There is nothing more frustrating than taking good care of the onion plants in your garden, only to find small onions when you harvest them. I did some research to find all the possible reasons that onions fail to grow large.
So, why are your onions so small? Planting a variety of onion that is incompatible with your climate will result in smaller onions. Also, early flowering due to fluctuating temperatures causes small onions. Finally, a lack of nutrients due to competition or poor soil also causes small onions.
We’ll look at each of these factors in turn, but let’s start with choosing an onion variety that is compatible with your climate.
Choosing an Onion Variety That Is Compatible With Your Climate
As you can guess, not all onions are alike. There are three basic varieties: short-day, intermediate-day, and long-day onions.
A short-day onion plant will start to form a bulb when there are 10 to 12 hours of daylight in a day. These are typically grown in the southern part of the U.S., where they can be planted in fall or winter.
If you plant these in the northern U.S., they won’t get as large as they would in the south. Varieties include Red Creole, Texas Super Sweet, White Bermuda, and Yellow Granex.
An intermediate-day (or day-neutral) onion plant will start to form a bulb when there are 12 to 14 hours of daylight in a day. These are typically grown farther north than short-day onion plants, but further south than long-day onion plants. Varieties include Candy, Super Star, and Red Candy Apple.
A long-day onion plant will start to form a bulb when there are 14 to 16 hours of daylight in a day. These are typically grown in the northern part of the U.S. Varieties include Ailsa Craig, Copra, Highlander, Red River, and Red Zeppelin.
All of these varieties are available from Dixondale farms, and you can check out their website here.
You can also choose your varieties of onion by the Plant Hardiness Zone where you live. You can check out the USDA Plant Hardiness Zone Map here.
For instance, Zones 6 and lower are considered part of the northern U.S., where you would want to plant long-day onions to get larger onions.
Zones 7 and higher are considered part of the southern U.S., where you would want to play short-day onions.
Intermediate-day onions can be planted anywhere, but will thrive in zones 5 and 6.
Growing Onions: Seeds, Transplants, or Sets
Finally, you will need to decide whether to start onions from seeds, or to use sets or transplants to grow them.
You can start seeds inside your house using a tray with some potting soil, artificial light, and water. One advantage is that you can get started indoors before the last frost in the spring. For more information, check out this frost date calculator from the Old Farmer’s Almanac.
A transplant is a more established onion plant which does not have a bulb yet. Essentially, a transplant is a sprouted seed that someone else started for you. Both of these options will yield larger onions than growing from an onion set.
An onion set is a small bulb that you plant, which will grow into a larger bulb. A set is taken from the previous season’s onion growth, and the resulting second-year onions won’t be as large as first-year onions.
Preventing Competition: Onion Spacing and Weeding
Choosing the right onion variety for your climate is important, but your work doesn’t end there. You still need to make sure that you aren’t overcrowding your onions when you plant them. You also need to prevent weeds from getting too close to your onions and competing for resources.
Ideally, you should plant your onions 6 inches apart in a row. This gives your onions plenty of room to grow, and ensures that the onion plants will not compete for water or nutrients in the soil.
You should plant your rows 12 inches apart. This will give you room to walk between rows of onions to do weeding, mulching, fertilizing, and watering.
The best way to keep weeds out of your garden is to let your garden weed itself. To do this, use mulch, compost, or grass clippings to spread around the onion plants. For more information on the difference between mulch and compost, check out my article on mulch versus compost.
This soil cover will prevent weeds from growing and competing with the onion plants. Just make sure not to cover the onion bulbs when they form.
If any weeds do appear, simply pull them at regular intervals to help your onion plants to grow as large as possible. You can always leave the weeds out in the sun to dry and then add them to your compost pile for next year.
Ensuring Proper Soil and Nutrition for Your Onions
Before you decide on onion varieties or where to plant, you should make sure that your soil conditions are ideal for growing onions.
First, you want loose soil, since the onion bulbs will be deformed or stunted if the soil is packed tightly and difficult to move.
