In Kentucky, onions go in the ground in early spring, typically late March through mid-April, as soon as the soil is workable and temperatures stay above 40 °F. You can also plant in fall for an earlier harvest the following year, with transplants going in during late October to mid-November. The right timing depends on whether you’re using seeds, sets, or transplants, and whether you want a spring or overwinter crop.
Spring Planting Dates
Most Kentucky gardeners plant onions in spring. Sets and transplants can go directly into the ground in late March to April, once the soil is no longer frozen or waterlogged. Onion seeds germinate in soil as cool as 40 °F, though they sprout fastest at around 75 °F. Since Kentucky’s last frost typically falls between mid-April and early May depending on your location, onions are one of the first crops you can get in the ground. They tolerate light frosts without damage.
If you’re starting from seed, sow indoors 4 to 6 weeks before you plan to transplant outside. That means starting seeds indoors around mid-February to early March for most of the state. Harden off seedlings by placing them outdoors for gradually longer periods during the week before transplanting.
Fall Planting for an Earlier Harvest
Fall-planted onions overwinter in the ground and mature earlier the following year, giving you a harvest in cooler temperatures before summer heat sets in. Research from the University of Kentucky tested this approach by transplanting onion seedlings in late October to early November. The results were promising: long-day varieties like Walla Walla and Olympic consistently had the highest survival rates through Kentucky winters.
If you go this route, plan to get transplants in the ground by late October. Row covers significantly improve yields and survival through the cold months. The long-day varieties Olympic, Ailsa Craig, and Walla Walla produced the highest yields under row covers. Olympic in particular combined strong survival, low bolting rates, and mild flavor, making it a top pick for overwinter production in Kentucky.
Choosing the Right Variety for Kentucky
Onion varieties are classified by how much daylight they need to form bulbs: short-day, intermediate-day, and long-day. Kentucky sits in a zone where variety selection matters a lot. Short-day onions will start bulbing too early in spring when the plants are still small, leaving you with golf ball or tennis ball-sized bulbs. That’s not what you want.
For spring planting, stick with intermediate-day or day-neutral varieties. Candy is a popular intermediate-day onion that performs well in Kentucky. Long-day varieties like Walla Walla and Sweet Spanish also work, but they won’t begin bulbing until June and continue maturing into early August. That longer timeline can be an advantage or a drawback depending on your goals.
For fall/overwinter planting, long-day types are actually the better choice. They showed higher survival rates, lower bolting (premature flowering that ruins the bulb), and better yields compared to intermediate-day types in Kentucky trials.
Planting Depth and Spacing
How you plant depends on whether you’re working with seeds, sets (small dry bulbs), or transplants.
- Seeds: Sow half an inch deep, either directly in the garden once soil is workable or indoors in trays for later transplanting.
- Sets: Plant 1 to 2 inches deep and 2 to 3 inches apart. Once they start growing, thin to 3 to 4 inches apart. The thinned plants make great green onions.
- Transplants: Bury the roots and about one inch of the lower stem in the soil. Space them 3 to 4 inches apart.
Onions have shallow root systems, so consistent moisture matters more than deep watering. They also compete poorly with weeds, so keep the bed clean throughout the growing season. Mulching between rows helps with both moisture and weed control.
Sets vs. Transplants vs. Seeds
Seeds are the cheapest option and give you the widest variety selection, but they require the most time and effort. Starting indoors in February and transplanting in late March means about six weeks of indoor care before your onions even see the garden.
Sets are the easiest for beginners. You push them into the soil in late March or April and let them grow. The tradeoff is a more limited selection of varieties at most garden centers, and sets are slightly more prone to bolting than transplants.
Transplants, whether store-bought or homegrown from seed, give you the best combination of variety choice and strong performance. They establish quickly in cool spring soil and generally produce the largest bulbs. Many Kentucky garden centers and online suppliers sell bundles of onion transplants timed for spring shipping.
What Affects Bulb Size
Onion bulbing is primarily driven by day length, but temperature, fertility, and watering all play a role. Larger, more established plants produce bigger bulbs once the photoperiod triggers bulb formation. This is why early planting matters: the more leaf growth your onions put on before bulbing starts, the bigger the final harvest. Every leaf corresponds to a ring in the bulb, so more leaves mean a larger onion.
Nitrogen is the most important nutrient during the leafy growth phase, roughly from planting through May. Once bulbing begins in June, stop fertilizing and let the plants direct energy into the bulb. Overwatering or fertilizing late in the season leads to thick necks that don’t cure well and shorten storage life.