Growing sweet onions

Onions are one of the few vegetables that can be planted from late September to late October in middle Georgia. Since Georgia has become internationally known for the world-famous Vidalia onion, and since they are so good, many gardeners have asked how to grow this unique onion.   

The characteristics (size, shape, color, sweetness, etc.) of the onion bulb that will be harvested next spring can be directly attributed to two factors: genetics and environment. We determine what those onions will be like by our selection of the appropriate genetic material (variety) and by properly managing the environment. 

Genetics first. There are many onion varieties available to gardeners. All of these fall into one of three day-length groups: short day, long day, or intermediate. The short day group contains the true Vidalia onion as well as the Vidalia types. 

Why is day length important when it comes to growing onions? Bulb formation in onions is primarily a day length response. In short day onions, bulbing begins when a day length of approximately 11 hours occurs. In Georgia, this day length is usually achieved in the spring during February and early March. 

Now for environment. Sweetness of onions is directly related to the amount or concentration of sulfur-containing compounds within the bulb. In order to grow a low pungency “Vidalia type” sweet onion, gardeners should apply only low to moderate amounts of sulfur to their onion soils. High rates of sulfur and late applications increase bulb sulfur content and therefore onion pungency. 

Following are some recommendations for growing sweet “Vidalia type” onions in your garden.           

Select a short day, sweet onion variety such as Granex 33, Granex 429, Sweet Vidalia, or Texas Sweet.

In middle and south Georgia, direct seed in the garden or set transplants in late September or early October.

Raised beds made by mounding up soil are ideal, especially if your soil is heavy clay. Mix a 2-inch layer of compost into the soil before placing an organic or timed-release fertilizer into planting furrows, following label rates. Set plants one inch deep, so that their roots are well covered with soil but the top of the transplant’s neck is not buried too deeply or rotting may occur. Space plants 6 inches apart in furrows 12 inches apart.   

Maintain a soil pH of 6.0 to 6.5 and provide an adequate supply of nutrients. Apply no more than one ounce of actual sulfur per 150 square feet and do not apply sulfur after December 31. Onions are heavy feeders, so don’t be shy with the fertilizer!

Onion roots are shallow and not very efficient at taking up moisture, so they need a steady supply of water to grow without interruption. Although they actually recover well from drought and start growing again when watered, it is best to keep the soil consistently moist until the bulbs enlarge. 

You may mulch with a light layer of weed-free and herbicide-free grass clippings or other fine mulch. Onions naturally push toward the surface as they form bulbs and it is best if the tops of the bulbs are allowed to bask in dry sun. Remove mulch that might keep the expanding bulbs excessively moist.

Some gardeners will find it difficult if not impossible to grow the very sweet, mild “Vidalia type” onion. Heavier soils tend to hold more sulfur and such soils may contain enough residual sulfur to make onions hot. Try it anyway; you might be pleasantly surprised! 

Tim Lewis is a Georgia Green Industry Association Certified Plant Professional, gardening writer, and former Perry High School horticulture instructor. He can be reached at (478)954-1507 or timlewis1@windstream.net