onions need to be raised out of the soil to finish growing. If using granular fertilizer, make sure you water it in after every application. I like to use fish emulsion as it is in a liquid form added to water, so you water and fertilize at the same time.
Water the onions at least an inch of water a week, especially when it has not rained. Onions will look fine during drought, but they will not be growing and their flavor will be strong instead of sweet. Water more often if you want sweeter onions or if you are not using mulch.
Pull any onions that send up flower stalks. This is called bolting and is a signal that the onion is done growing. This can be caused by alternating hot and cold temperatures. These onions should be eaten immediately as they may rot in storage.
When onions begin to mature, the tops will turn yellow and fall over. This should occur in late summer. You can loosen the soil around the onion to help speed up the drying process. After a few days, take a potato fork or shovel and gently loosen them from the soil but leave them there to dry until their tops are completely brown. This is part of the curing process. At this stage the onions are very tender and can be bruised easily so be careful. Use any damaged onions right away since they will rot quickly. Once the tops are brown, pull up the onions and allow them to dry on screens for several weeks in a cool dry place. This curing process will ensure that they will keep longer in storage.
When it is time to store your onions, cut off the roots and most of the stem (leave about 1”) and brush off any dried dirt, careful not to remove any of the onion skins. Store your onions between 40-50 degrees F in a dry place away from apples and potatoes. To ensure that onions will store for a long time, store the onions on screens and don’t let them touch one another. Alternately, you can store them in an old pair of stockings. Simply place an onion in one of the legs, tie a knot close to the onion, add another onion, and tie a knot, and so on until the stocking is full. If you notice any blemishes or soft spots on the onions, use them right away as they will only rot in storage. If you don’t have a root cellar to store your onions, wrap them individually in aluminum foil and store them in the fridge for up to a year. Sweet varieties will not store as long as more pungent varieties.
Onions should not be grown in the same place year after year, so rotate where they grow in your garden every year. Since beans will add nitrogen to the soil, I like to grow my onions in the same spot that beans grew the year before. Read more about crop rotation here.