How to Grow Onions Successfully (And Actually Get Big Bulbs)
- Jan 10
- 9 min read
IT'S TIME TO PLANT ONIONS!!!
Here in Texas, fall going into early winter is hands down the best time to plant onions. I’m located along the Texas Gulf Coast in the Houston metro area in zone 9a, where summers are brutal and winters are… basically a suggestion. Because of that, timing and variety matter a lot when it comes to growing onions successfully.
So instead of just telling you what to do, I want to walk you through why onions behave the way they do, how to choose the right type based on where you live, and how I personally set up my soil to grow big, healthy, properly bulbing onions.
This blog post follows along with my YouTube onion growing guide, where I plant onions in real time and explain everything step by step.
Table of Contents
Best Time to Plant Onions Based on Your Climate
How to Choose the Right Onion Type for Where You Live
How to Start Onions: Seeds vs Starts vs Sets
Soil Preparation for Growing Large, Properly Bulbing Onions
Fertilizing Onions: Why Nitrogen Is the Key to Bigger Bulbs
How to Fertilize Onions (Simple Step-by-Step)
How to Space and Plant Onions Correctly
Watering Onions Properly
Sunlight Requirements
Common Onion Pests and How to Treat Them
Common Onion Diseases and How to Manage Them
When to Harvest and Cure Onions
Best Time to Plant Onions Based on Your Climate
Onions are a long-season crop, and when you plant them depends heavily on your climate.
In warmer regions where the ground doesn’t freeze, onions are typically planted in fall to early winter so they can establish slowly and bulb in spring. In colder regions, onions are usually planted in early spring as soon as the soil can be worked. Planting at the right time gives onions enough opportunity to grow strong green tops before bulbing begins. The below chart is a general guide on when to plant onions by growing zone. Research for your area or follow onion packaging instructions for exact planting times.
How to Choose the Right Onion Type for Where You Live
Onions are daylight sensitive, meaning they require a certain number of daylight hours to trigger bulbing. That daylight requirement depends on the type of onion being grown, which in turn depends on where you live.
There are three types of bulbing onions, and the difference between them comes down to how many hours of daylight they need before they start forming a bulb.
Short-Day Onions
Short-day onions begin bulbing at about 10–12 hours of daylight. These are best suited for southern regions, generally below about 30–32° latitude, where winter and early spring days are shorter.
Common short-day onion varieties include:
Texas Sweet
Yellow Granex
White Bermuda
Red Burgundy
Intermediate-Day Onions
Intermediate-day onions begin bulbing at around 12–14 hours of daylight. These perform well in regions roughly between 32–40° latitude and are a good option for many gardeners in the middle of the country.
Common intermediate-day onion varieties include:
Candy
Super Star
Red Candy Apple
Sierra Blanca
Long-Day Onions
Long-day onions require 14–16 hours of daylight before bulbing begins. These are best suited for northern regions, typically above 40° latitude, where summer days are long.
Common long-day onion varieties include:
Walla Walla
Yellow Sweet Spanish
Red Wing
Copra
Some regions fall into a transitional range, especially in the mid-latitudes. If you live in one of these areas, you may be able to successfully grow more than one type of bulbing onion, particularly short-day and intermediate-day varieties, depending on planting time and how quickly day length increases in your area. Below is a US map that illustrates onion planting zones.
📌 In the Northern Hemisphere, gardeners in southern regions typically grow short-day onions, while northern regions grow long-day onions. In the Southern Hemisphere, this pattern is reversed because day length follows the opposite seasonal cycle.
How to Start Onions: Seeds vs Starts vs Sets
There are three ways to start onions, and while all three work, each one serves a different purpose. The best option depends on how much time you have, how much control you want over variety, and whether your goal is large bulbing onions or something else.
Starting Onions From Seed
Widest variety selection
Largest bulb potential
Lowest disease risk
Cons
Longest time investment
Requires starting early
Seedlings need hands-on care
Best for
Gardeners who can start seeds indoors
Those who want specific onion varieties
Onion seeds are planted and grown into seedlings before being transplanted into the garden.
