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Super-Size Your Onion Harvest: 15 Pro Tips and Tricks for a Record-Breaking Crop

Fresh harvest onions and in raised beds.Onions come in many sizes, from tiny wild varieties to the massive sweet onions found in supermarkets.

With the proper garden techniques, you can improve the yield and size of your harvest. Keep in mind different factors so you can plan ahead better when you’re planting your garden. This includes the time you have available, as well as resources. Finally, consider the space you have, and if you want to use greenhouses for a better grow.

1. Choosing the Right Variety

Large variety of onions grows in raised bed.

One of the essential factors in getting big onions is to choose a suitable variety. There are many varieties of onion, each with its own strengths and weaknesses. Some varieties are better suited to certain climates, while others have been bred for high yields or large sizes.

Long day onions are a good choice for gardeners in the northern US. This is because they begin to produce bulbs when the length of the day is around 14 hours. On the other hand, short-day onions are better suited to southern gardens. They start to form bulbs when the days are shorter, around 10-12 hours long.

Intermediate day onions can be grown in both northern and southern regions. They produce bulbs in a range of day lengths, from 12-14 hours. While there are many different varieties of onion, not all of them will produce large bulbs. For the best chance of harvesting big onions, choose varieties listed as suitable for “storage” or “bunching.”.

These varieties tend to be larger than others. Some good storage onion varieties include “Candy,’” “Yellow Globe,” and “Supersweet.” Bunching onion varieties that are good for large sizes include “Beltsville Bunching,” “White Lisbon,” and “Evergreen Hardy White.”

Size – You’ll want to choose an onion variety that is known for producing large onions. There are also intermediate varieties that will do well in either region.

Other essential factors to consider when choosing an onion variety include: Days to maturity – The time from planting to harvest can vary significantly between different onion varieties.

If you want to get big onions quickly, choose a variety with few days to maturity. However, if you’re willing to wait a little longer for your onions to reach full size, go for a variety that matures in a longer time.

2. Planting at the Right Time

Give your onions a 10-15 week head start before putting them in the ground or a bed. The ideal germination temperature is between 45°F to 85°F. They may take between 4 days to 20 days to germinate. Seeds are viable for between one to four years.

The ideal time to plant them is after the frost melts. However, this can vary in different climates or growing conditions, like using greenhouses. 

3. Growing Via Seed

Onion seedlings in the raised bed.

When growing onions, you can sow them by seed or transplant bulbs. Start the seedlings off in a seed tray if you choose to grow them by seed. Once the seedlings have grown large enough, transplant them into the ground.

Onions grown by seed will take longer to mature than those grown from bulbs, but they will typically be larger. When transplanting onions, make sure to space them 8 to 10 inches apart in rows 18 to 24 inches apart.

4. Growing Via Bulb

If you choose to grow your onions from bulbs, also called sets, plant them in the early spring as soon as the ground can be worked. To plant, dig a large enough hole to accommodate the onion set.

Place the onion set in the hole, making sure the pointy end points up. Backfill the hole and water them well. Onions grown from bulbs typically mature faster than those grown from seed but will be smaller.

5. The Right Garden Bed

Farmer's hand planting onion in raised bed.

Giving onions the right environment will do wonders for their growth. For example, onions do well in raised beds. Raised beds offer good drainage, which onions need. They also make it easier to keep the bed weed-free. Onions also prefer a sunny spot in the garden. If you live in a hot climate, provide some afternoon shade to prevent the bulbs from overheating.

6. The Right Temperature

These plants are cool-season vegetables, which means they prefer cooler temperatures. Onions will start to form bulbs when the temperature drops below 60°F at night. If the temperature gets too hot, over 95°F, the onion will stop growing and begin to flower.

Once an onion flowers, it will not form a bulb. To get large onions, make sure the temperature stays between 60 and 70°F during the day and 50 and 60°F at night.

7. Soil Quality and Improving Soil Conditions

Onions will only grow as large as their root system allows. A healthy, deep root system is essential for producing big onions. To encourage strong growth, make sure your soil is rich in organic matter.

Compost or manure added to the planting bed will help improve drainage and aeration while adding essential nutrients. In addition, make sure the pH of your soil is between 6.5-6.8. Testing your soil gives more concrete data to help you have a better yield.

8. Fertilizing

Miracle-Gro Water Soluble All Purpose Plant Food

Whether you’re growing your onions from seed or bulb, you’ll need to fertilize them regularly. One way to do this is by side-dressing, which means applying fertilizer to the soil around the plants when they’re about 6 inches tall.

You can also top-dress onion plants by applying a layer of compost around them when they’re about 12 inches tall.

Notes about plant nutrients:

Phosphorous helps onions form large, sturdy bulbs. You can add phosphorous naturally with bone meal or compost.

Potassium helps onions resist disease and bolsters overall plant health. You can add potassium naturally by adding wood ash to the soil.

Nitrogen is also a key nutrient for onions. It helps the plants form healthy leaves, which in turn produce more energy for bulb formation. It can be found in compost, manure, or commercial fertilizer.

Many gardeners swear by nitrogen being the key ingredient for healthy, large onion growth.

9. Weeding

Onions are relatively low-maintenance plants, but they do need to be kept weed-free. Weeds compete with onions for water, space, and nutrients, so it’s important to remove them as soon as you see them.

One way to do this is by hoeing around the plants regularly. You can also use mulch to help prevent weeds from germinating in the first place. Organic mulches like straw or shredded leaves work well.

10. Making the right soil mixture

Onion in container and a shovel in the soil mixture.

Onions need soil that is rich in organic matter and drains well. To improve drainage, add sand to your soil. To improve fertility, add compost. You can also use a commercial potting mix made for vegetables.

Rich, fertile soil is critical for healthy harvests. Some gardeners mix wood chips with the soil, which is called hugelkultur. This improves drainage and aeration while also providing nutrients over time as the wood chips break down.

11. Watering

Farmer watering onion plant.

Onions need about 1 inch of water per week, either from rainfall or irrigation. They’re drought-tolerant plants, so they can survive if you miss a week of watering. But if you want large onions, it’s essential to keep them evenly watered. Watering in the morning is best because it gives the plants time to dry off before nightfall. 

Damp leaves overnight can lead to fungal diseases. To test the moisture level of your soil, stick your finger in it. If it feels dry several inches below the surface, it’s time to water. Overwatering can lead to disease, so be sure not to give your onions too much H2O.

12. Thinning

Thinning is essential because it allows the onion plants that remain to have more space and resources. When the seedlings are about 4 inches tall, thin them to be 8 to 10 inches apart. You can eat the seedlings you thin out or compost them.

13. Pest control

Onions are relatively pest-free, but you may still need to protect them from critters like rabbits, deer, and gophers. You can build a fence around your garden or use netting or cages to keep pests away.

14. Monitoring the tops of the leaves

If you wait to harvest until the leaves of the onions are paper-like, then the harvest will be bigger. This usually happens around the middle of summer. Each leaf the onion sprouts is a ring of the onion. So, the more leaves, the bigger the onion will be. The ideal harvest time is about half of the leaves have fallen over.

15. Harvesting

A farmer's hand holding fresh harvest onions.

Onions are typically ready to harvest about 100 days after planting. To tell if they’re ready, wait until the tops have fallen over and begun to turn brown. Then, gently lift one of the onions out of the ground to check its size.

To harvest your onions, use a garden fork to loosen the soil around them and then pull them out by hand. Be careful not to damage the bulbs as you harvest them.

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