When To Plant Onions In Tennessee

If you’re wondering when to plant onions in Tennessee, timing is everything for a successful harvest. Tennessee’s ideal planting schedule depends on whether you start from seeds, sets, or transplants, and your specific region within the state plays a huge role.

Getting your onions in the ground at the right moment ensures they develop strong roots before the summer heat arrives. This guide will walk you through the best dates, methods, and tips tailored for Tennessee gardeners.

Onions are a cool-season crop, meaning they thrive in the cooler temperatures of spring and fall. In Tennessee, the primary planting season is early spring. However, the exact timing shifts by a few weeks depending on where you live.

Understanding Tennessee’s Growing Zones

Tennessee spans USDA Plant Hardiness Zones 5b through 8a. This variation means winter lows and last frost dates differ significantly from the Smoky Mountains to the Mississippi River.

East Tennessee (Zones 6a-7a): This mountainous region has the latest spring frost. Aim to plant onions outdoors between March 15 and April 1.
Middle Tennessee (Zones 7a-7b): The central basin warms up a bit earlier. Your target planting window here is March 1 to March 20.
West Tennessee (Zones 7b-8a): This area has the mildest winters and earliest springs. You can safely plant onions from mid-February to early March.

Key Dates Based on Planting Method

How you start your onions changes your calendar. Here’s a breakdown:

Onion Sets & Transplants: These are the most common methods. Plant them in your garden 4-6 weeks before your last expected spring frost. The soil should be workable—not frozen or soggy.
Onion Seeds: For a wider variety, start seeds indoors 10-12 weeks before your last frost date. This means sowing seeds indoors in January (West TN) to early February (East TN). You can then move them outside at the same time as sets and transplants.
Fall Planting: In milder parts of West and Middle Tennessee, you can plant a fall crop of short-day onions in October. They’ll overwinter and produce an early summer harvest.

Using the Last Frost Date as Your Guide

Your local last frost date is your best friend. You can find it through your county extension office. Once you have it, count backwards to schedule your planting.

For example, if your last frost is April 10, you should plant sets around March 1. This gives the onions time to establish before the warm weather triggers bulbing.

Soil Temperature Matters

Onion roots develop best in cool (not cold) soil. A soil thermometer is a handy tool. Aim for a soil temperature of 50°F or higher at planting depth. If the ground is still frozen or waterlogged, wait a little longer.

What Happens if You Plant Too Early or Too Late?

Too Early: Planting in cold, wet soil can cause sets to rot or seeds to fail to germinate. A hard freeze can also damage young plants.
Too Late: Planting after the ideal window means onions won’t have enough time to grow sufficient foliage before long days trigger bulb formation. This results in smaller onions.

Choosing the Right Onion Type for Tennessee

Day length is crucial for onion bulbing. Tennessee sits in the intermediate-day onion zone. Choosing the correct type is as important as timing.

Short-Day Onions: Bulb when days are 10-12 hours long. Better suited for deep South, but can work for fall planting in TN.
Intermediate-Day Onions: Bulb when days are 12-14 hours long. These are the perfect choice for most of Tennessee. Popular varieties include ‘Candy’, ‘Super Star’, and ‘Red Candy Apple’.
Long-Day Onions: Bulb when days are 14-16 hours long. Best for northern states and will not bulb properly here.

Always check the seed packet or plant tag to ensure you’re getting intermediate-day onions.

Step-by-Step Planting Guide

Once your timing is set, follow these steps for a great start.

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. The soil pH should be between 6.0 and 6.8. A simple soil test from your extension office can provide specific recommendations.

2. Planting Onion Sets & Transplants

Space sets or transplants 4-6 inches apart in rows that are 12-18 inches apart.
Plant them so the tip is just barely visible above the soil surface. For sets, the pointed end should be up.
Firm the soil gently around them and water thoroughly.

3. Planting Onion Seeds

For direct seeding outdoors, sow seeds ½ inch deep and 1 inch apart in rows.
Once seedlings are a few inches tall, thin them to the final 4-inch spacing. You can use the thinned greens as scallions!

4. Essential Care After Planting

Watering: Provide 1 inch of water per week. Keep the soil consistently moist but not waterlogged, especially during bulb formation.
Weeding: Onions have shallow roots. Hand-pull weeds carefully to avoid damaging the bulbs. Mulching with straw can help suppress weeds and retain moisture.
Fertilizing: Use a nitrogen-rich fertilizer early in the growing season to encourage green growth. Switch to a balanced fertilizer as bulbing begins. Stop fertilizing once the bulbs start to swell.

5. Knowing When to Harvest

Harvest time in Tennessee is typically late June through July. Signs your onions are ready include:

The tops (necks) begin to soften and flop over.
The leafy tops turn yellow and start to brown.

When about half the tops have fallen over, gently lift the bulbs with a garden fork. Let them cure in a warm, dry, airy place for 2-3 weeks before storing.

Common Problems and Solutions

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

Bolting: This is when an onion sends up a flower stalk, often caused by temperature fluctuations. Choose bolt-resistant varieties and plant at the correct time. If a plant bolts, harvest and use it immediately.
Pests: Onion thrips and maggots can be a nusiance. Use floating row covers as a barrier and practice crop rotation each year.
Diseases: Fungal issues like downy mildew can occur in wet weather. Ensure good air circulation and avoid overhead watering to minimize risk.

FAQ: Planting Onions in Tennessee

Can you plant onions in the fall in Tennessee?

Yes, in parts of West and Middle Tennessee, you can plant short-day or some intermediate-day onion sets in October. They will establish roots before winter, go dormant, and resume growth in spring for an early harvest.

What is the best month to plant onions in Tennessee?

For a spring planting, the best month is generally March for most of the state. West TN can start in late February, while East TN should wait until mid-to-late March.

How long do onions take to grow in Tennessee?

From planting sets or transplants, onions typically take 90-110 days to reach full maturity. Those grown from seed will take longer, often 120-150 days total from sowing.

Can I grow onions in containers?

Absolutely. Choose a container at least 10 inches deep with good drainage. Use a quality potting mix and follow the same spacing and care instructions. This is a great option if your garden soil is heavy clay.

Why are my onions not forming bulbs?

The most common reasons are planting the wrong day-length type for Tennessee (use intermediate-day), planting too late, or not providing enough water and nutrients during the early growth stages. Crowding can also prevent proper bulbing.

By following Tennessee’s ideal planting schedule and these simple steps, you’ll be well on your way to a bountiful harvest of crisp, homegrown onions. Remember, the key is to get them in the ground early so they can grow while the weather is still cool. With a little planning, you’ll have onions to enjoy all year long.