Onions (Allium spp.) are cool-season crops that are best planted in early spring. They perform well when temperatures are between 55 to 75 degrees Fahrenheit and will tolerate a light frost. Onions will survive in a wide range of soil types but prefer a loose, well-draining soil that has a slightly acidic pH level (5.5 to 6.5). They can be cultivated from either seeds, sets (bulbs), or seedlings; the method will depend on the variety of onion. Many large onion varieties such as “Benny's Red” and “Golden Globe” grow best when started from bulbs.
Sweet Spanish onions come in several different yellow and white varieties, with one of the best-known being "Walla Walla Sweet". They have a sweet, mild flavor that is less pungent that other onions. They are globe-shaped and capable of reaching 4 to 6 inches wide at maturity. Sweet Spanish onions are typically cultivated from seeds or seedlings. They require a long growing period and can take up to 130 days before being ready to harvest.
Like sweet Spanish onions, red onions varieties are also sweet and mild in flavor. Red onions have a red to purple skin but are white- or pinkish-fleshed in the middle. They can grow up to 6 inches across and are often established from sets. “Red Southport Globe” is a popular red variety because of its great taste and long shelf life. Red onions can be harvested in 100 to 120 days after planting.
Common onion varieties, also known as yellow onions, make up the bulk of what is grown in the home garden. These onions are strong in flavor and can be stored for a long period of time before use. When grown from bulbs or seedlings, most yellow onions can be harvested in about 95 to 100 days. Yellow onions varieties such as “Frontier” and “Prince” typically grow to 3 to 4 inches across but may vary in size from season to season, depending on soil conditions and planting time.
White onions varieties are globe-shaped and appear white on both outside and inside. They are crisp and more pungent in taste and smell than most red and yellow onion types. “Super Star” is an award-winning white onion that is capable of weighing 1 pound or more at maturity. According to Colorado State University Extension, "Super Star" onions taste sweeter and grow larger than "White Sweet Spanish". They can be harvested about 100 days after planting.