The common onion is a staple of gardens and kitchens, but successfully growing large, harvestable bulbs depends entirely on precise timing. In a region like Arkansas, planting too early or too late can mean the difference between a bountiful harvest and a small, underdeveloped crop. The single most important factor determining when to plant is the plant’s biological response to the changing length of the day. Achieving a satisfying yield requires understanding this natural clock and coordinating it with the local growing season.
Understanding Photoperiod Requirements
Onion bulb formation is governed by photoperiodism, the plant’s reaction to the duration of daylight hours. The plant first develops a large, leafy top. Only when the day length reaches a specific threshold does it trigger bulb enlargement. This dependence classifies onions into three categories: short-day, intermediate-day, and long-day varieties.
Short-day onions require 10 to 12 hours of daylight to begin bulbing, making them ideal for southern latitudes. Long-day onions need 14 to 16 hours of daylight and are best suited for northern states. Intermediate-day onions need between 12 and 14 hours of daylight.
Arkansas is situated in a transitional zone, meaning both short-day and intermediate-day varieties are adapted for cultivation. Short-day cultivars are suitable for the entire state. Intermediate-day cultivars are best reserved for the northern half of Arkansas, where the summer day length is longer.
Planting the wrong variety is the primary reason for a failed bulb crop. If a gardener in Southern Arkansas plants a long-day variety, it will not receive enough daylight hours to trigger bulbing before the summer heat arrives. Planting a short-day variety too far north causes early bulbing, resulting in small onions because the plant lacked time to develop a substantial leafy top.
Optimal Planting Timelines
The primary goal for planting bulb onions is to establish large, healthy top growth before the critical day length arrives. This requires planting the onions in the cool weather of late winter or early spring as soon as the soil is workable. The planting window must allow several weeks of vegetative growth before the lengthening days signal the start of bulb formation.
In Southern Arkansas, the planting window opens earlier, often beginning in late January and continuing through February, due to milder winter temperatures. Northern Arkansas gardeners wait slightly longer, with the optimal period stretching from late February into early March. Planting during this time allows the roots and foliage to develop under cool, moist conditions before the heat of late spring arrives.
Planting too late in the spring is detrimental because the plant will not have sufficient time to develop a large top before the critical bulbing day-length is reached. If the plant is small when bulbing is triggered, the final bulb size will be significantly reduced.
A secondary planting timeline exists for overwintering green onions or scallions. This involves planting in the fall, allowing the plants to establish before going dormant over the winter. This method is primarily for harvesting green tops rather than large, dry bulbs.
Selecting the Best Starting Material
Onions can be started using three materials: seeds, sets, or transplants, each with its own advantages and drawbacks. Transplants, which are young seedlings generally eight to ten weeks old, are the most reliable option for producing large, dry onions. They provide a head start on the growing season and are the most common way to plant short-day varieties suited for Arkansas.
Transplants are typically bundled and purchased from garden centers or catalogs, and should be planted soon after the soil can be worked. Growing from seed is the least expensive method, but it requires starting seeds indoors as early as December to produce a transplant-sized seedling by late winter or early spring.
Onion sets are small, dormant bulbs grown from seed the previous year. While convenient and quick to establish, they carry a higher risk of bolting, or sending up a flower stalk, especially if the sets are larger than a dime. Bolting stops bulb enlargement and greatly reduces the onion’s storage life. Sets are often better utilized for a quick crop of green onions than for developing large, storable bulbs.