Pros
Most cost-effective optionfor their region
Anyone trying to maximize bulb size
Using Onion Starts (Transplants)
Onion starts are onions grown from seed and pulled while still young—before bulbing begins. Because of this, they are considered first-year onions, which is important for proper bulb development.
Pros
Faster than starting from seed
More predictable results
Still capable of producing large bulbing onions
No indoor seed starting required
Cons
Limited variety selection
Higher cost than seeds
Low to moderate disease risk depending on source
Best for
Gardeners who want a head start without seed starting
Those with limited time
Anyone who wants large, reliable bulbing onions without being variety-specific
Planting Onion Sets
Onion sets are second-year onions grown the previous season and stored as small bulbs for replanting. Because onions are biennial, second-year onions are more likely to bolt.
Pros
Easiest and fastest option to plant
Widely available at nurseries and big box stores
Less expensive than onion starts
Cons
Produce the smallest bulbs
Onion type is often not clearly labeled
Higher likelihood of bolting
Highest disease risk of the three methods
Best for
Growing green onions
Interplanting as a natural pest deterrent
Areas with short growing seasons
I personally grow onion sets, but not for bulbing onions. I use them for harvesting green onion tops and for interplanting throughout the garden to help deter pests.
Soil Preparation for Growing Large, Properly Bulbing Onions
Soil is literally the lifeline to your onions. If your soil isn’t right, you can do everything else perfectly and still end up with disappointing bulbs.
Onions prefer a well-draining, sandy loam soil that’s rich in organic matter. Drainage is important because onions do not tolerate waterlogged soil, and loose soil is critical because the bulb needs room to expand as it grows. The ideal soil pH for onions falls between 6.0 and 7.0.
But beyond texture and pH, what really matters is nutrition, especially early in the onion’s life.
Onions grow in two main phases:
The vegetative phase, where the plant’s only job is to grow green tops
The bulbing phase, where the onion uses stored energy to form the bulb
During the vegetative phase, the onion is focused entirely on leaf production. This stage is critical because each green leaf directly corresponds to one ring of the onion. More leaves mean more rings, and more rings mean a larger onion.
This is where many people go wrong. They focus too early on bulbing, without realizing that bulbing is completely dependent on how much energy the plant stored during vegetative growth.
How to Space and Plant Onions Correctly
Spacing plays a big role in how large your onions can get. Crowded onions compete for nutrients and space, which limits bulb size.
When spacing onions, always consider the expected size at maturity.
For bulbing onions, I plant mine 6–8 inches apart. When planting onion starts or transplants, you want the roots and base covered, but most of the stem above the soil line (about ½ to 1 inch deep). Follow packaging instructions for correct planting depth for sets. Planting too deep can restrict bulb expansion later on. Proper spacing and planting depth give each onion enough room to grow and bulb properly.
Fertilizing Onions: Why Nitrogen Is the Key to Bigger Bulbs
There’s a common belief that phosphorus is the most important nutrient for growing big onions, because phosphorus supports root development, flowering, fruiting, and bulbing. And while that’s technically true, it’s only part of the picture.
Phosphorus helps with bulbing—but bulbing can only happen if the plant already has enough stored energy. That energy comes from photosynthesis, and photosynthesis depends on healthy green growth.
This is where nitrogen comes in. Nitrogen fuels leafy growth, and leafy growth is what allows the onion to capture energy from the sun and store it for later use. Without enough nitrogen during the vegetative phase, the onion simply doesn’t have the reserves needed to produce a large bulb, no matter how much phosphorus you add later.
For that reason, nitrogen is the most important nutrient early on when growing bulbing onions.
For my onions, I start by heavily amending the soil with compost. My compost of choice is composted chicken manure because it naturally contains higher nitrogen levels than many other composts, but any quality compost will work.
