How to Plant Onion Plants: Step-by-Step Guide for Success

Plant onion sets 1-2 inches deep with 4-6 inches between plants in well-drained soil with pH 6.0-7.0, ideally 4-6 weeks before your last spring frost. Water consistently but avoid overwatering, and harvest when tops fall over naturally in mid-to-late summer.

Discover exactly how to plant onion plants for a bountiful harvest with this practical guide. Whether you're growing onions from sets, seeds, or transplants, proper planting technique directly impacts your yield, flavor, and storage potential. Get science-backed timing recommendations, spacing requirements, and soil preparation methods that work for both beginner and experienced gardeners.

When to Plant Onions: Timing Is Everything

Timing your onion planting correctly makes the difference between small, weak bulbs and robust, flavorful harvests. Onions are cool-season crops that need specific day lengths to form bulbs properly. In most regions, plant onion sets 4-6 weeks before your last expected spring frost date when soil temperatures reach at least 50°F (10°C). Fall planting works in zones 6 and warmer, typically 4-6 weeks before first frost.

For precise timing, consult your local agricultural extension service's frost date calculator. The University of Georgia Extension provides a reliable frost date tool that factors in your specific zip code and historical climate patterns.

Onion Type	Best Planting Time	Days to Maturity	Ideal Climate Zone
Short-Day Onions	Late fall to early winter	90-110 days	Zones 7-10
Intermediate-Day Onions	Early spring	100-120 days	Zones 5-7
Long-Day Onions	Late winter to early spring	110-130 days	Zones 2-5

This fact comparison table shows why matching your onion variety to your climate zone matters. Planting the wrong type for your region results in poor bulb formation. Short-day varieties need 10-12 hours of daylight to bulb, while long-day types require 14-16 hours. Check seed packets for "day length requirement" to select appropriate varieties.

Soil Preparation: The Foundation for Healthy Onions

Onions thrive in loose, well-drained soil rich in organic matter. Prepare your planting bed 2-3 weeks before planting by:

• Removing all weeds and debris
• Amending soil with 2-3 inches of compost or well-rotted manure
• Testing soil pH (ideal range: 6.0-7.0)
• Adding bone meal if phosphorus is low

Avoid fresh manure which can cause excessive top growth at the expense of bulb development. The USDA Natural Resources Conservation Service confirms that onions perform best in soils with at least 2% organic matter content for optimal moisture retention without waterlogging.

Planting Onion Sets Step-by-Step

Planting onion sets (small dormant bulbs) is the easiest method for beginners. Follow these steps for success:

1. Choose firm, disease-free sets larger than ½ inch in diameter
2. Plant with pointed end up, 1-2 inches deep in heavy soil, 2-3 inches in sandy soil
3. Space sets 4-6 inches apart in rows 12-18 inches apart
4. Gently firm soil around each set without compacting
5. Water thoroughly after planting

Many gardeners make the mistake of planting sets too deep, which delays emergence and reduces bulb size. Research from University of Minnesota Extension shows sets planted at 1-inch depth produce 25% larger bulbs than those planted at 3 inches. For seed planting, sow ¼ inch deep with 1 inch between seeds, thinning to final spacing as plants develop.

Onion Growth Timeline: What to Expect

Understanding the onion growth cycle helps you provide appropriate care at each stage:

• Weeks 1-2: Root development begins; green shoots emerge
• Weeks 3-6: Top growth accelerates; consistent watering critical
• Weeks 7-10: Bulb formation starts as day length increases
• Weeks 11-16: Bulb swelling phase; reduce watering as harvest approaches
• Week 16+: Tops fall over naturally; time for harvest

This timeline varies based on variety and climate. In cooler northern regions, long-day varieties take 110-130 days from planting to harvest, while short-day types in southern climates mature in 90-110 days. Track your specific variety's days to maturity for accurate harvest timing.

Watering and Fertilizing for Maximum Bulb Development

Consistent moisture is crucial for onion growth. Water 1-2 inches per week, keeping soil moist but not soggy. Increase watering during bulb formation (when tops reach 6-8 inches tall), then gradually reduce irrigation 2-3 weeks before expected harvest to encourage proper curing.

