When To Plant Onions In Zone 7b

Getting your onion planting timing right is the single biggest factor for a successful harvest. If you’re wondering when to plant onions in zone 7b, you’ve come to the right place for clear, actionable advice.

Zone 7b, with its average minimum winter temperatures of 5 to 10°F, offers a long growing season that’s perfect for onions. But to hit that optimal harvest window, you need to work with two distinct planting times: fall and spring. This guide will walk you through both, ensuring you get the biggest, best-storing bulbs from your garden.

For gardeners in zone 7b, the calendar offers two excellent windows for planting onions. Your choice depends on the type of onion you want to grow and your harvest goals.

The primary planting is in early spring. However, a fall planting is also highly successful for certain varieties. Let’s break down the specific timing for each.

Spring Planting: The Main Event

Spring planting is the most common method. You’re putting small onion plants (sets, transplants, or seeds) into the ground as it becomes workable.

Onion Sets & Transplants: Plant these in late February through mid-March. The goal is to get them in the ground 4-6 weeks before your last expected spring frost. For most of zone 7b, the last frost date falls around mid-April.
Onion Seeds (Direct Sown): For starting from seed directly in the garden, sow them as soon as the soil can be worked, often in late February. They are more frost-tolerant than you might think.
Onion Seeds (Started Indoors): Start your seeds indoors about 10-12 weeks before your planned spring transplant date. That means sowing seeds in flats around early January.

Fall Planting: For an Early Start

Fall planting is a fantastic secret for zone 7b gardeners. You plant in autumn, the onions establish roots, go dormant in winter, then explode with growth first thing in spring.

Best Timing: Plant onion sets or transplants in October. The exact date is less critical than the soil temperature. Aim for a date about 4-6 weeks before your first hard freeze, which allows time for root establishment.
Variety is Key: Fall planting is best for overwintering varieties, which are specifically bred to survive the cold. Short-day and some intermediate-day types often perform well. Always check the seed packet or plant tag.

Understanding Onion Types: Day Length Matters

You can’t just plant any onion anytime. Onions are classified by the ammount of daylight they need to form bulbs. Matching the type to our zone is non-negotiable.

Short-Day Onions: These bulb when daylight lasts 10-12 hours. They are ideal for southern zones. In zone 7b, plant them in fall or very early spring. They’ll mature earlier, often by late May or June.
Intermediate-Day (Day-Neutral) Onions: These need 12-14 hours of daylight. They are the perfect choice for zone 7b and offer the widest selection of varieties. Plant them in early spring for a summer harvest.
Long-Day Onions: Require 14-16 hours of daylight and are best for northern zones. They typically won’t bulb properly in zone 7b and should be avoided.

Step-by-Step Planting Guide

Once you’ve got your timing and type sorted, follow these steps for success.

1. Preparing Your Soil

Onions need loose, well-draining soil rich in organic matter. They are heavy feeders, especially needing phosphorus and potassium for root and bulb development.

Work the soil to a depth of at least 8 inches, removing any rocks or debris.
Mix in several inches of finished compost or well-rotted manure.
Add a balanced, slow-release fertilizer or a specific bulb fertilizer according to package directions. Avoid high-nitrogen mixes, which promote leaf growth at the expense of the bulb.

2. How to Plant Correctly

Proper planting depth and spacing is crucial for bulb formation.

Sets & Transplants: Plant them so the tip is just barely visible above the soil surface. If you bury them to deep, bulb formation can be hindered. Space them 4-6 inches apart in rows that are 12-18 inches apart.
Seeds: Sow seeds about ½ inch deep and 1 inch apart. You can thin them later to the proper spacing, using the thinned green onions in your kitchen.

3. Essential Care Through the Season

Consistent care is what turns a small set into a large, storage-worthy onion.

Watering: Onions have shallow roots. They need consistent moisture, especially during bulb formation. Provide about 1 inch of water per week. Reduce watering as the tops begin to yellow and fall over, signaling maturity.
Weeding: Keep the area meticulously weeded. Onions don’t compete well for nutrients and water.
Fertilizing: Side-dress with a nitrogen fertilizer about 3 weeks after planting, and again when bulbs begin to swell. Stop fertilizing once the bulbs have formed.

Harvesting and Curing for Storage

Knowing when to harvest is as important as knowing when to plant. Harvest timing depends on your planting time and onion type.

Signs Your Onions Are Ready

The green tops (necks) become soft, yellow, and start to fall over naturally. Do not bend them over yourself.
Once about half the tops in a patch have fallen, it’s time to harvest.

The Curing Process

Curing is essential for long-term storage. It dries the outer layers and the neck, sealing the bulb.

Gently lift the bulbs from the soil on a dry, sunny day. Brush off excess dirt, but don’t wash them.
Lay them in a single layer in a warm, dry, well-ventilated place out of direct sun. A covered porch, garage, or shed is perfect.
Let them cure for 2-3 weeks. The necks should be completely dry and papery, and the outer skins rustle when touched.
Once cured, trim the roots and cut the tops down to about 1 inch. Store in a cool, dry, dark place in mesh bags or a single layer in a box.

Common Problems and Solutions in Zone 7b

Every garden has it’s challenges. Here are a few to watch for.

Bolting: This is when an onion sends up a flower stalk. It’s often caused by temperature fluctuations (a warm spell followed by a cold snap). Once an onion bolts, the bulb stops growing. Choose bolt-resistant varieties and follow planting timelines to minimize risk.
Pests: Onion thrips and onion maggots are the main culprits. Use floating row covers to prevent maggot flies from laying eggs. For thrips, a strong spray of water or insecticidal soap can help.
Diseases: Fungal diseases like downy mildew or botrytis can be a problem in our humid zone 7b springs. Ensure good air circulation by proper spacing, rotate your onion bed each year, and avoid overhead watering.

FAQ: Your Onion Planting Questions Answered

Can I plant onions in the fall in zone 7b?

Absolutely. Planting overwintering onion sets or transplants in October is a great strategy for an earlier summer harvest. Just make sure you select varieties labeled for overwintering.

What is the best month to plant onions in my zone?

For spring planting, March is your prime month for sets and transplants. For fall planting, October is ideal. If direct sowing seeds in spring, late February is a good target.

How long do onions take to grow?

From planting sets or transplants, onions typically take 3 to 4 months to reach full maturity. From seed, it takes 5 to 6 months. Fall-planted onions will be ready for harvest earlier the following summer.

Should I grow onions from seeds or sets?

Sets are easier and faster, offering a head start. Seeds provide a much wider variety selection and are often more economical for large plantings. Transplants offer a good middle ground.

Can I grow onions in containers?

Yes. Choose a container at least 10 inches deep and wide. Use a well-draining potting mix and ensure it gets full sun. Water more frequently, as containers dry out faster.

By aligning your planting schedule with the unique climate of zone 7b and understanding the needs of different onion types, you set the stage for a bountiful harvest. Remember, the key is in the timing: get those plants in the ground during those sweet spots of early spring or fall, provide consistent care, and you’ll be rewarded with a cellar full of homegrown onions that will last for months.