Onions are a garden favorite, and successfully cultivating them in Kentucky relies heavily on precise timing to manage the state’s fluctuating climate. The onion plant is cold-tolerant, allowing for an early start, but its growth and eventual bulb formation are controlled by both the calendar and the sun. Getting the planting date right is the most significant factor in whether you harvest small scallions or large, mature bulbs.
Seasonal Timing for Onion Planting in Kentucky
The primary planting window for onions in Kentucky is late winter to very early spring, generally from late February through mid-March. Onions are a cool-season crop, thriving in mild spring temperatures and handling light frosts. The goal is to establish the plants in cool soil well before the onset of long, warm summer days, which trigger the bulbing process.
The specific timing depends on the region, with Western Kentucky planting earliest, around the first week of March, and Central and Eastern Kentucky following by mid-March. This schedule aims to plant the onions approximately four to six weeks before the average last spring frost date. Planting should only occur once the soil is workable and not overly saturated from winter rains.
Fall planting is a less common option used for overwintering varieties, typically for a very early spring harvest of green onions. For mature bulb production, the spring planting window is far more reliable and is the standard practice. Planting early allows the tops to grow large before the day length cues the formation of the bulb, contributing to a larger final harvest.
Understanding Onion Types and Day Length
Onion bulb formation is a photoperiodic process, triggered by the number of daylight hours, not just temperature. This dependency categorizes onions into three main types: short-day, intermediate-day, and long-day varieties. Kentucky’s mid-latitude position dictates that specific varieties must be chosen to ensure proper bulbing.
Intermediate-day onions, sometimes called day-neutral, are the best choice for Kentucky gardeners, requiring approximately 12 to 14 hours of sunlight to begin forming a bulb. Long-day varieties, needing 14 to 16 hours of light, can be successful but may suffer from increased disease risk due to high summer temperatures before maturity. Short-day varieties, which bulb with only 10 to 12 hours of light, are ill-suited for Kentucky because they will bulb prematurely when the plant tops are still small, resulting in tiny onions.
Recommended intermediate-day varieties include ‘Candy,’ ‘Red Candy Apple,’ and ‘Walla Walla,’ which will begin to bulb in mid-May and mature by mid-July. Selecting the correct day-length type is a more significant factor in bulb size than any other cultural practice. Using the appropriate variety ensures the plant develops a large, leafy top before the genetic trigger for bulbing is activated.
Comparing Planting Methods
Onions can be started using three materials: sets, transplants, or seeds, each offering a different trade-off between convenience and variety. Onion sets are small, dormant bulbs grown the previous year and are the fastest and easiest method for beginners. Sets should be planted one to two inches deep, pointed end upward, and spaced two to four inches apart, though they offer a limited selection of varieties.
Transplants, or slips, are young seedlings started from seed several weeks before planting and offer a wider selection of day-length-appropriate varieties. These small plants should be planted about an inch deep, covering the roots and the base of the stem. Transplants give the gardener a head start without the long lead time required for growing from seed, and they are generally less prone to bolting than sets.
Growing from seeds offers the widest variety selection but requires the longest lead time, as seeds must be started indoors about eight to ten weeks before the outdoor planting date in late January. Direct-sowing seeds outdoors is possible but is often less reliable due to slow germination in cool soil. Regardless of the method, the final spacing for mature bulbs should be three to four inches apart to allow for proper bulb expansion.
Soil Preparation and Early Growth Requirements
Onions have a shallow, fibrous root system, making proper soil preparation necessary for successful growth. The soil must be loose, well-draining, and rich in organic matter to prevent compaction, which can deform the growing bulbs. Onions thrive best in a slightly acidic to neutral soil environment, with an ideal pH range between 6.0 and 6.8.
Amending the bed with compost or well-rotted manure before planting improves the soil structure and provides necessary nutrients. Onions are heavy feeders, especially needing nitrogen during their early growth phase to develop large, leafy tops. A balanced fertilizer, such as a 5-10-10 or 10-20-10, can be incorporated into the soil at planting time.
Once planted, consistent moisture and high nitrogen fertilization are important until the bulbing process begins. Applying a high-nitrogen fertilizer, such as a 33-0-0, every two to three weeks after planting helps promote vigorous leaf growth. Because onions compete poorly with weeds, shallow weeding must be performed regularly, taking care not to damage the shallow roots.