Planting onions in June is achievable, though it is late for the traditional planting window, which typically occurs in early spring or fall for bulb production. Success requires specific adjustments to the planting material and understanding the gardener’s ultimate goal. The feasibility of growing a large, mature bulb versus harvesting tender green tops depends on employing the right strategies for late-season planting.
Understanding Photoperiod and Bulbing
The primary constraint when planting onions late is their dependence on day length, known as photoperiodism. Onions are categorized into short-day, intermediate-day, and long-day groups based on the daylight hours required to trigger bulb formation. If the plant does not receive the appropriate day length signal, it will continue to produce only green tops instead of developing a bulb.
Long-day varieties, which are typically grown in northern latitudes, require 14 to 16 hours of daylight to begin bulbing. Intermediate-day onions, suited for middle latitudes, initiate bulbing when day length reaches 12 to 14 hours. Short-day onions, grown in the South, only need 10 to 12 hours of light to start the process.
Planting in June, especially in northern regions, means the longest days are either underway or have just passed. This is the time the onion needs to be well-established for maximum leaf growth. The potential size of the mature bulb is directly related to the number and size of leaves developed before bulbing is triggered. Since each leaf layer becomes one ring of the final onion, a late-planted onion has less time to develop the foliage necessary to support a large bulb.
Late planting pushes the final stages of growth into the high heat of mid-to-late summer, which onions generally do not prefer. Temperatures consistently above 85°F can stress the plant, potentially causing it to prematurely send up a flower stalk, a process called bolting. Once an onion bolts, it stops forming a usable bulb, resulting in a small, tough, and less flavorful harvest.
Planting Materials That Work in June
To overcome photoperiod limitations, gardeners aiming for a full bulb harvest in June should avoid starting from seed. Onion seeds take too long to germinate and develop into a seedling large enough to support bulbing for a successful harvest the same year. The most practical inputs for late planting are onion sets or established transplants.
Onion sets are small, dormant bulbs grown from seed the previous year, offering a substantial head start. Transplants are live seedlings started indoors by a grower, typically offering a six-week advantage over direct-sown seeds. Both options drastically reduce the time needed to develop the foliage required for a decent-sized bulb before the season ends.
When planting sets or transplants in June, specific adjustments must be made to mitigate the stress of high summer temperatures. Unlike early spring planting, the soil is much warmer, necessitating consistent moisture to prevent the shallow root system from drying out. Watering should be frequent and uniform, especially during establishment and periods of intense heat, as dry soil can trigger premature bolting.
Planting sets or transplants slightly deeper than normal (about two inches for sets) can help insulate the developing root structure from surface heat. Providing temporary afternoon shade in the hottest climates may also prevent heat stress and reduce the risk of premature bolting. Choosing an intermediate-day variety, if appropriate for the latitude, offers the greatest flexibility for a successful bulb harvest. This type requires a moderate day length, making it less sensitive to the precise timing of the solstice.
Planting for Green Onions
If the goal is to harvest the flavorful green tops rather than a mature bulb, planting in June is straightforward and reliable. Green onions, also known as scallions, are immature onion plants harvested before bulbing is complete. This method effectively bypasses complicated photoperiod requirements and the race against the seasonal clock.
For this purpose, sets or seeds can be used, with seeds offering the most economical choice for continuous planting. Green onion seeds can be sown directly outdoors throughout the summer, benefiting from afternoon shade during the hottest periods. They typically germinate within one to two weeks, and the initial harvest can begin in 8 to 12 weeks from sowing.
The technique for growing green onions differs from bulbing onions in that dense planting is encouraged to prioritize leaf growth. Seeds should be sown about a quarter-inch deep and spaced closely, sometimes as little as one inch apart. This tight spacing focuses the plant’s energy on producing tall, slender green stalks instead of a large underground bulb.
A useful strategy is to harvest green onions by cutting the foliage about an inch above the soil line, leaving the root plate intact. This allows the plant to regrow new shoots, providing multiple harvests from the same planting. This method makes June planting a practical way to ensure a fresh, steady supply of mild onion flavor throughout the summer and into the early fall.