How To Grow Bigger & Sweeter Onions

To grow bigger onions plant onions at the right time, choose varieties of onions that are known to produce large onions, make sure the soil has all the nutrients it needs, and give the onion plants enough space. Growing sweeter onions requires its own steps and can be measured scientifically.

How To Grow Bigger Onions

Plant Onions at the Right Time

Onion plants will begin to develop bulb size when daylight hours reach a certain length. Short day onions start to grow the bulb when daylight hours reach between 10-12 hours a day and long day onions will start to develop bulb size when day lengths reach 14-16 hours a day.

For more on this check out “Long Day Onions vs. Short Day Onions

To grow a large onion bulb a plant first has to develop good green growth above the ground. This is why planting time is important to grow big onions.

The size of the bulb is directly related to the amount of leaves above ground. The more leaves the onion plant has when it starts to develop the bulb the more energy the plant can create through photosynthesis to put into bulb development.

Mature onion bulb size is highly correlated to the size of the onion plant at the time bulb formation begins.

Bulb Onion – Arizona University

You might also be interested in “When To Plant Onions“.

Grow Large Varieties of Onions

  • Ailsa Craig – This onion variety can grow up to 8″ in diameter and can weigh up to 6 lbs. It’s a yellow onion with a tear drop shape. The Ailsa Craig is a long day onion that doesn’t store well long term.
  • Globo Onion – This onion is similar to the Ailsa Craig but is more of a white onion than yellow. The Globo Onion is said to have a complex flavor that is sweet but finishes spicy.
  • Sierra Blanca – Large white onions that are adapted to grow anywhere in the country. White skinned onions with white interior, Sierra Blanca onions have a mild flavor and don’t do well for long term storage.
  • Walla Walla Sweet – This large onion variety is better known for its sweetness. It’s a long day onion that gained its name and reputation from its origin in Walla Walla, Washington.
  • Yellow Granex – This is another sweet onion that grows in the jumbo-colossal range. This is a short day onion variety that should be grown in the south.
  • Rossa Di Milano – Most large sized onions are white or yellow but this red onion fits the bill and grows in the large-colossal range. It’s a long day onion that can store for over 6 months.
  • Expression – Another large sized yellow sweet onion this one falls under the intermediate category and grows well in the middle of the country.

Give Onions More Space For Bigger Onions

The tighter the spacing between onions the smaller the onions will be. Typical spacing between onion plants grown for bulbs is 3″-4″ but to grow colossal size onions spacing should be 6″-8″ between onion plants. The bulbs need the space to grow into their maximum size.

For more information about how plant spacing affects onion yields check out “How Far Apart To Plant Onions“.

Grow Onions in Great Soil

Growing onion in soils without nutrient deficiencies will give the onion plants the best shot at maximizing their genetics, translating to bigger and better tasting onions!

In this study by Yara, the company compares the weight of onions when grown in soils with various nutrient deficiencies. One of the following categories: bulb weight, diameter, and yield were measured when soils were deficient in either nitrogen, phosphate, potassium, sulfur, and boron.

Long story short, onions were bigger and heavier when the soil was at its best.

To check on the nutrient levels of your soil do a soil test. This soil test checks the nutrient levels for 13 different nutrients and the pH level of the soil. University Extensions also offer soil tests, usually for a similar price to the test linked above.

How To Grow Sweeter Onions

Growing sweeter onions is somewhat out of our hands and up to the weather of the season but we can definitely control some factors. We can control what onion varieties we grow, soil nutrition, watering, and the timing of our planting.

We can also use a brix refractometer to accurately test the sugar levels of onions. An average onion scores between 6-10 on the brix scale with anything over 10 considered a very sweet onion.

Production factors such as temperature and light levels and soil moisture and fertility status influence crop °Brix levels.

Using Brix as an Indicator of Vegetable Crop Quality – Ohio State University Extension

The article linked and quoted above says some interesting things about increasing brix levels.

