How To Plant Onion Plants In The Garden: Step-by-Step

What is the best way to plant onion plants in the garden? The best way to plant onion plants in the garden is to prepare your soil well, choose the right time to plant, and give them enough space to grow. This guide will walk you through everything you need to know.

How To Plant Onion Plants In The Garden
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Getting Started with Onion Plants

Growing onions is a rewarding experience for any gardener. From the satisfying crunch of fresh onion rings to the foundational flavor they add to countless dishes, homegrown onions are a staple. Whether you’re a seasoned gardener or just starting out, this guide will provide clear, actionable steps to ensure a successful onion harvest. We’ll cover everything from selecting your onion varieties to the final steps of harvesting onions.

Choosing Your Onion Plants: Sets vs. Seeds

You have two main options when starting onions: onion sets and seeds.

Onion Sets: These are small, immature bulbs that were grown from seed the previous year. They are the easiest and quickest way to grow onions.
- Pros: Faster growth, higher success rate for beginners, less prone to disease.
- Cons: Limited variety selection, can sometimes bolt (go to seed) prematurely, bulbs may be smaller than those grown from seed.
Seeds: Growing from seed offers a wider array of onion varieties and can result in larger bulbs.
- Pros: Huge variety selection (including heirloom types), potentially larger bulbs, more cost-effective for large quantities.
- Cons: Takes longer to mature (often require starting indoors 8-10 weeks before the last frost), more susceptible to disease and pests, requires more patience.

For beginners, onion sets are often the preferred choice due to their simplicity and speed. If you’re looking for specific flavors, sizes, or colors, seeds are the way to go.

When to Plant Onions

The timing of your onion planting depends on your climate and the type of onion you are growing.

Cool-Season Crop: Onions are cool-season crops. They grow best when temperatures are between 55-75°F (13-24°C).
Spring Planting: In most temperate climates, onions are planted in early spring, a few weeks before the last expected frost. This allows the young plants to establish roots before warmer weather arrives.
Fall Planting (Overwintering): In milder climates with short winters, some onion varieties can be planted in the fall. These plants overwinter and are ready for harvest in late spring or early summer. Research varieties suited for overwintering in your region.

Preparing Your Garden Bed for Onions

Proper soil preparation is crucial for healthy onion growth. Onions thrive in well-drained, fertile soil with a slightly acidic to neutral pH (6.0-7.0).

Soil Amendments

Before planting, it’s beneficial to amend your soil.

Compost: Mix in a generous amount of well-rotted compost. Compost improves soil structure, drainage, and provides essential nutrients. Aim for about 2-4 inches of compost worked into the top 6-8 inches of soil.
Fertilizers: Onions are heavy feeders. If your soil is not rich, consider adding a balanced fertilizer. Look for one with a good amount of phosphorus and potassium, as these are important for bulb development. A soil test can help determine specific nutrient needs.

Ensuring Good Drainage

Onions do not like to sit in waterlogged soil, which can lead to bulb rot.

Raised Beds: If you have heavy clay soil or experience frequent rainfall, consider planting in raised beds. This naturally improves drainage.
Adding Organic Matter: As mentioned, compost helps improve drainage in clay soils. For sandy soils, compost helps retain moisture and nutrients.

Soil pH Check

Testing: Use a soil test kit to check your soil’s pH.
Adjusting: If your soil is too acidic (pH below 6.0), you can add lime. If it’s too alkaline (pH above 7.0), you can add sulfur or organic matter like peat moss. Follow the recommendations on your soil test report for accurate application rates.

Planting Onion Sets: A Step-by-Step Guide

Planting onion sets is a straightforward process. Follow these steps for the best results.

Step 1: Prepare the Sets

Inspect: Before planting, check your onion sets for any signs of rot or mold. Discard any that look unhealthy.
Soaking (Optional): Some gardeners soak their sets in water for a few hours before planting. This can help rehydrate them. You can add a bit of seaweed extract or a weak fertilizer solution to the water.

Step 2: Digging the Furrows or Holes

Furrows: For planting in rows, create shallow furrows about 4-6 inches apart. The furrow depth should be around 2-3 inches.
Individual Holes: If planting in a scattered manner or in smaller beds, dig individual holes that are about 2-3 inches deep.

Step 3: Planting Depth and Spacing

This is where planting depth and spacing onions become critical.

Planting Depth

The Rule: Place each onion set pointy-end up in the furrow or hole. The tip of the bulb should be about 1 inch below the soil surface.
Why it Matters: Planting too deep can delay growth. Planting too shallow can cause the sets to dry out or be dislodged.

