Despite belonging to the same species, bulb onions (Allium cepa var. cepa) stand out from their cousins the scallion and bunching onion with their large, overdeveloped bulbs. They require moderate care and the right growing conditions to produce a bountiful harvest. However, the most important aspect of growing bulb onions is understanding how to plant them and what types will work best in your garden.
Successful bulb onion cultivation depends greatly on choosing the right type of onion for the local climate. Short-day onions such as "Yellow Bermuda" and "Ebenezer" require 11 to 13 hours of daylight for bulb development, making them appropriate for warmer climates with shorter summer days. Long-day varieties such as "Yellow Globe Danvers" and "Sweet Spanish" need 14 or more hours of daylight in summer, which makes them a suitable crop for northern climates with long summer days. Intermediate cultivars include "Early Yellow Globe" and "Southport Red Globe," both of which will grow anywhere with roughly 13 to 14 hours of daylight during the growing season.
Sets or Seeds
Onions grow from seeds or sets, which are immature bulbs from the previous season. Seeds are more economical but take longer to grow, and they are also more prone to failure. Sets cost more but will produce a crop faster. Bulb onions are a cool-season crop yet they require a somewhat lengthy growing season ranging from 100 to 120 days. For that reason, sets are the best option in cooler climates. When choosing sets, look for bulbs that measure no more than 3/4 inch in diameter because larger sets often yield fewer onions. Another option is starting onion seeds indoors eight to 10 weeks before the last frost to grow transplants.
A planting site with full sun exposure and loamy or sandy, fast-draining soil provides the ideal conditions for growing bulb onions. As a subterranean crop, onions need loose soil to form their characteristic bulbs. Amend the planting site with a 2- to 4-inch layer of compost worked into the top 12 to 15 inches of soil. Use neutral compost to keep the soil pH between 6.2 and 6.8, and remove any sticks, stones or other pieces of debris that might interfere with bulb development or drainage. Another factor to consider when planting bulb onions is what crops were previously grown at that site. Avoid planting where other alliums have been grown in the previous three to four years to prevent bacterial and fungal growth in the bulbs.
Planting and Spacing
Bulb onions need temperatures between 45 and 60 degrees Fahrenheit when developing and 60 to 75 degrees F when their bulbs form and cure, so planting at the correct time is vital. Plant bulb onion sets in spring as soon as the soil warms to around 45 degrees F and plant seeds when soil temperatures reach 50 degrees F. Plant bulb onion sets by grasping the pointed end and pressing the blunt root end into the tilled soil until the whole bulb is barely covered. When planting smaller sets, plant the bulbs 1 to 2 inches deep into the soil. Space them in rows 4 to 6 inches apart. Sow bulb onions seeds 1/4 inch deep and 1/2 inch apart in straight rows spaced 12 to 18 inches apart. When the seedlings emerge, thin them to one every 2 to 4 inches depending on the mature size of the onion bulb.
Bulb onions possess a shallow root system that needs regular watering starting at planting. Water to a 12-inch depth at planting, which is roughly 1 inch of water. After the initial watering, spread a 3- to 4-inch layer of straw or herbicide-free grass-clipping mulch over the onion bulbs, leaving a narrow strip directly above them. Water only if less than 1 inch of rain falls each week, watering to a 12-inch depth each time. Sandy soil drains more quickly than loamy soil, so check the moisture level frequently and water when it feels mostly dry just beneath the surface. Onions feed heavily once their bulbs begin to expand, so side-dress once with 10-10-10 analysis fertilizer at a rate of 2 1/2 pounds per 100 square feet, applying it to moist soil after watering.