Some onion varieties can be picky about day length, soil pH

Question: I would like to know how to make my onions get bigger in the ground so I can store them over the winter.

Answer: Onions are a staple crop in many gardens. They're quite easy to grow, as long as you provide proper growing conditions and take care in selecting the variety. If you want to grow big, top-size onions, there are a few things you're going to have to do.

First, be careful about what type of onion you are growing. If you don't grow varieties that have been bred to perform well in Pennsylvania's northern climate, you'll end up with small bulbs that never reach their full potential.

Onions come in three types. Which type you should plant depends on where you live. Just buying whatever generic onion sets they've got at the grocery store or a big-box store might mean the difference between ping-pong balls and soft balls.

Here in the North, you'll want to choose either Long Day or Intermediate Day onion varieties. If you plant a Short Day variety, you'll end up with very small bulbs because these types stop growing as soon as the days reach 12 hours in length. Short Day onions are best for the South, where the days are slightly shorter. (These varieties include ‘Vidalia,' ‘Southern Belle,' ‘White Bermuda,' and many others.)

Long Day onions, on the other hand, are perfect for gardens in the North because they continue to grow until the day length reaches 14 to 16 hours. They're the best choice for our slightly longer days and will produce large bulbs. Good Long Day varieties include ‘Red Zeppelin,' ‘Yellow Spanish' and ‘Walla Walla.'

Intermediate Day (or Day-neutral) onions are varieties that are less sensitive to day length and will produce decent-size bulbs almost anywhere, especially across the middle of the country. Three popular varieties of Intermediate Day onions grown in Pennsylvania are ‘Candy Apple,' ‘Candy' and ‘Superstar.'

Another important step in growing onions is making sure your soil's pH is on the money. A soil pH that's too high or too low does not allow for maximum nutrient absorption (particularly phosphorous). The best pH for growing onions is between 5.8 and 6.5. A soil test from the Penn State Extension Service will accurately tell you the soil's pH and make recommendations for amending it.

Onions, and other root crops, require ample phosphorous for good root production, so making sure the soil's pH is correct will do wonders for phosphorous absorption. You also should add bonemeal to the planting area in the fall but only if your soil test indicates a deficiency in phosphorous.

Onions need to be planted early in the spring for the best bulb set. Plant them as soon as the soil can be worked; usually in mid- to late March. If you wait too long to plant them, they won't have a chance to set the bulb before the days reach the critical day length.

You can grow onions from seeds, sets or transplants. I have the best luck when I grow my onions from transplants, rather than from sets. Many local, independent garden centers sell onion transplants in the early spring.

If you grow onions from seed and you use grow lights, make sure the lights are on for only 10 to 12 hours per day. If you leave them on longer, it will initiate early bulb set and result in small onions.

If you grow from sets, make sure you pick the right variety, rather than just grabbing the first sets you can find. Contrary to popular belief, smaller sets yield significantly bigger onions. Plant the smallest sets you can find.

Regardless of whether you use seeds, sets or transplants, remove the flower stalk as soon as it emerges. Doing so diverts the plant's energy back into the formation of the bulb rather than the flower.

Onions are edible at all stages of maturity so they don't have to be officially “ripe” to enjoy a harvest. But, if you plan to store them for winter use, wait until three quarters of the foliage has browned and fallen over, stop watering for about a week to dry the skins, then pull them up and begin the curing process. Place the onions in a warm, dry, well-ventilated location for two or three weeks to fully cure them before storing them in a mesh onion bag in a cool, dry place.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 503 Martindale St., Third Floor, Pittsburgh, PA 15212.