Spacing Onions

Row Spacing: Space rows about 12-18 inches apart to allow for good air circulation and easy access for weeding and harvesting.
Within the Row: Space the onion sets about 4-6 inches apart in the row.
- Closer Spacing (4 inches): If you plan to harvest green onions or “bunching onions” early, you can plant them closer together.
- Wider Spacing (6 inches): For larger bulb onions, 6 inches apart provides enough room for them to swell.

Step 4: Covering the Sets

Gently cover the onion sets with soil. If you planted in furrows, simply pull soil back into the furrow. If you used holes, backfill them with soil.

Step 5: Watering

Initial Watering: Water the newly planted area gently but thoroughly. This settles the soil around the sets and helps them begin to root.

Planting Onion Seeds or Transplants (For the More Adventurous Gardener)

If you’ve chosen to grow from seed or have purchased onion transplants, the process is slightly different.

Starting Seeds Indoors

Timing: Start seeds indoors 8-10 weeks before your last expected frost date.
Containers: Use seed trays or small pots filled with a seed-starting mix.
Sowing: Sow seeds about 1/4 inch deep.
Light and Water: Keep the soil consistently moist and provide plenty of light (a sunny windowsill or grow lights).
Thinning: Once seedlings have a few true leaves, thin them to about 1 inch apart.

Transplanting Onions (Seedlings)

Hardening Off: Before transplanting onions outdoors, you need to “harden them off.” This means gradually exposing them to outdoor conditions over a week or two. Start with a few hours in a sheltered spot and increase the time and exposure gradually.
Soil Prep: Prepare the garden bed as described earlier.
Planting Depth: Plant seedlings so that the roots are covered, and the bulb just begins to form at the soil surface. The planting depth is shallower for transplants than for sets, with the bulb itself sitting just at or slightly above soil level.
Spacing: Space transplants about 4-6 inches apart in rows that are 12-18 inches apart.
Watering: Water thoroughly after transplanting.

Onion Care Throughout the Growing Season

Consistent onion care will lead to a bountiful harvest.

Watering

Consistency: Onions need consistent moisture, especially during the bulb formation stage. Aim for about 1 inch of water per week, either from rain or irrigation.
Avoid Overwatering: While they need moisture, avoid waterlogged conditions.
Reduce Watering: As onions approach maturity and their tops start to yellow and fall over, gradually reduce watering. This helps the bulbs cure properly.

Weeding

Competition: Onions are poor competitors with weeds. Keep the planting area meticulously weeded.
Gentle Weeding: Be careful when weeding, as onion roots are shallow and easily damaged. Hand-pulling is often best. Mulching can help suppress weeds and retain moisture.

Fertilizing

Early Growth: In the first few weeks after planting, a nitrogen-rich fertilizer can encourage leafy growth.
Bulb Development: Once the bulbs start to form, switch to a fertilizer that is lower in nitrogen and higher in phosphorus and potassium. This promotes bulb size and quality. Follow package directions for application rates and frequency.

Mulching

Benefits: Applying a layer of organic mulch (like straw or shredded leaves) around your onion plants offers several benefits:
- Suppresses weeds.
- Retains soil moisture.
- Keeps soil temperatures more even.
- Prevents soil splashing onto the leaves, which can reduce disease spread.
Application: Apply mulch when plants are about 4-6 inches tall. Leave a small gap around the base of the plants to prevent rot.

Pests and Diseases

Common Pests: Onion flies, thrips, and slugs can be problematic.
- Onion Flies: Row covers can help prevent adult flies from laying eggs.
- Thrips: These tiny insects cause silvery streaks on leaves. Good garden hygiene and beneficial insects can help.
- Slugs: Use slug bait or beer traps.
Common Diseases: Downy mildew and purple blotch are common fungal diseases.
- Prevention: Good air circulation (proper spacing onions), avoiding overhead watering, and choosing disease-resistant onion varieties are key preventive measures. Remove and destroy any infected plant parts immediately.

Companion Planting with Onions

Companion planting is a gardening technique where certain plants are grown together because they benefit each other. Onions can be good companions for many vegetables.

Beneficial Companions for Onions

Carrots: Onions deter carrot rust flies, and carrots can deter onion flies.
Cabbage family (Broccoli, Cauliflower, Brussels Sprouts): The strong smell of onions can help deter cabbage moths.
Lettuce and Spinach: These leafy greens appreciate the shade provided by taller onion plants and benefit from the onions deterring pests.
Strawberries: Onions can help deter strawberry weevils.
Chamomile and Dill: These herbs can attract beneficial insects that prey on onion pests.

