Homegrown onions are the real garden game-changer. (Step aside, tomato!) Not only are they easy to grow and store, but they’re also a kitchen staple. Onions are packed with nutrients like B6, B1, and folic acid, plus lots of antioxidants that fight free radicals. Eating them raw daily is even better for you—just kiss your honey first!
But growing large, beautiful onions doesn’t happen by accident. It takes the right preparation and a little know-how to avoid some common pitfalls. Let’s dive into the top mistakes to avoid so you can grow your biggest, best onions yet!
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MISTAKE #1: CHOOSING THE WRONG ONION VARIETY
Growing large onion bulbs starts with selecting the right variety for your garden. One of the most common mistakes gardeners make is planting onions not suited for their region, which can result in small bulbs or poor growth.
Onions are categorized into three types based on their response to the number of daylight hours:
Short-Day Onions
Do best in regions with mild winters and require 10–12 hours of daylight to form bulbs. In the US, this would be the southern states. Examples include: Texas Early Grano, Yellow Granex (Super Sweet), White Granex, White Bermuda, Red Grano, and Red Creole.
Long-Day Onions
Need 14–16 hours of daylight and are ideal for northern regions. Varieties like Walla Walla and Yellow Sweet Spanish fall into this category.
Day-Neutral (Intermediate-Day) Onions
Adapt to most regions, needing 12–14 hours of daylight. Options like Rainbow Trio and Red Candy Apple are excellent choices.
How to Choose the Right Onion Variety for your Garden:
MATCH YOUR REGION AND CLIMATE
Use the above map to help research your growing zone and choose onions that do best with your local day length. Planting an onion variety incompatible with your region can result in small bulbs or your onion plants bolting (flowering prematurely).
CHOOSE LARGE BULB VARIETIES
Some onion varieties are naturally bred for larger bulbs. Look for varieties specifically noted for their size potential, such as Ailsa Craig or Sweet Vidalia.
START WITH QUALITY SEEDS, TRANSPLANTS, OR SETS
Your onion’s size starts with its beginning. Purchase seeds, sets, or transplants from a trusted source to ensure they’re healthy and disease-free. Avoid planting sets that are too large (over ¾ inch in diameter) as they’re more prone to bolting.
MISTAKE #2: POOR SOIL PREPARATION
One of the most common mistakes gardeners make when growing onions is neglecting their soil preparation. Onions are heavy feeders that require rich, loose, and well-draining soil to develop large, healthy bulbs.
Learn more about the right type of soil for a raised bed garden.
Here’s how to prepare your soil for onion success:
ENRICH WITH ORGANIC MATTER
Onions thrive in nutrient-rich soil. Incorporate plenty of well-rotted compost or aged manure to improve fertility and soil structure.
Avoid fresh manure, as it can burn the plants and introduce harmful pathogens.
ENSURE PROPER DRAINAGE
Compacted or heavy clay soil can lead to waterlogged roots, which stunt onion growth and increase the risk of rot.
To improve drainage, consider adding coarse sand or growing onions in raised beds filled with loose, well-draining soil.
Learn more about amending clay soil for vegetable growing.
AVOID OVER-FERTILIZING WITH NITROGEN
While nitrogen is essential for healthy leaf growth, too much can result in lush green tops at the expense of bulb development. Balance nitrogen-rich fertilizers with phosphorus and potassium as the bulbs begin to form.
Look for a balanced fertilizer, such as a 3-8-3 Microlife Maximum Blooms blend, during the bulb development stage.
LOOSEN THE SOIL BEFORE PLANTING
Onions need space to grow, so loosen the top 8–12 inches of soil before planting. Remove any rocks or debris that could obstruct bulb growth.
MISTAKE #3: PLANTING AT THE WRONG TIME OR DEPTH
Timing and planting depth are critical when growing large onions. Planting too early, too late, or at the wrong depth can prevent onions from forming healthy, full-sized bulbs. Here’s how to get it right:
PLANT AT THE RIGHT TIME FOR YOUR REGION
Onions are sensitive to day length, so planting at the correct time is crucial for bulb development. Avoid planting too late, as onions need time to develop leaves before bulb formation begins.
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- Short-Day Onions: Plant in fall or early winter in mild climates.
- Long-Day Onions: Plant in early spring in northern regions.
- Day-Neutral Onions: These can be planted in a broader range of climates but still need the correct season based on local conditions.
DON’T PLANT TOO DEEP OR TOO SHALLOW
Onions need to be planted at just the right depth to thrive. Planting too deeply can prevent bulbs from swelling, while planting too shallowly can leave them exposed to the elements.
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- Seeds: Sow them about ¼ inch deep.
