Growing MASSIVE Onion Bulbs

Onions are a popular plant in most gardens. They can be started from seed or onion sets. You can plant them deep to create scallions or “Spring Onions” for early harvest, or plant them shallower to let them grow into thick bulbs you can use all year round.

As simple as they might seem, some techniques will make all the difference when it comes to enjoying a big onion harvest.

If this is your first time growing onions, or you’ve been underwhelmed by your past harvests, you need to try these tips to give you a bountiful onion harvest this year.

Tip 1: Companion Planting with Nightshades

Companion Planting with Nightshades

Onions do very well when companions are planted in the same gardening bed with nightshade plants like tomatoes and peppers. There are several reasons for this, starting with the way the two types of plants develop their root base.

Onions tend to have shallow roots that draw nutrients from the upper layers of the soil. Whereas tomatoes, peppers, and other nightshades tend to create deep root systems. This keeps the two plants from directly competing, even if you plant a row of onions within 12 inches of a row of peppers.

They both share similar fertilizing needs at the same time of year. Early on onions benefit from nitrogen to help develop their green grass-like tops. Peppers and onions also benefit from nitrogen in the early stages of development to grow the kind of thick foliage they need to prevent their fruits from suffering sunscald.

The foliage of nightshade plants also helps shade the soil, to keep it from drying out. Not only does this help onions draw up the moisture they need, but it also helps keep the soil soft so onion bulbs can expand to their maximum potential.

As a bonus, the odor of onions also helps deter common night shade pests. Allowing the two plant species to grow together as happy companions.

Pro Tip

If you’re going to plant onion sets deep for harvest, be sure to water the soil before harvesting. This will reduce the risk of root damage to the tomatoes and peppers when you pull the deep-planted scallion out of the soil.

Tip 2: Change Your Fertilizing Strategy with the Season

Change Your Fertilizing Strategy with the Season

Onion’s fertilizing needs change as the season progresses. Especially if you’re growing onions from seed, rather than sets.

This process starts early in the spring when you’re preparing your planting beds. Onions grow best in soil with pH between 6.0 and 7.0 in soil that is loose with a high volume of organic matter. Then you can augment the soil with a phosphorus-rich fertilizer, which promotes good root development.

If you’re companion planting with nightshades, additional soil phosphorus will also promote flowering and fruit setting.

After planting your onion seedlings or sets, you want to side-dress the rows with additional phosphorus-rich fertilizer (like superphosphate and potassium sulfate). Do this again in July with food that’s good for growing bulbs, without nitrogen, so you get big bulbs instead of too many leaves.

Pro Tip

If you’re growing onions from seeds and transplanting seedlings into the bed, you can also add a little bio-available nitrogen to the planting bed. This will give the onion seedlings the little boost of nitrogen they need to rapidly develop strong, green tops.

You don’t want to use a delayed-release nitrogen fertilizer, as too much nitrogen in the peak of the growing season encourages the onions to put their energy into top growth when you want them to be developing large bulbs.

Tip 3: Follow The Best Watering Schedule

Watering onions during their initial two to three months is crucial. Neglecting proper watering in this timeframe can result in undersized bulbs and diminished flavor.

Watering schedule details:

  • In the initial month, ensure weekly watering, followed by immediate soil loosening and application of fine mulch. Large mulch pieces can harbor pests and foster fungal growth, whereas fine mulch prevents the topsoil from drying out and decomposes efficiently with moisture. During this phase, aim to moisten the soil up to a depth of 10 cm.
  • By June, as the bulbs begin to develop, reduce watering frequency to once every ten days, ensuring the moisture reaches a depth of 20-25 cm. Use a gentle spray to water, preventing water from pooling.
  • In July, adjust watering to once every 8-10 days, based on the need to keep the root zone from drying out.
  • From mid-July, maintain only the necessary moisture in the soil, transitioning to “dry watering” by loosening and mulching the soil, and removing weeds.
  • Cease watering 2-3 weeks prior to harvesting and carefully clear the soil around the bulb shoulders. This aids in bulb maturation, particularly the base stem, which is prone to fungal and bacterial decay if immature.

Tip 4: Use Drip Irrigation

Use Drip Irrigation

Onions need moist soil to take advantage of the bioavailable nutrients, while also making sure the soil is soft enough for the bulbs to expand to their largest growth potential.

Watering with a drip irrigation system also reduces the risk of fungal diseases developing on the nearby nightshade plants. Things like early blight on tomatoes and peppers promoted by wet foliage from sprinklers can transfer to the onion tops, killing them, which stunts bulb growth.

So, drip irrigation systems help both types of plants simultaneously, and is a better option than sprinkler irrigation or spraying with a hose.

Frequently Asked Questions

Is it better to plant onion seeds or onion sets?

Onion seeds are more cost-effective than onion sets, but you need to start them several weeks if not months earlier. They’re only really a good option if you have the space and time, as they are the more labor and resource-intensive methods.

Onion sets only cost a little more and are very easy to plant. They also tend to be more robust in times when a freak spring cold snap hits. Making them the better option for zones 6 and colder.


By following these simple tips you can enjoy the best onion crops year after year. This starts with preparing the soil in the planting bed.

Augmenting with bioavailable nitrogen gives a boost to onions started from seed. Mild phosphorus fertilizer side dressed along rows then promotes bulb growth throughout the growing season.

Companion planting onions next to nightshades like onions and peppers helps both plants. The foliage of the nightshades helps keep the soil from drying out, while the aroma of the onions deters common nightshade pests.

Since the two plants develop root bases at different depths, they don’t compete for available soil nutrients. A drip irrigation system is the best way to water onions.

Not only does it keep the soil from drying out, but the soft soil makes it easy for onion bulbs to expand in the soil. It also reduces the risk of many fungal diseases that can affect onion tops and the foliage of the nearby nightshades.

Amber Noyes

Written By

Amber Noyes

Amber Noyes was born and raised in a suburban California town, San Mateo. She holds a master’s degree in horticulture from the University of California as well as a BS in Biology from the University of San Francisco. With experience working on an organic farm, water conservation research, farmers’ markets, and plant nursery, she understands what makes plants thrive and how we can better understand the connection between microclimate and plant health. When she’s not on the land, Amber loves informing people of new ideas/things related to gardening, especially organic gardening, houseplants, and growing plants in a small space.

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