University of Vermont Extension 
Department of Plant and Soil Science

Spring News Article

Dr. Leonard Perry, Extension Professor
University of Vermont 

If you’re a gardener, you may already be growing onions.  If not, or new to gardening, and you like to use onions in cooking, consider growing your own.  They don’t take up much room, are an easy and tough crop, and by growing your own you can get cultivars (cultivated varieties) not available at markets.  There are a few facts on onion growing which can make your onions excellent, instead of just good.
First, you’ll need to choose which onions to grow.  Consider the color—they come in reds, yellows, and whites.  Consider the size, which often relates to use, such as the small white pearl onions for stews or large ones for slicing.  Consider if you want to eat and use them fresh or store them, as some store much longer than others.  Of course consider taste—do you like sweet or strong flavors?
Perhaps most important culturally is their day length requirement.  Onions, depending on cultivar, require various amounts of light in order to form bulbs.  For northern gardens look for “long day” onions which require at least 14 hours of light daily in summer to “bulb up”.  Or, you can choose “day neutral” ones, for which day length doesn’t matter.
Next, figure how you’ll start your onions.  They can be grown from seeds, plants, or “sets”.  The latter are small bulbs which you plant with their pointed side up, tips just level with the soil surface, about 3 to 4 inches apart in a staggered spacing between rows.  Contrary to what you might think, smaller sets are better as they are less likely to “bolt” than larger ones (flower and stop forming bulbs prematurely). Sets are often recommended as the easiest method.

You also can start onions from purchased small plants, which are easy and what many do, but plants or sets offer you fewer choices than seeds.  If you’re starting from both sets and plants, put them in different areas, as the sets may be more prone to diseases.  Space plants in rows similar to sets.
Perhaps the most variety is from starting plants yourself from seeds.  Since they are a long season crop, taking over 3 months and often 4 months from sowing to harvest, it is best to start seeds indoors in the north.  Seeds are short-lived, so start with fresh ones each year.
Sow seeds in flats mid-February to mid-March-- about 8 to 10 weeks before planting out.  Sow them just below the soil surface in rows and, when they are about 5 to 6 inches tall, thin to one-quarter inch apart or transplant into individual cells in 6-packs.  If tops droop, cut them back to about 4-inches high.  Use a heating mat to keep soil temperatures above 65 degrees (F)—75 to 85 degrees is best.
Whether you’re planting sets, purchased plants, or those you started yourself from seeds, plant out about 2 to 4 weeks before the last usual frost date in your area.  For northern gardens, this may mean planting out late-April to mid-May.  To avoid problems and get the best crop, don’t plant them in the same area onions or their relatives have grown the last couple years, nor where legumes (peas and beans) have recently been grown.  Onions grow best in a well-drained, loose soil with lots of organic matter.  A sandy loam with additional compost is ideal.
If starting your own seedlings, fertilize with a half-strength fertilizer when you thin or transplant into cells.  Then, in late spring after you plant out, “sidedress” (apply fertilizer along the side of the rows) with compost.  If leaves aren’t a dark green, you can use an organic fertilizer such as fish emulsion once a month.  A key is not to over fertilize with nitrogen, as this will result in great leaves but small bulbs.  Onions do need sufficient potassium, or they’ll have hard “necks” (the stem above the bulb) and bulbs won’t store as well.
Usually onions have no problems, except for weeds and perhaps for onion maggots.  To discourage this pest, don’t plant too early in spring, and use row covers for the first few weeks to discourage the flies laying eggs.  Plants don’t compete well with weeds, so keep them weeded, particularly when young.  Mulching after planting with straw or grass clippings reduces weeding.
Another key to an excellent crop is to keep the soil moist during the first couple weeks after planting out.  Then, at the end of the season, the opposite applies.  As plants mature—no more new leaves and tops fall over—reduce watering, and pull soil away from the tops of the bulbs.  This encourages them to go dormant and start the drying process.
Plants should be dormant before harvest, or the bulbs won’t store well.  The sign to harvest is when most the tops have fallen over.  Harvest when the weather is dry, remove loose soil, then “cure” onions-- place in a warm and airy place, out of direct sun, for 3 weeks before storing.  Then, you can cut tops back to an inch above the bulb, unless you want to leave them on for braiding many together as you often see in farmer’s markets.   
Store bulbs cool (32 to 40 degrees F is ideal) and dry, with good air circulation.  You can do this in mesh bags, or shallow boxes with newspaper layers between bulbs.  Check periodically to make sure they aren’t getting soft spots or rots.  Generally the large, white, sweet onions only store for a few weeks to a couple months from harvest.  Those with yellow or brown skin, with hard necks and pungent flavor, store for months.  Those grown from sets tend not to store as long as those grown from plants.  Keep stored onions away from apples and tomatoes, as these give off a gas (ethylene) which makes onions sprout.  Onions are easily frozen, too.  Merely peel, chop, and loosely pack in freezer bags.

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