The soil should also be well-draining, since soil that stays wet can lead to root rot, which will lead to stunted growth or even death of the plant. If you have trouble with dry soil, check out my article on how to treat dry soil.
The soil pH should be in the range of 6.0 to 6.8 to allow for ideal growth of onions. A pH that is too low or too high can prevent the onion plant from absorbing nutrients from the soil. In this case, your onions could end up with a nutrient deficiency, even if there is plenty of that nutrient in the soil.
To test your pH, you can buy a soil test kit online or at a garden center. You can also send a sample of your soil to your local agricultural extension to get it tested. They may also be able to tell you if your soil is lacking in certain nutrients. To learn more, check out my article on soil testing.
If your soil pH is too low (acidic), add lime (calcium carbonate). If your soil pH is too high (alkaline or basic), add sulfur.
Finally, keep a sharp eye on nutrition for your onion plants. Start off the season by adding compost or a balanced fertilizer to your soil, and mix it in well.
While your onion’s leaves are growing, be sure to use a high-nitrogen fertilizer, since this will promote green growth in a plant. Ideally, you will see 13 green leaves, signaling a “perfect” onion plant.
Just remember that it is possible to over fertilize your plants, especially when using too much high-nitrogen fertilizer all at once. For more information, check out my article on over fertilizing your plants.
Eventually, you will see an onion bulb start to form, as the energy is moved from the plant’s leaves to its bulb. Part of the onion bulb will be above the soil.
This is normal, so don’t cover the top of the onion bulb with more soil. However, you can stop adding fertilizer when you see the onion bulb form.
Keeping Your Onion Plants Watered
Now that you have prepared your soil, chosen your plant varieties, and planted your onions, now it is time to make sure that they get the proper watering.
You don’t want to let the soil get bone-dry, so feel the soil every day and water if it feels dry. You should water in the morning, when temperatures are cool and the sun won’t evaporate the water too quickly.
Also, avoid getting the onion’s leaves wet, since this can cause diseases and fungus to infect the plant or the bulb.
Keep an eye on the weather forecast, and don’t water at night, or if rain is expected. If the soil stays too wet for too long, you can end up with root rot, which will stunt the onion’s growth or kill the plant outright. For more information, check out my article on over watering your plants.
Giving Your Onion Plants Enough Time to Grow to Maturity
Depending on your climate and location, your onion plants will need varying time to grow to maturity.
If you are growing short-day onions, they will need about 110 days to maturity in the southern U.S., and 75 days in the northern U.S. In the southern U.S., you can plant onions in the fall, grow them in winter, and harvest in the fall.
However, in the northern U.S., you cannot plant short-day onions in the fall, since the winter temperatures will be too cold to allow the onions to survive.
If you are growing intermediate-day onions, they will need 100 days to reach maturity. They tend to be sweet, and so they won’t store as well. Plant these onions in the spring in colder northern areas, and in the fall in warmer southern areas.
If you are growing long-day onions, they will need about 110 days to maturity. Long-day onions will not usually form bulbs in the southern U.S.
In the northern U.S., long-day onions should be planted in the early spring, after danger of frost has passed, and harvested in late summer. They tend to be less sweet, and so they store better.
When to Harvest Onions and How to Store Them
It is tempting to pull up an onion when the bulbs start to get large, but don’t do it – they may not be finished growing yet!
Eventually, the onion’s leaves will fall over, turning yellow and then brown. After this happens, then you can harvest knowing that the bulbs have grown as large as they are going to get.
Also, resist the temptation to break off the onion’s leaves early, since this will prevent the plant from getting as large as possible.
After you harvest your onions, give them some time to dry out in the sun. Cut off most of the leaves to prevent them from sprouting in storage.
If any of the onions “bolt” (form flowers), set these ones aside and use them right away in your cooking, since they will not store as well as onions that have not flowered.
By now, you should have a good idea of the factors that will affect the size of the onions you can grow in your garden. Hopefully, you also got some helpful advice to help you to grow larger onions.
If you have any questions to ask or advice to share about growing onions, please leave them in the comments below.