When To Plant Onions In Illinois

If you’re planning your garden in the Prairie State, knowing when to plant onions in Illinois is the first step to a succesful harvest. Getting the timing right is crucial because onions are sensitive to day length and frost.

This guide will walk you through the optimal planting seasons for different parts of the state, from Chicago to Carbondale. We’ll cover everything from choosing the right type of onion to the simple steps for getting them in the ground.

Illinois spans several USDA hardiness zones, from 5a in the north to 7a in the south. This means your planting dates shift by a few weeks depending on where you live. Onions can be grown from sets, seeds, or transplants, and each has its own ideal schedule.

Understanding Onion Types: Long-Day vs. Short-Day

Onions form bulbs based on day length. Choosing the wrong type for your latitude is a common mistake that leads to small bulbs.

Long-Day Onions: These need 14-16 hours of daylight to bulb. They are perfect for northern Illinois. Popular varieties include ‘Walla Walla’, ‘Yellow Sweet Spanish’, and ‘Red Zeppelin’.
Intermediate-Day Onions: They require 12-14 hours of daylight and perform well in central Illinois. Try ‘Candy’, ‘Super Star’, or ‘Red Candy Apple’.
Short-Day Onions: These bulb with 10-12 hours of daylight and are best for southern Illinois. Good choices are ‘Texas Sweet’, ‘Granex’ (Vidalia type), and ‘Red Burgundy’.

Spring Planting Timeline by Region

Onions are cool-season crops that can tolerate a light frost. The goal is to get them in the ground as soon as the soil is workable in spring.

Northern Illinois (Zones 5a-5b)

Sets & Transplants: Plant from late March to mid-April. The soil temperature should be at least 50°F.
Seeds Directly Outdoors: Sow as soon as the ground can be worked, often early to mid-April.
Seeds Started Indoors: Begin 8-10 weeks before your outdoor transplant date, around late January to early February.

Central Illinois (Zones 5b-6a)

Sets & Transplants: Aim for late March through early April.
Seeds Directly Outdoors: You can sow in early April.
Seeds Started Indoors: Start in mid-January to get a head start on the season.

Southern Illinois (Zones 6b-7a)

Sets & Transplants: Planting can often begin in early to mid-March.
Seeds Directly Outdoors: Sow in late March.
Seeds Started Indoors: Start in early January for the strongest transplants.

Fall Planting for a Head Start

In central and southern Illinois, you can plant a crop of overwintering onions. These are planted in fall, survive the winter, and are ready for an early summer harvest.

Choose the Right Variety: Select specific overwintering varieties like ‘Yellow Potato Onion’ or ‘Egyptian Walking Onions’. Some intermediate-day types also work.
Timing is Everything: Plant them about 4-6 weeks before your first hard freeze. This is usually late September to mid-October.
Provide Protection: Mulch heavily with straw after the ground freezes to prevent heaving during winter thaws.

Step-by-Step Planting Guide

Follow these steps for a healthy onion patch.

1. Prepare Your Soil

Onions need loose, well-draining soil with plenty of organic matter. Work in several inches of compost or aged manure a few weeks before planting. A soil pH between 6.0 and 7.0 is ideal. A simple test kit can tell you if you need to adjust it.

2. Choose Your Planting Method

Sets (Small Bulbs): The easiest method. Plant them root-side down, about 1 inch deep and 4-6 inches apart in rows 12 inches apart.
Transplants (Seedlings): These give you a wider variety choice. Plant them at the same depth they were growing, spacing them 4-6 inches apart.
Seeds: Sow seeds ½ inch deep and thin seedlings to the proper spacing later. This method is cheapest but takes the longest.

3. Provide Consistent Care

Onions have shallow roots and need consistent moisture, especially during bulb formation. Water deeply when the top inch of soil feels dry. Keep the area weed-free, as onions don’t compete well. A light layer of mulch helps with both moisture and weeds.

4. Know When to Harvest

Your onions are ready when the tops begin to yellow and fall over. Gently lift the bulbs from the soil. Let them cure in a warm, dry, airy place for about two weeks until the necks are tight and the outer skins are papery. Then you can store them.

Common Problems and Solutions

Even with good timing, you might face a few challenges.

Bolting (Flowering): This happens when onions are exposed to a cold snap after growth has started. It can ruin the bulb. To prevent it, choose the right day-length type and avoid planting too early.
Small Bulbs: Often caused by incorrect day-length type, overcrowding, or insufficient nutrients. Ensure proper spacing and side-dress with a nitrogen fertilizer early in the growing season.
Pests: Onion thrips and maggots are the main culprits. Use floating row covers as a barrier and practice crop rotation each year to disrupt their life cycle.

Frequently Asked Questions (FAQ)

What is the best month to plant onions in Illinois?

For most of Illinois, the prime planting month is April. In southern regions, late March is possible, and in the north, you may wait until late April. Fall planting for overwintering types happens in October.

Can you plant onions in early spring in Illinois?

Yes, absolutely. Onions are one of the first crops you can plant in early spring, as soon as the soil is no longer soggy and can be worked. They tolerate cool weather well.

How late is too late to plant onions in Illinois?

If you’re planting sets or transplants, getting them in by early May is usually the cutoff for a decent harvest. Planting later than that doesn’t give them enough time to form good-sized bulbs before the summer heat triggers bulbing.

Do onions need full sun in Illinois?

Yes, onions require a minimum of 6-8 hours of direct sunlight per day to produce well. More sun is always better for robust growth and large bulbs.

Can I grow onions from grocery store scraps in Illinois?

You can regrow the green tops from a scallion or onion bottom in water for fresh greens, but it’s very difficult to grow a full, new bulb from a store-bought onion. They are often hybrids or treated for long storage. For full bulbs, its best to use purchased sets, seeds, or transplants meant for gardening.

By following this regional guide for when to plant onions in Illinois, you’ll set your crop up for success. Remember to match your onion type to your location, prepare the soil well, and keep them watered. With a little planning, you’ll be harvesting a bounty of homegrown onions to enjoy for months to come.