When to Plant Onions for the Most Productive Harvests, According to Horticulturists

Onions can be a great vegetable for backyard and container gardens. They take up minimal space, and are relatively forgiving for beginners; plus, they attract few pests. They also make excellent companion plants for a variety of herbs and veggies.

However, choosing the right onion variety and knowing when to plant them can be a confusing process to navigate. To help guide first-time growers, we spoke with experts about how to determine which type of onion is best for you—along with tips and tricks for when to grow these delicious alliums.

Onion Varieties

Onions are categorized differently than many vegetables, says Kassi Kuppinger, an avid gardener and the marketing manager for Bath Garden Center. There are short-day onions, intermediate-day (or day-neutral onions), and long-day onions, with different varieties in each of the three categories.

It's wise to choose the right variety for your region, says Sara Rubens, owner of Seed to Sanctuary. Short-day or intermediate-day onions do best in the south, and long-day onions thrive in the North.

“Some can grow very large, like mammoth or colossal varieties, and some can be especially sweet,” adds Sarah Akovic, a regenerative farmer and instructor at Lorain County Community College. “For beginners, I would recommend using varieties that have the most disease and pest resistance and steering away from large onion varieties."

Best Time to Plant Onions

The best time to plant onions depends on your zone. “In warmer zones, such as 7 and up, you can plant in late fall or winter,” says Anna Ohler, owner of Bright Lane Gardens. “Cooler zones like 6 and below should wait until early spring—as soon as the soil is workable.” 

The worst time to plant is late spring—especially in warmer regions. “The heat can cause bolting before the bulbs size up,” says Ohler. 

Rubens recommends the following guidelines:

Hardiness Zones	Onion Category	Planting Time
7 through 10	Short-Day	Sept. through Feb.
5 through 8	Intermediate-Day	Jan. through March
3 through 6	Long-Day	Feb. through April

How to Plant

“The best planting tip I can offer is starting your onions indoors, as they are slow to germinate and start off,” says Akovic. “Since I live in the long-day onion zone, I start my onions indoors from late January to early February and aim to transplant them outdoors in late March through mid-April.”

Rubens recommends the following steps for planting onions:

1. Plant onion sets or seedlings 1 inch deep and 3 to 4 inches apart. 
2. Ensure they receive six to eight hours of direct sunlight daily. 
3. Water consistently when the top inch of soil is dry, but avoid overwatering to prevent rot. A drip irrigation system can help maintain even moisture. 
4. Fertilize every two to three weeks with a balanced liquid fertilizer or nitrogen-rich options like fish emulsion. 
5. If growing from seed, thin seedlings once they are a few inches tall to avoid overcrowding.
6. Harvesting depends on the type of onion. Green onions can be picked at 6 to 8 inches tall, while bulb onions should be harvested when their tops fall over.

Container Gardening

Growing onions in containers is a great way to maximize space and control soil conditions. Ohler has grown green onions in deep pots with good drainage and watched them do surprisingly well with consistent moisture and full sun.

“Bunching onions and shallots also thrive in containers,” says Rubens. “Use a deep, well-draining pot (10 to 12 inches minimum) with a mix of potting soil, compost, and sand for optimal growth.”

Tips and Tricks

• Don’t crowd your plants. Onions need space to bulb up properly. Kuppinger recommends using a ruler to make sure you get the correct depth and spacing before planting. 
• Pay attention to your soil. Consider using organic potting soil in containers, not garden soil, says Adam Weiss, master gardener and founder of Pike Lane Gardens. “Be sure to mulch your plants to minimize the chance of disease and help prevent weeds," he adds.
• Don’t harvest too early. “Onions are ready to harvest when their green tops have fallen over,” says Akovic.