The USDA Hardiness Zone 8 designation covers regions with average minimum winter temperatures between 10°F and 20°F, creating a climate suited for a long growing season. This mild winter allows planting onions in both the fall and the spring. Successfully growing large, sweet onions depends heavily on aligning your planting schedule with the variety chosen, specifically its sensitivity to day length.
Selecting Onion Varieties for Zone 8
The success of growing onions is entirely dependent on a biological process called photoperiodism, which dictates when the plant switches from growing foliage to forming a bulb. Onions are categorized based on the number of daylight hours required to trigger this bulbing process. Choosing the wrong type for your latitude is the most common reason for a failed onion harvest.
Zone 8, which spans the mid-temperate latitudes, can successfully grow both short-day and intermediate-day varieties. Short-day onions, which require 10 to 12 hours of daylight to bulb, are often planted in the fall for an early spring harvest. Examples include varieties like Texas Grano, Granex, and Red Creole, which are favored for their mild flavor and ability to mature quickly in the shorter days of winter and early spring.
Intermediate-day onions are also well-suited for Zone 8, needing 12 to 14 hours of daylight to begin bulbing. These are planted in the late winter or early spring for a summer harvest, as they are less likely to prematurely form a flower stalk, or “bolt,” in the mild climate. Popular intermediate-day varieties include Candy, Superstar, and Red Candy Apple. Long-day varieties, which require 14 to 16 hours of light, are best reserved for northern zones and do not perform well in Zone 8.
Optimal Planting Windows
Gardeners in Zone 8 have two primary windows for planting onions, each linked to a specific type of onion and a different harvest time. The first opportunity is the fall planting window, which is reserved for short-day varieties intended to be overwintered. Planting should occur between mid-October and mid-November, ideally after the hottest summer temperatures have broken but well before the onset of any sustained, hard freezes. This timing allows the onion plants to develop a robust root system and a few inches of green growth during the mild winter before they initiate bulbing in the lengthening days of early spring.
The second opportunity is the spring planting window, used for intermediate-day varieties. These should be planted as soon as the soil is workable, typically from late February through early March, allowing sufficient time to establish large foliage. Since the size of the final bulb directly correlates with the amount of leaf growth, planting early maximizes growth before longer days trigger bulb formation. This early spring planting ensures the bulbs mature before the intense heat of mid-summer arrives, which can stress the plants.
Planting Techniques
Onions can be planted using seeds, sets, or transplants, with each method offering different advantages. Onion sets are small, immature bulbs from the previous year, easiest for beginners, and offer a quick start but often lack specific variety names. Transplants are small, bare-root seedlings, often sold in bunches, and are the most reliable option for producing large, uniform bulbs.
To plant sets or transplants, you should aim to place them about one inch deep in the soil, ensuring the root base is covered but the tip of the set or the neck of the transplant is not buried too deeply. Overly deep planting can inhibit the plant’s ability to form a proper bulb. Spacing is crucial for bulb size; plants should be spaced four to six inches apart in the row to allow for full expansion, with rows spaced 12 to 18 inches apart. If you wish to harvest some as green onions, you can initially space them closer and thin them out as needed.
Essential Post-Planting Care
Onions are shallow-rooted and require consistent moisture, especially as the bulbs begin to swell. The soil should be kept uniformly moist but never waterlogged, as poor drainage is a common cause of rot and disease in Zone 8’s mild climate. Raised beds are often recommended to improve drainage and prevent the roots from sitting in standing water.
These plants are heavy feeders and benefit significantly from regular applications of a high-nitrogen fertilizer throughout the early and middle stages of growth. Weed control is also important because the onion’s shallow roots compete poorly with weeds for water and nutrients. Harvest is indicated when the tops of the plants begin to yellow and fall over, at which point watering should stop to prepare the bulbs for curing.