Onion gardening in Connecticut requires precise planting times and specific onion types because successful bulb development is strictly governed by the amount of daylight available. This northern climate demands the right timing to ensure a bountiful harvest. Preparing the soil and providing consistent care throughout the season are also necessary steps to move from a small seedling to a mature, storage-ready onion.
Timing is Key: Understanding Onion Types for Connecticut
Onions are categorized into three groups—short-day, intermediate-day, and long-day—based on the number of daylight hours required to trigger bulb formation (photoperiodism). Due to Connecticut’s northern latitude, gardeners must exclusively choose long-day onion varieties. These types begin forming a bulb when the day length reaches approximately 14 to 16 hours.
Planting the wrong variety, such as a short-day onion, results in disappointing growth. Short-day onions begin bulbing too early in the Connecticut season before the plant develops sufficient foliage. Without large top-growth, the resulting bulb will be extremely small or nonexistent. Long-day varieties, such as ‘Copra’ or ‘Walla Walla’, grow their foliage during the increasing daylight of spring and early summer, storing energy before the summer solstice triggers bulbing.
The CT Planting Calendar: When to Start and Set Out
The optimal planting strategy depends on starting with seeds, sets, or transplants, as each method has a distinct timeline. Starting with seeds offers the greatest variety selection and is the most economical option, but requires the earliest start indoors. Sow onion seeds indoors about 8 to 10 weeks before the average last frost date, typically from late January into early February. Seedlings should be grown under lights and can be trimmed back to about three inches if they grow too tall before transplanting.
For the easiest start, gardeners can use onion sets, which are small, dormant bulbs from the previous season. Sets can be planted directly outdoors as soon as the soil is workable in the spring, generally from late March to mid-April. Choose sets smaller than a dime, as larger sets are more prone to bolting (prematurely flowering), which stops bulb development.
Transplants, which are purchased seedlings, should be set out shortly after the soil is ready, typically mid-April to early May. Plant these seedlings before the summer heat arrives to ensure they have time to establish themselves. Extended exposure to temperatures below 45°F can cause the plants to bolt. Whether using sets or transplants, position them about one inch deep and space them three to four inches apart.
Preparing the Ground for Optimal Onion Growth
Onions are heavy feeders with shallow root systems, requiring proper soil preparation before planting. The planting site must receive full sun exposure for at least six to eight hours each day, as sunlight is the primary driver for bulb formation. The soil must be loose, crumbly, and extremely well-drained, as onions are susceptible to rot in heavy clay or compacted soil.
Aim for a slightly acidic to neutral soil pH between 6.0 and 7.0; a soil test can confirm the current pH and nutrient levels. Before planting, incorporate a generous amount of aged compost or other organic matter to improve texture and water retention. Also, work in a balanced fertilizer, such as a 10-10-10 blend, to provide the necessary nutrients for initial growth.
Ongoing Care and Harvesting
Consistent moisture is necessary for developing large, healthy onion bulbs, especially during the spring and early summer. Onions require about one inch of water per week; dry conditions during bulbing can cause the bulbs to split. Because their roots are shallow, onions compete poorly with weeds, so the planting bed must be kept consistently weed-free.
The size of the final bulb relates directly to the amount of green top growth. Gardeners should side-dress with a nitrogen-rich fertilizer in late spring and early summer to encourage maximum foliage development. Stop feeding the plants once the bulbs begin to swell in late June or early July. Onions are ready for harvest when their leafy tops begin to turn yellow and approximately two-thirds of the foliage has naturally fallen over.
After lifting the bulbs, harvested onions need to be cured for long-term storage. Curing involves drying the bulbs in a warm, dry, well-ventilated area, such as a covered porch or shed, for about three to four weeks. This process allows the neck of the onion to dry and seal shut, and the outer skin to form a protective, papery layer. Once the necks are completely dry and tight, the tops can be trimmed to an inch or two above the bulb, and the onions are ready for storage.