Plants to Avoid Planting Near Onions

Beans and Peas: Legumes can stunt the growth of onions.
Asparagus: Asparagus can inhibit onion growth.
Sage: While some herbs are beneficial, sage can negatively impact onion growth.

Harvesting Onions

Knowing when and how to harvest is the final step in the onion-growing process.

Signs of Maturity

Fallen Tops: The most common sign that onions are ready for harvest is when the green tops begin to yellow, wither, and fall over. This typically happens when the bulbs have reached maturity.
Neck Softening: The “neck” of the bulb, where the leaves emerge, will soften.
Bulb Size: While tops falling is a good indicator, you can also gently dig up a few to check the bulb size you desire.

How to Harvest

Stop Watering: A week or two before you plan to harvest, stop watering the onions. This helps the bulbs start to cure in the ground.
Loosen the Soil: On the day of harvest, gently loosen the soil around the bulbs with a garden fork or trowel. Be careful not to damage the bulbs.
Pull or Lift: Carefully pull the onions from the ground by their tops. If the soil is very dry, they should come out easily.

Curing Onions

Proper curing is essential for long-term storage.

Drying: Once harvested, spread the onions in a single layer in a warm, dry, well-ventilated area. A garage, shed, or covered porch works well.
Conditions: Ideal curing conditions are around 75-85°F (24-29°C) with good air circulation.
Duration: Cure the onions for 1-2 weeks, or until the necks are completely dry and the outer skins are papery and crisp. The tops should be brittle.
Trimming: After curing, trim the roots and cut the tops, leaving about 1 inch of the stem attached to the bulb.

Storing Onions

Conditions: Store cured onions in a cool, dry, dark place with good air circulation. Mesh bags, crates, or even old pantyhose work well for hanging.
Temperature: Aim for temperatures between 40-50°F (4-10°C). Avoid storing onions with potatoes, as they release moisture that can cause onions to spoil.
Longevity: Properly cured and stored onions can last for several months.

Troubleshooting Common Onion Growing Issues

Onions Not Forming Bulbs

Too Much Nitrogen: Excessive nitrogen fertilizer can promote leafy growth at the expense of bulb formation.
Incorrect Day Length: Onions are sensitive to day length. “Long-day” varieties need longer daylight hours to trigger bulbing, while “short-day” varieties bulbing is triggered by shorter days. Ensure you’ve chosen a variety suited to your climate’s day length.
Overcrowding: If spacing onions too closely, they won’t have enough room to form large bulbs.
Stress: Drought stress or extreme heat can prevent bulbing.

Onions Bolting (Going to Seed)

Stress: Bolting is often triggered by stress, such as sudden cold snaps after a warm spell, or drought followed by heavy rain.
Variety: Some onion varieties are more prone to bolting than others.
Set Size: Very large or very small onion sets can sometimes be more likely to bolt.

Frequently Asked Questions About Planting Onions

Q: Can I plant onion sets too close together?

A: While you can plant them closer for green onions, for bulb onions, planting them too close (less than 4 inches apart) will result in smaller bulbs due to competition for space, water, and nutrients.

Q: My onion tops are yellowing, but they haven’t fallen over. Should I harvest?

A: Yellowing tops are a good sign, but the fallen tops are a more reliable indicator of maturity for bulb onions. If the tops are still standing, the bulbs may not be fully developed. Give them a bit more time.

Q: What is the best type of soil for growing onions?

A: Onions prefer well-drained, fertile soil with good aeration and a pH between 6.0 and 7.0. Sandy loam is often considered ideal.

Q: How deep should onion sets be planted?

A: Onion sets should be planted about 1-2 inches deep, with the pointy end facing upwards, so the tip is just covered by soil.

Q: Can I plant onions in pots?

A: Yes, you can grow onions in pots, especially smaller varieties or for green onions. Ensure the pot is deep enough (at least 6-8 inches) and has good drainage. Use a well-draining potting mix.

Q: What is the difference between short-day, intermediate-day, and long-day onions?

A: This refers to the number of daylight hours required to trigger bulb formation.
* Short-day: Need about 10-12 hours of daylight. Best for southern regions.
* Intermediate-day: Need about 12-14 hours of daylight. Good for middle latitudes.
* Long-day: Need about 14-16 hours of daylight. Best for northern regions.

Choosing the right type for your latitude is crucial for successful bulbing.

By following these detailed steps, you’ll be well on your way to enjoying a plentiful harvest of delicious, homegrown onions. Happy gardening!