Types of Onions and Varieties

Choosing the best onion for you and your farm is very important for maximizing onion quality, yield, and ultimately, profits. Certain onion types and cultivars grow best at particular latitudes of the United States, thus the geographic location of your onion crop should determine what kind of onion should be planted.

Bulb onions (Allium cepa) are separated into three types: short-day, medium/intermediate-day, and long-day. These onion types best correspond with latitude (geographical locations). Picture the United States divided into three horizontal zones—short-day onions grow best in southern latitudes, medium-day onions grow best in central latitudes, and long-day onions grow best in northern latitudes.

Short-day onions, such as Vidalia onions from Georgia, require the shortest day length to form bulbs—typically under 12 hours. They are traditionally grown in southern states during the late winter to be harvested the following winter or early spring (Shock, 2013). For example, south Texas grows short-day onions such as the famous yellow 1015 “Million Dollar Baby” onions (Bond, 2013). Medium-day onions, or intermediate-day onions, require 12 to 14 hours of daylight for bulb formation and can be grown in the aforementioned “central latitudes” of the United States. A few examples of intermediate onions are the white Superstar onions, Utah Sweet Yellow Spanish onions, and Sweet Red onions. These are planted in the fall and harvested in late spring/early summer. Long-day onions require 14 or more hours of daylight to initiate bulb formation and can be grown in the northernmost states like Oregon and Washington. The most popular long-day onion variety in Malheur County and Southwest Idaho is ‘Vaquero’.

Onion bulbs come in three different colors: red, yellow, and white. Some red and yellow varieties are known to be very mild and sweet in flavor, while others tend to be more pungent because of their higher thiol (a sulfhydryl compound) contents. Pungency and sweetness differ within onion color classifications and are not always properly judged; for example, an onion with high sugar content should be considered sweet but may not taste sweet if its pungency is overbearing. If you are looking for onions with a certain flavor but do not know what kind to buy, National Onion Labs, Inc. in Georgia tests sweetness and pungency of onions. Other important variety attributes include storability and susceptibility to disease. Greater storability provides the growers and packers more opportunities for profitable sales, contributing to economic sustainability. Varietal resistance to disease can help reduce pesticide use.

Growers are often tasked with selecting from many varieties within each onion type, and an informed choice of variety can maximize the value of the crop. For example, some onion varieties tend to yield more single-centered bulbs than other varieties. Knowledge of these onion varieties is highly advantageous if you are contractually obligated to provide a certain percentage of single-centered bulbs. Variety trials done at experiment stations, such as the Malheur Experiment Station, provide variety performance information.