When to Plant Onions & Garlic

Onions (Allium cepa) and garlic (Allium sativum) can be planted in a variety of forms, so when you buy sets, cloves, seeds or transplants, make sure you know what you’re getting. Planting onions is complicated by their varying need for daily hours of sun, making it necessary to match an onion variety to your climate. When you plant onion or garlic depends on the form and variety you are planting.

You may see an online nursery selling onion or garlic "seeds." What they usually mean are onion or garlic cloves from a bulb.

Planting cloves is easier than the complications of germinating actual seeds. You plant the cloves and they grow into plants, a straightforward chore.

Onions and garlic do yield actual seeds that are less often planted by home gardeners. You may see them referred to as true seed, meaning they are not cloves.

Onions and garlic are both biennials that are typically grown as annuals. Beginning with a seed, they produce a mature bulb in their first growing season. After winter dormancy, they begin growing the next spring when they flower and yield seeds.

Nurseries and garden supply centers also sell transplants or seedlings that mature into onions in one growing season. The slender green starts that you buy for planting are transplants. They are sometimes, erroneously, called sets.

An onion or garlic set is a small bulb that has entered its second growth. Sets mature rapidly.

Onions come in short-day, intermediate-day and long-day varieties. Short-day varieties need 12 of sun to form bulbs. Long-day onions need 13 to 16 hours of sun. Short-day varieties planted northern latitudes form bulbs early, resulting in small onions. Long-day varieties planted in southern latitudes may never form bulbs. Thirty-five degrees north latitude is the rough dividing line between growing short-day and long-day onions.

This means that the variety you should choose depends on where you live. If you live in U.S. Department of Agriculture plant hardiness zone 7 or warmer, plant short-day onion cloves in the fall. If you live in USDA zone 6 or colder, plant long-day onion cloves in April and May.

Garlic cloves can be planted in the spring or fall. Plant them in the spring as early as you can work the soil. Plant them with the root side down and cover with 1 to 2 inches of fine soil in rows that are from 18 to 24 inches apart. You can also plant them in the middle of autumn. If you live in the northern part of the garlic variety's USDA growing range, cover the cloves with 6 inches of mulch to protect them from winter cold.

Plant onion transplants four to six weeks before the last expected spring frost in your area. If you buy these from an online source, they make look dried out. Don’t be alarmed. They have enough energy in their immature bulbs to last three weeks.

Onion bolting, typically caused by winter temperatures of 40 to 45 degrees Fahrenheit, is the quick production of flowers that result in small bulbs that are prone to rotting. To avoid bolting, plant onion or garlic sets in late winter or early spring.