When To Plant Onions In Zone 7

Knowing when to plant onions in zone 7 is the single most important factor for a succesful harvest. Get the timing right, and you’ll be rewarded with large, firm bulbs that store for months. This guide will walk you through the optimal planting windows for both seeds and sets, so you can plan your garden calendar with confidence.

Zone 7 offers a generous growing season with average winter lows between 0°F and 10°F. This climate is excellent for growing onions, as they need a long, cool period to establish roots and foliage before the warm weather triggers bulb formation. Planting at the correct time is non-negotiable for good results.

The best time to plant onions in zone 7 depends on your chosen method: starting from seed, using transplants, or planting sets (small dormant bulbs). Each has its own ideal schedule.

Planting Onion Seeds

For the widest variety of onion types, starting from seed is the way to go. You have two main options:

Indoor Seed Starting: Begin sowing onion seeds indoors 10-12 weeks before your last expected spring frost date. For most of zone 7, this means starting seeds in mid-to-late January. This gives the seedlings a strong head start.
Direct Sowing Outdoors: You can also sow seeds directly in the garden as soon as the soil is workable in late winter or early spring, typically 4-6 weeks before your last frost. This is often around late February to early March.

Planting Onion Sets & Transplants

This is the most common and easiest method for home gardeners. Onion sets and nursery-bought transplants are planted in early spring.

Optimal Window: Plant onion sets and transplants 4-6 weeks before your last spring frost date. In zone 7, this usually falls between late February and mid-March.
Soil Condition is Key: The soil must be dry enough to work without clumping. If its too wet and muddy, wait a bit longer to avoid rot.

Fall Planting for Overwintering

An advanced technique in zone 7 is fall planting for a very early summer harvest the next year.

Timing: Plant short-day onion varieties from seed or sets in October. The goal is to get them established before winter dormancy.
Considerations: This method carries more risk of winter loss, but can yield larger, earlier bulbs. Mulching heavily is essential for protection.

Understanding Onion Types: Day Length Matters

Choosing the right type of onion for your location is as crucial as timing. Onions are categorized by the ammount of daylight they need to form bulbs.

Long-Day Onions: Need 14-16 hours of daylight to bulb. These are the best choice for zone 7 gardeners. They are typically planted in spring and mature in mid-to-late summer.
Short-Day Onions: Bulb with 10-12 hours of daylight. Better suited to southern climates, but can be used for fall planting in zone 7.
Day-Neutral (Intermediate-Day) Onions: A flexible option that bulbs with 12-14 hours of daylight. They perform well across a wide range, including zone 7.

Step-by-Step Planting Guide

Follow these steps for sucessful planting and growth.

1. Preparing Your Garden Bed

Onions need loose, well-draining soil with plenty of organic matter. A few weeks before planting, work in 2-3 inches of compost or aged manure. Ensure the planting site gets full sun—at least 6-8 hours daily.

2. How to Plant Correctly

For Sets & Transplants: Space plants 4-6 inches apart in rows that are 12-18 inches apart. Plant sets and transplants just deep enough so the base is covered, with the very tip of the set or the top of the transplant’s white shank showing above the soil. Never bury them to deep.
For Seeds: Sow seeds ½ inch deep and 1 inch apart. Thin seedlings to the proper spacing of 4-6 inches once they are a few inches tall.

3. Essential Care Through the Season

Watering: Onions have shallow roots and need consistent moisture, especially during bulb formation. Provide about 1 inch of water per week, including rainfall. Reduce watering as the bulbs mature and the tops begin to fall over.
Weeding: Keep the area meticulously weeded. Onions don’t compete well for nutrients and water.
Fertilizing: Use a nitrogen-rich fertilizer early in the season to promote green growth. Switch to a balanced or lower-nitrogen fertilizer once bulbing begins (when you see the soil start to crack around the forming bulb).

Common Mistakes to Avoid

Even experienced gardeners can make these errors. Here’s what to watch out for:

Planting Too Deep: This is the most common mistake. It can restrict bulb development and cause rot.
Using the Wrong Onion Type: Planting a short-day variety in spring in zone 7 will often result in small bulbs or premature bolting (flowering).
Inconsistent Watering: Letting the soil dry out completely during bulbing can lead to split or smaller bulbs.
Ignoring Soil Fertility: Onions are heavy feeders. Poor soil will produce a poor harvest without proper ammendments.

Harvesting and Curing Your Onions

Knowing when to harvest is the final step. Your onions are ready when the tops (necks) become soft and flop over naturally. Once about half the tops in your patch have fallen, it’s time to act.

Gently lift the bulbs from the soil on a dry, sunny day.
Lay them out in a single layer in a warm, dry, well-ventilated spot out of direct rain and sun. A covered porch or garage works well.
Let them cure for 2-3 weeks until the necks are completely dry and papery, and the outer skins rustle.
Once cured, trim the roots and cut the tops back to about 1 inch. Store in a cool, dry, dark place in mesh bags or a single layer.

Frequently Asked Questions (FAQ)

What is the latest you can plant onions in zone 7?

For spring planting, aim to get sets and transplants in the ground by early April at the absolute latest. Planting later significantly reduces bulb size, as they won’t have enough time to grow adequate foliage before the long days trigger bulbing.

Can I plant onions in the fall in zone 7?

Yes, but it requires careful variety selection and timing. Plant short-day onion sets or seeds in October, protect them with a thick layer of mulch, and they will resume growth in early spring for a June harvest.

How do I know if my onions are long-day or short-day?

Always check the seed packet or plant label. Reputable suppliers will clearly state the onion’s day-length requirement. When in doubt for a spring planting in zone 7, choose long-day or day-neutral varieties.

Why are my onions bolting (flowering)?

Bolting is often caused by stress, such as temperature fluctuations (a late cold snap after warm weather) or inconsistent watering. Once an onion sends up a flower stalk, the bulb stops growing. Harvest and use bolted onions immediately, as they won’t store well.

What should I plant after onions are harvested?

Onions are light feeders, making them a great crop for rotation. After harvest in mid-to-late summer, you can plant a fast-growing fall crop like lettuce, spinach, radishes, or bush beans in the vacated space.

By following this guide for when to plant onions in zone 7, you set the foundation for a bountiful harvest. Remember the golden rules: plant long-day varieties in early spring, keep them weeded and watered, and be patient. With good timing and basic care, you’ll be pulling up plump, storage-worthy onions by midsummer. There’s nothing quite like the satisfaction of adding homegrown onions to your kitchen.