It goes over the many factors that can influence the brix levels of produce at harvest time but it says onion variety selection, amount of watering, and amount of nutrients in the soil are the most controllable factors to increase sugar levels in produce.

Select Sweet Varieties of Onions

For the most part sweet onions have just slightly sweeter sugar levels than other types of onions, about an 18% increase in sugar content. The main reason sweet onions taste sweeter is that they have sulfur levels that are about 50% less than other onions.

Although lower sulfur levels give sweet onions they’re pleasant taste it also means they won’t store as long because the high sulfur levels in other types of onions is what helps them keep for longer.

Here are a few varieties of onions that are known for their sweetness: Walla Walla Sweet, Candy, Texas Supersweet, Sweet Oversize, Yellow Granex.

Benefits of High Brix Levels in the Garden

High brix levels mean sweeter produce but also mean more nutritious, full flavored produce with better storage capabilities. It also means overall healthier plants in the garden.

Healthy plants look great, are able to tolerate more extreme temperatures, and don’t attract the pests and disease that less healthy plants do.

This article, “Fix Your Brix” is well written and chalked full of information on how to improve brix levels. The main way they state is to improve soil health by adding organic materials.

A simple soil recipe would be to mix 1/3 compost, 1/3 peat or coco coir, and 1/3 vermiculite as your base.  Then add beneficial microbes, and food for those microbes.  Earthworm castings, mycorrhizal inoculants, compost teas, humus, amino acids, humic and fulvic acids, rock dust, and seaweed are the best soil additives to use if you want higher brix.

Here’s an article where a gardener does a test on brix levels in onions. She grew three different varieties of onions and grew some with a soil amendment and some without. The results loosely pointed to the onions grown with the soil amendments having higher brix levels.

Watering for High Brix Levels

The amount we water is something we can control. It’s a little less known but pretty standard practice to stop watering watermelons 7-10 days before harvesting them. The reason is that the sugar levels will concentrate before harvesting and not be diluted by more water.

The same practice can apply to all garden grown produce. After the fruits of a plant reach their full size watering can be slowed down and 7-10 days before harvest can be stopped completely.

How To Grow More Flavorful Onions

The flavor intensity of onions is determined by two factors: The variety of the onion (called a cultivar), and the sulfur content of the soil.

The Science of Onion Flavor

The article that quote come from is interesting. It states all the chemicals that are present in onions and activated when an onion is cut. The same chemicals cause the onions pungent smell and taste.

Two other ways that the flavor of onions can be measured are pyruvate levels and lachrymatory factor. Scroll to page 7 for Pungency and Flavor Characteristics of the Georgia Onion Research – Extension Report.

“Individual onion (Allium cepa L.) bulbs were evaluated for pungency by measurement of enzymatically produced pyruvate and by flavor perception. In four separate experiments, pyruvate values were highly and significantly correlated to mean sensory ratings. The high correlations indicate that pyruvate analysis can be used as a reliable selection technique for pungency in onion breeding programs.”

Relationship between Pyruvate Analysis and Flavor Perception for Onion Pungency Determination

The lachrymatory factor has something to do with the chemicals in onions that cause our eyes to water and is a recent discovery as a tool used to measure the pungency of onions.

I read somewhere that onion plants that come to maturity in cooler weather are more sweet and onion plants that finish up in warmer temperatures are more pungent.

In cooler temperatures the rate at which a plant burns sugars is slowed down, especially with cool temperatures at night. The plant is able to rest and is burning less of it’s sugars to complete processes like rapid respiration necessary in hotter weather.

In warmer temperatures a plant might be able to absorb more sulfur.

Can I Eat Onions After They Flower?

You can and should eat onions after they flower. Having an onion plant flower before harvest is not what we want as gardeners unless we want to harvest the seeds from the flower.

An onion that flowers will weigh about half of what an onion that doesn’t flower weighs and it won’t store well after flowering because of the stalk growth inside the middle of the bulb.

Temperature fluctuations and other stresses can cause plants to bolt (flower prematurely).

Please comment below with your own thoughts on growing bigger and sweeter onions!

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