Why Are My Onions So Small

If you’ve ever pulled up your onion crop only to find a bunch of tiny bulbs, you’re not alone. It’s a common frustration, and understanding why are my onions so small is the first step to growing bigger ones next season.

Onions are sensitive growers. Several key factors, from planting time to watering habits, determine their final size. The good news is that with a few adjustments, you can significantly improve your harvest.

Let’s look at the main reasons for small onions and how to fix them.

Small onions are usually a symptom of stress. The plant didn’t get what it needed when it needed it, so it focused on survival rather than growing a large bulb. Pinpointing the exact stressor is your key to success.

1. Incorrect Onion Type for Your Location

This is the most critical factor. Onions are classified by their daylight requirements:

Long-Day Onions: Need 14-16 hours of daylight to form bulbs. Best for northern states (north of 36th parallel).
Short-Day Onions: Need 10-12 hours of daylight to form bulbs. Best for southern states (south of 36th parallel).
Day-Neutral (Intermediate) Onions: Form bulbs with 12-14 hours of daylight. Good for many middle regions.

Planting a short-day variety in the north means it will try to bulb too early, resulting in a small onion. The reverse is also true; a long-day variety in the south may never get enough light to bulb properly.

2. Planting at the Wrong Time

Timing is everything. Onions need a long growing season to reach a good size.

In most areas, onions are best planted in early spring, as soon as the soil can be worked.
In mild-winter climates, they are often planted in fall or winter for a spring harvest.

Planting too late gives the plant less time to grow green tops before bulb formation begins. Since each leaf creates one ring in the bulb, fewer leaves means a smaller bulb.

3. Crowded Planting

Onions need space. If they’re planted too close together, they’ll compete for nutrients, water, and sunlight. This competition stunts their growth.

Follow spacing guidelines carefully. For full-sized bulbs, plant sets or transplants 4-6 inches apart in rows 12-18 inches apart. If you want to harvest some as green onions, you can plant those closer.

4. Poor Soil Conditions

Onions are heavy feeders with shallow root systems. They demand loose, fertile, and well-draining soil.

Compacted or Clay Soil: Roots can’t expand easily, restricting growth.
Low Fertility: Onions need a steady supply of nutrients, especially nitrogen early on.
Incorrect pH: Onions prefer a soil pH between 6.0 and 7.0. Too acidic or alkaline soil locks up nutrients.

5. Inconsistent Watering

Water stress is a major cause of small bulbs. The onion bulb is mostly water, so a steady supply is crucial.

Early Growth (Leaf Stage): Keep soil consistently moist.
Bulb Formation & Swelling: This is peak water need. Don’t let the soil dry out.
Late Season (Bulb Maturation): As tops begin to yellow and fall over, gradually reduce watering to cure the bulbs.

Deep, infrequent watering is better than frequent sprinkles, which encourage shallow roots.

6. Weed Competition

Weeds are stealthy thieves. They compete fiercely with onions for resources. Because onion roots are shallow and weak, they lose this battle easily. A weedy patch will always produce smaller onions.

Regular, careful weeding is essential. Mulching with straw or shredded leaves can supress weeds and help retain soil moisture.

7. Pest and Disease Pressure

Problems like onion thrips, maggots, or fungal diseases (like downy mildew) weaken the plant. A stressed plant diverts energy away from bulb growth to just staying alive.

Inspect plants regularly. Use row covers to prevent pest issues, and ensure good air circulation to reduce fungal disease risk.

8. Using the Wrong Planting Material

You can start onions from seed, sets (small bulbs), or transplants. Each has pros and cons for bulb size.

Seeds: Offer the most variety and potential for large bulbs but have the longest growing season.
Sets: Easiest and quickest, but can sometimes bolt (flower) prematurely, which stops bulb growth. Choose sets smaller than 3/4 inch in diameter to reduce bolting risk.
Transplants: These are young seedlings. They give you a head start and are reliable for good bulb development.

Your Action Plan for Bigger Onions

Now that you know the “why,” here’s the “how” to get a better crop. Follow these steps for success.

Step 1: Choose the Right Variety

Check your location’s latitude. Select a long-day, short-day, or day-neutral variety accordingly. When in doubt, ask your local nursery or extension service for recomendations. Popular large varieties include ‘Ailsa Craig’ (long-day), ‘Walla Walla’ (long-day), and ‘Texas Grano’ (short-day).

Step 2: Prepare the Soil Thoroughly

Do this a few weeks before planting.

Loosen the soil to a depth of at least 8 inches.
Mix in 2-3 inches of well-rotted compost or aged manure.
Add a balanced, slow-release fertilizer according to package directions.
Test your soil pH and adjust if needed using lime (to raise pH) or sulfur (to lower it).

Step 3: Plant Correctly and On Time

Get them in the ground early. For spring planting, that’s often 4-6 weeks before your last frost date. Plant seeds 1/4 inch deep. Plant sets and transplants about 1 inch deep. Remember the 4-6 inch spacing. Firm the soil gently around them.

Step 4: Feed Them Well

Onions need a consistent nutrient supply.

Early Season: Feed with a high-nitrogen fertilizer every 2-3 weeks until bulbs begin to form.
Bulb Formation: Switch to a fertilizer higher in phosphorus and potassium (like a 0-10-10) to support bulb development. Stop fertilizing once the bulbs start to swell significantly.

Step 5: Water Deeply and Consistently

Aim for about 1 inch of water per week, including rainfall. Use a rain gauge. Water deeply at the base of the plants. Soaker hoses or drip irrigation are ideal as they keep foliage dry, preventing disease. Reduce watering when tops begin to fall over naturally.

Step 6: Stay on Top of Weeding

Weed carefully by hand to avoid damaging shallow onion roots. Apply a light mulch after planting to supress weeds and conserve moisture. Just keep mulch away from direct contact with the onion stems.

Step 7: Know When to Harvest

Harvest at the right time for the best storage. When about half the tops have fallen over and yellowed, gently push over the remaining tops. Wait 1-2 weeks, then lift the bulbs on a dry, sunny day. Let them cure in a warm, dry, airy place for 2-3 weeks until the necks are tight and the outer skins are papery.

FAQ: Getting Larger Onions

Q: Can I use fertilizer to make my onions bigger?
A: Yes, but it must be part of the whole plan. Fertilizer alone won’t fix issues like wrong variety or poor spacing. Use it to supplement good growing practices.

Q: How long does it take to grow a full-sized onion?
A> From planting to harvest, most onions need 90-120 days. This is why starting early is so important for bulb development.

Q: Should I bend the onion tops over myself?
A: No. Let the tops fall over naturally. Forcing them over can damage the neck and invite rot. It doesn’t speed up bulbing in a helpful way.

Q: What’s the secret to huge onions?
A: There’s no single secret. It’s the combination of all the steps: perfect variety selection, flawless soil preparation, perfect timing, consistent watering, and diligent care. Championship growers pay fanatical attention to every detail.

Q: Can I replant small onions to let them grow bigger?
A: Unfortunately, no. Once an onion bulb has formed and the plant has matured, its growth cycle is complete. It won’t get any bigger if replanted. Focus on improving conditions for your next crop instead.

Growing large onions is a rewarding achievement. It requires attention to detail, but the process is straightforward. By adressing each potential stress point—choosing the right type, preparing the soil, spacing well, and providing consistent water and food—you give your onions the foundation they need to reach their full potential. Don’t get discouraged by one season of small bulbs. Take notes, adjust your methods, and look forward to a much more impressive harvest next time you dig in the garden.