- Transplants or Sets: Plant them with the tip slightly above the soil surface, about 1 inch deep.
SPACING MATTERS
Onions need enough room to grow without competing for nutrients. Space them 4–6 inches apart in rows that are 12–18 inches apart.
MISTAKE #4: NEGLECTING WATERING NEEDS
Onions are sensitive to inconsistent watering, and improper moisture levels can lead to undersized bulbs, stress, or even diseases. To grow large, healthy onions, it’s essential to manage watering effectively.
PROVIDE CONSISTENT MOISTURE
Onions need about 1 inch of water per week, either from rainfall or supplemental irrigation. In hotter climates or sandy soils, they may require more frequent watering.
Avoid letting the soil dry out completely, as this can stunt growth and cause bulb development to halt.
DON’T OVERWATER
While onions need consistent moisture, too much water can lead to rot and fungal diseases. Ensure the soil is moist but not soggy.
Well-draining soil and raised beds can help prevent waterlogged roots.
WATER AT THE BASE OF PLANT
Wet leaves can encourage fungal diseases like downy mildew. Use drip irrigation or a soaker hose to deliver water directly to the soil.
ADJUST WATERING DURING BULB DEVELOPMENT
Once onion bulbs start to swell, reduce watering slightly to encourage proper development. Too much water during this stage can cause splitting or delay maturity.
About two weeks before harvest, stop watering entirely to allow the bulbs to cure in the ground. This step enhances their storage life.
MISTAKE #5: IGNORING WEEDS AND PESTS
Weeds and pests can wreak havoc on your onion crop, stealing nutrients, space, and moisture needed for healthy bulb development. By staying on top of both, you’ll help ensure your onions grow large and strong. Here’s how to prevent and manage common threats:
CONTROL WEEDS EARLY
Weeds compete with onions for nutrients and water, stunting their growth. Onions have shallow roots, which makes them especially vulnerable to weed pressure.
Keep the area around your onions weed-free by hand weeding or using a hoe, but be careful not to damage the onion roots.
MONITOR FOR COMMON ONION PESTS
Several pests target onions, and it’s important to catch infestations early to prevent major damage.
Onion Maggots: These larvae burrow into onion roots, causing stunted growth. Use floating row covers to protect young plants or apply beneficial nematodes to control larvae.
Thrips: These tiny insects feed on onion leaves, causing silver streaks and deformities. Insecticidal soap or neem oil can help manage thrip populations.
Onion Root Fly: This pest attacks onion roots and bulbs, especially in damp conditions. Rotate crops each year and remove any plant debris to reduce attraction.
PROTECT AGAINST DISEASES
Onions are susceptible to fungal diseases like downy mildew, rust, and botrytis, especially in humid or overly wet conditions.
Prevent these diseases by ensuring proper spacing, avoiding overhead watering, and removing any infected plants immediately.
If necessary, use an organic fungicide to control fungal outbreaks.
MISTAKE #6: FORGETTING TO HARVEST AT THE RIGHT TIME
Harvesting onions at the right time is crucial for achieving large, healthy bulbs and ensuring they store well. If you pull them too early or too late, you could lose out on the size, flavor, and longevity of your onions. Here’s how to tell when it’s time to harvest:
LOOK FOR SIGNS OF MATURITY
Onions are ready to harvest when the tops begin to yellow and fall over. This is a sign that the plant has stopped growing and the bulb is reaching its full size.
If the leaves are still green and upright, your onions are not yet fully matured, and they need more time to develop.
In some cases, you may need to gently dig around the bulb to check its size. If it’s large enough, it’s time to harvest.
TIMING IS KEY FOR STORAGE
Harvesting onions too early can result in small, underdeveloped bulbs that won’t store well. If you wait too long, the bulbs can begin to split or rot in the ground.
Aim to harvest when the bulbs have reached full maturity, but before they start to deteriorate. Generally, this is 100 to 120 days after planting, depending on the variety.
CURE ONIONS AFTER HARVESTING
Once your onions are harvested, they need to be cured to dry out the outer skin and allow the bulb to store properly.
Lay your onions in a warm, dry, well-ventilated area for 2 to 3 weeks. This will help the outer skins toughen up and the inner necks dry out.
Make sure the onions are not in direct sunlight, as this can cause them to become overly dried or sunburned.
TRIM TOPS AND ROOTS
Once onions are fully cured, trim the roots to about 1 inch and cut the tops back to 1–2 inches. Be sure not to damage the bulb while handling.
Proper trimming helps prevent moisture from entering the bulb and extends storage life.
Ready to start growing your own onions?
Growing large onions takes a bit of know-how, but avoiding these common mistakes will help you grow successfully from the start. Happy gardening!
