Planting onion sets or slips in autumn has some distinct advantages and is much like planting garlic or shallots. But success isn't always guaranteed. Timing, along with your specific growing zone, and choosing the right onion variety all make a difference between big, flavorful bulbs and small bulbs or ones that fail to develop.

This guide tells you when and how to plant autumn onions for a bountiful harvest the following spring and summer.

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Why Plant Onions in Fall?

Onions establish and develop green tops and roots in cooler weather which translates to bigger and more flavorful bulbs. In northern zones, spring planted onions need to get into the ground early which can be challenging unless you have already prepared a raised bed or garden area. Setting them out in autumn gives plants a head start, allowing them to establish before winter frost and freezing.

After several frosts, plants go dormant. When temperatures start to rise in spring and day hours lengthen, your onions are ready to direct energy into growing bulbs.

Keeping autumn-planted onions free of weeds is easier as many garden weeds have died back.

How to Plant Onions in Fall

Both sets and slips (young plants) need four to six weeks to establish tops and roots before frost sets in. Find the first frost date in your zone and back time to find out the best day/s for planting.

1. Start by digging a trench 2 to 3 inches deep and 2 inches wide.
2. Fill the bottom of the trench with an inch or two of well-aged compost.
3. Place the sets or slips about 1/2 inch into the compost layer allowing 4 to 6 inches in between each onion. Place sets with the pointed end facing up.
4. Fill the rest of the trench with soil, covering the onion with about 1/2 inch of soil. If you start with slips, leave any green tops above the soil surface.
5. Water and mulch with a thin layer of straw or dried leaves.
6. Once new green growth appears, add more mulch and continue to water every few days. You'll need a mulch layer of at least 6 inches to protect roots when ground freezes.

Best Onions for Fall Planting

Onion sets are widely available in spring, sold as yellow, white, or red, and often not identified by variety. Try searching online or in garden catalogs to find sets or slips for autumn planting.

Look for varieties most successful in your growing zone. Onions are categorized as short-day, intermediate, or long-day. Keep in mind that sets need 80 days and slips need up to 110 days to mature from the time they're planted.

Here are a few onion varieties to consider for autumn planting.

• 'Electric': Winter hardy with shiny red skin and pink-tinged flesh. Mild, sweet flavor.
• 'Radar': Yellow, globe-shaped Japanese variety. Mild flavor and good bolt resistance.
• 'Snowball': A short-day variety with medium-sized, slightly flattened bulbs. A mild but tangy onion best for eating fresh.
• 'Granex': Large, globe-shaped bulbs are mild and sweet. A short-day variety and a popular choice for southern gardens.
• 'Walla Walla': Extra large bulbs mature in 90 days. This long day type is comparable to Vidalia onion in flavor.

Spring and Summer Harvesting

Once soil warms in spring and day time hours increase, autumn planted onions start forming bulbs. Short day onions are the quickest to bulb up and ready to harvest in as few as 75 days in late April and early May.

Intermediate and long day types will be ready to harvest from early to mid summer. The sooner daylight hours reach the time needed for bulbing, the sooner you'll be able to harvest good size onions.

When bulbs are mature, they sit partially above soil level and the green tops turn yellow and flop over. Mature bulbs can remain in the ground for one or two weeks but if left too long they become vulnerable to thrips and can soften and rot.