At planting, I also add:
Bone meal, which provides phosphorus to support later bulbing
A well-balanced organic fertilizer, because all nutrients still matter
How to Fertilize Onions (Simple Step-by-Step)
Step 1: Amend the soil before planting (if possible) If you can, mix your compost and fertilizers into the soil 1–2 weeks before planting. This gives nutrients time to break down and become available to the plant.
👉 If you’re short on time, it’s completely fine to amend the soil at planting. Just keep in mind nutrients may take a little longer to become available.
Step 2: Feed regularly during early growth After planting, fertilize onions with blood meal and a well-balanced organic fertilizer every 2–3 weeks while they are actively growing green tops. During this vegetative phase, continue supplying nitrogen, since this is what supports leaf growth and sets the stage for large bulbs.
👉I don't add any additional bone meal beyond the initial application at planting.
Step 3: Stop fertilizing when bulbing begins When the onion starts to form a bulb, you’ll see the base of the plant pushing up out of the soil and the soil cracking around it. This is your signal to stop fertilizing.
Adding fertilizer after this point won’t make the onion bigger and can actually interfere with proper bulb development.
How to Water Onions Properly
Onions need consistent moisture, but they do not tolerate soggy soil. Too much water can lead to rot, while inconsistent watering can stress the plant and affect bulb development.
A simple way to know when to water is the finger test. Stick your finger about 2 inches into the soil:
If the soil still feels moist, hold off on watering.
If it feels dry, it’s time to water.
The goal is to keep the soil evenly moist, especially during the vegetative phase when the plant is actively growing green tops. As onions get closer to maturity and bulbing is well underway, you can begin easing back slightly on watering.
Sunlight Requirements (With a Heat Disclaimer)
Onions grow best in full sun, meaning at least 6–8 hours of direct sunlight per day.
However, in warm climates, especially areas with intense summer heat, onions can benefit from a little afternoon shade. This helps reduce stress during the hottest part of the day and can prevent premature bolting.
The key is plenty of light without prolonged extreme heat stress.
Common Onion Pests and How to Treat Them
One advantage of onions is that they naturally deter many pests, but they aren’t completely pest-proof. The most common issues you may encounter include:
Thrips and Aphids
These pests feed on the foliage and can cause discoloration or stunted growth.
How to treat:
Spray with soapy water
A few teaspoons of Castile soap mixed with water works well
Reapply as needed
Cutworms
Cutworms chew through young plants at the soil line.
How to treat:
Remove any visible worms by hand
Apply BT (Bacillus thuringiensis), which is effective for caterpillars
Reapply weekly as needed
Onion Maggots
Onion maggots attack the roots and bulbs, causing wilting and rot.
How to treat:
Use row covers to prevent egg-laying
Treat soil with beneficial nematodes
Common Onion Diseases and How to Manage Them
Diseases are often related to excess moisture, poor airflow, or infected planting material. Choosing healthy starts and managing water properly goes a long way in prevention.
Downy Mildew
Appears as pale spots or fuzzy growth on leaves, often in cool, damp conditions.
Prevention & treatment:
Improve airflow
Avoid overhead watering
Remove affected plants if severe
Fusarium Basal Rot
Causes yellowing, wilting, and rot at the base of the bulb.
Prevention:
Use disease-free planting stock
Rotate crops
Avoid overwatering
Neck Rot
Usually shows up after harvest and affects storage onions.
Prevention:
Stop watering before harvest
Cure onions thoroughly
Only store fully cured, undamaged bulbs
Good soil drainage, proper spacing, and careful watering are the best defenses against most onion diseases.
When to Harvest and Cure Onions
Onions are ready to harvest when the tops flop over and begin turning brown. I prefer to wait until tops start to turn brown
Harvest with stems attached and cure onions in a dry, well-ventilated area out of direct sunlight for 2–3 weeks, until necks are dry and skins become papery.
Final Thoughts on Growing Onions
Even though I grow onions in raised beds, the same principles apply to containers. Onions grow exceptionally well in containers, making them a great option for balconies or small spaces. Onions aren’t difficult—but they are specific. When you match the right onion type to your location and give the plant what it needs early on, growing large, healthy onions becomes straightforward.
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