Fertilize with nitrogen-rich fertilizer every 2-3 weeks until bulb formation begins, then switch to a balanced fertilizer. The Oregon State University Extension recommends 100-150 pounds of nitrogen per acre for optimal onion production, which translates to about 2-3 tablespoons of 10-10-10 fertilizer per 10 square feet for home gardens.

Common Planting Mistakes to Avoid

Even experienced gardeners make these onion planting errors:

• Planting too early in cold, wet soil causing sets to rot
• Overcrowding plants reducing bulb size and increasing disease risk
• Planting sets too deep delaying emergence and reducing yield
• Using poor quality sets that are sprouted or shriveled
• Planting the wrong variety for your day length requirements

Context matters significantly for onion success. In regions with heavy clay soil, raised beds improve drainage. In areas with intense summer heat, plant earlier to complete bulb formation before extreme temperatures. The USDA National Agricultural Statistics Service reports that soil temperature below 50°F significantly slows onion root development, making proper planting timing essential.

Harvesting and Curing Your Onion Crop

Harvest onions when at least 50% of the tops have fallen over naturally. Gently lift bulbs with a garden fork, then cure them in a warm, dry, well-ventilated area for 2-3 weeks. Properly cured onions develop dry, papery skins that protect the bulb during storage.

Store cured onions in mesh bags or crates in a cool, dry location (32-45°F with 65-70% humidity). Under ideal conditions, long-day storage varieties can last 6-8 months. Check periodically for soft spots or sprouting, which indicate spoilage.

Troubleshooting Common Onion Problems

Address these issues promptly to save your crop:

• Poor bulb formation: Usually caused by wrong variety for your day length or overcrowding
• Thick necks: Result from excess nitrogen late in season or improper curing
• Yellowing leaves: Could indicate onion thrips, nutrient deficiency, or overwatering
• Rotting sets: Typically from planting in cold, wet soil or using damaged sets

For pest management, practice crop rotation (don't plant onions in the same spot more than once every 3 years) and use floating row covers to prevent insect damage. The Oregon State University Extension confirms that proper spacing (4-6 inches) reduces fungal disease incidence by improving air circulation around plants.

Frequently Asked Questions

Can I grow onions in containers? Yes, use containers at least 8 inches deep with drainage holes. Plant sets 4 inches apart in quality potting mix, keeping soil consistently moist. Container-grown onions need more frequent watering and fertilizing than garden-planted ones.

How do I know when onions are ready to harvest? Onions are ready when the tops fall over naturally and begin to yellow. Stop watering 2-3 weeks before expected harvest to encourage the necks to dry. Harvest when about half the tops have fallen; waiting too long risks bulbs re-sprouting in the ground.

Should I remove the flower stalks if my onions bolt? Yes, immediately cut off flower stalks at the base when they appear. Bolting diverts energy from bulb development to seed production, resulting in smaller, less storage-stable onions. Bolting often occurs after temperature fluctuations or planting sets too early.

What's the best depth for planting onion sets?

Plant onion sets 1-2 inches deep in heavy soil and 2-3 inches deep in sandy soil, with the pointed end facing up. Planting too deep delays emergence and reduces bulb size, while planting too shallow exposes bulbs to sunscald.

How far apart should I space onion plants?

Space onion sets 4-6 inches apart in rows 12-18 inches apart. Proper spacing ensures adequate room for bulb development while allowing good air circulation to prevent disease. Crowded plants produce smaller bulbs and are more susceptible to fungal issues.

Can I plant onions in the fall for spring harvest?

Yes, in zones 6 and warmer, plant short-day onion varieties 4-6 weeks before first fall frost for overwintering. Mulch heavily after planting to protect from extreme cold. These will resume growth in spring and typically harvest 2-3 weeks earlier than spring-planted onions.

How often should I water newly planted onions?

Water newly planted onions thoroughly after planting, then maintain consistent moisture with 1-2 inches of water per week. During bulb formation (when tops reach 6-8 inches), increase watering slightly, then gradually reduce irrigation 2-3 weeks before harvest to encourage proper curing.