Growing Sweet Onions in Ohio

Climate and Growth

• Onions are mostly frost-tolerant and can survive in a wide range of temperatures. Spring planting is best when the temperature is 55 to 75 degrees Fahrenheit, according to an Ohio State University Extension website article, so that roots and foliage can develop before bulb formation. Along with temperature, the amount of daylight, or photoperiod, controls when onion plants form bulbs; short-day varieties need 12 or fewer hours of sunlight daily and long-day plants require 15 or more hours of daylight daily. Newly planted onions first grow roots into the ground while developing tops and green leaves above soil. In Ohio, onions grow best from sets or plants. Depending on the variety and planting time, onions typically need to grow 100 or more days before the bulbs are ready for harvest. Peak times for onions in Ohio are from Aug. 1 to Oct. 15.

Environment

• In Ohio, onion bulbs cannot tolerate highly acidic soil; they grow best in soil with a pH level between 6.2 and 6.8. Silt loams and clay-based soils are too heavy for onions. So planting them in loose, well-draining, organic media helps with oxygen and water movement. Providing a 5-10-10 fertilizer helps the onions' tender roots grow and thicken. Applying mulch keeps moisture in and weeds out. Onion plants attract insects, such as thrips, and fly larvae, which are maggots. Onion bulbs are susceptible to mildew and root diseases. Effective control measures depend on the onion variety and growing environment.

Sweet Spanish

• Long-day, perennial, yellow sweet Spanish and white sweet Spanish onions require at least 15 hours of summer daylight in Ohio’s USDA zones 5 and 6. Place the onion seeds about 1 inch deep and 2 to 4 inches apart when the soil temperature is about 55 degrees Fahrenheit. Do not transplant indoor seedlings or bulbs until after the danger of frost passes. Full-size onion bulbs are ready for harvest when their tops become brown and fall over. As you dig up the onions from the ground, do not remove their leaves or roots, instead, shake off the soil, and store the bulbs for a couple of weeks to cure. Onions are usually ready to harvest by early fall in Ohio.

Fresh Varieties

• Short-day Bermuda (Allium cepa “Bermuda”), “Red” (Allium cepa “Red”), “Vidalia” (Allium cepa “Vidalia") and “Walla Walla” (Allium cepa “Walla Walla”) onion seeds may be embedded directly into the ground or started indoors. Seedlings can be transplanted into the garden after the average last frost date in the particular Ohio location. Also suitable for Ohio gardens are Egyptian (Allium cepa var. viviparum) onions. Although they are not as tender as other bulb varieties, Egyptian onions, also called winter onions, are the most cold-hardy of onions and can be planted in late fall. Unlike other fresh varieties, Egyptian onions are not started from seeds; they produce bulbils that form at the top of the plants.

Harvest and Storage

• Sweet onion bulbs from Ohio gardens are best if eaten within a couple of days after removing them from the ground. They can be cured and stored, however, by setting them in a warm area with good air circulation and low humidity for 10 to 14 days. Store onions in cool, dry containers; they can be placed in a refrigerator, but do not put them in plastic bags because air cannot circulate in such bags. Cured onion bulbs also may be stored in mesh bags, or their dried tops can be braided into strings and the bulbs hung from the braided strings. Cured onions stored in dry areas with a temperature between 32 and 45 degrees Fahrenheit may stay firm and tasty for about three to six months.