Onions (How to Plant, Grow & Harvest) – GIY Plants

Onion plants growing in field

Growing onions has been an art for over the last 5000 years, possibly originating in either Central Asia, Iran, or Pakistan. But in today’s time, the flavorful root vegetable is cultivated in six of the seven continents[1]. If you want to grow onion plants in your home garden, you have come to the right place!

Let’s dive deeper into onion plant care, general facts, and how you can grow the plant from seeds or sets.

Scientific name Allium cepa
Common names Onion, bulb onion, common onion.
Family Amaryllidaceae
Genus Allium
Species Cepa
Origin Central Asia
Growth habit A perennial that grows a bulb below the soil.
Leaves Long and pale green/gray leaves that sprout from the edible bulb.
Flower Pink or white ball-shaped flower clusters called umbels.
Height 1′-1.5′
Soil type/pH Sandy loamy soil with a pH of 6.0 to 7.0
Germination time 6-12 days.
Hardiness 4-10 (USDA)

Onion Plant Care

Onion plant care

To successfully grow onion plants, you need to know the fundamentals of their care. The slightly intricate details of their care are what will ensure you have a healthy harvest.

If you are unsure of how to grow and care for onion seeds, don’t worry, we’re here to help!


The best soil type for growing onions is a sandy loam that also has a high amount of organic matter. This soil type allows for drainage which can help prevent fungal diseases. You can also add well-rotted manure to your soil to improve its overall quality. Although, it is important to keep in mind that you should be aiming for a soil pH of 6.0 to 7.0 when growing onions.


Onions need one inch of water per week. When using proper sandy loam soil, your onion watering schedule should be more than once per week. That’s because this soil drains well, which is great for preventing fungal diseases. But due to the drainage, the soil does not retain water as well.


The lighting requirement for Allium cepa onions is full sun for six to ten hours a day. Although some varieties enjoy 14 hours or more of sunlight, including walla wallas.

Humidity & Temperature

Onion seeds grow best in soil temperatures of 75°F or 23.89°C. Although the ambient temperature must only be above 40°F or 4.4°C, but temperatures above 50°F or 10°C are ideal.

When it comes to proper humidity levels for onions, 30% to 50% is optimal for growth.


Before applying a fertilizer you will want to take a soil test. The test’s results will give you the information necessary to choose a fertilizer. Although, it’s important to note that onions need high amounts of nitrogen. Some of the best nitrogen fertilizers are ammonium sulfate and ammonium nitrate.

Additionally, the fertilizer application time should be three weeks after planting. Following the initial application, you can apply more every three weeks. As for fertilizer dosing, one cup of granules per twenty feet of row is sufficient.


Propagating onions is easy and simple. All you need to do is take a harvested onion plant and cut 1″ off of the bottom. Once cut, let the cut piece dry cut side up for twelve to twenty-four hours. After, poke a toothpick into each side of your onion; four picks in total. Once the picks are in place, put the onion over top of a full glass or bowl of water for three to four days.

After the three to four days you can place the onion 1″ deep into sandy loam soil and follow general care practices.

Diseases & Pests

Allium cepa plants can deal with their fair share of diseases and pests. Fungal diseases are some of the most prevalent. Some of those fungal diseases include downy mildew, black mold, fusarium basal plate rot, and purple blotch. Fungicides and proper care practices are best for preventing and mitigating these diseases[2].

As for pests, thrips, onion maggots, leafminers, and mites are prevalent for the plant. For thrips, you can use insecticide to mitigate them. Onion maggots are best dealt with using insecticide and sanitization methods. But for leafminers, it is best to look for signs of the unwanted insect before planting. And mites can be prevented by not planting successive crops.

Days to maturity

Allium cepa plants reach maturity between 85 and 150 days after planting. By this time an onion reaches maturity, you can harvest it.

Harvesting Onions

Once an onion reaches maturity in 85 to 150 days, you can harvest them. To harvest an onion, you will need to dig them out and place them on a drying rack outdoors for a week. Once dry you can store them in the fridge or pantry.

How to Plant & Grow Onion

If you haven’t already begun planting your onions, you may be unsure of where to start. The good news is, we are here to break down how to plant and grow the delicious root veggie. Keep reading to find out when, where, and how to plant onions.

  • When to Plant

Before deciding when to plant, you first need to decide on your planting methods. That’s because when you can directly sow versus when you can transplant differs.

If you are directly sowing seeds into a garden, you can do so at the end of winter to early spring. But you must wait until the soil is no longer frozen and is workable.

Although, if you are transplanting onion sets, the temperatures can be 50°F or 10°C. Onion sets and transplants are young but established plants that tolerate frost more. That is why they are able to be planted in climates where frost is still possible.

  • Where to Plant

You should plant Allium cepas where they can receive six to ten hours of full sun a day. Also, the soil in the location should be a sandy loam type, with high organic matter, and a 6.0 to 7.0 pH.

  • How to Plant

Now you know when and where to plant, let’s discuss how to plant onions seeds and sets.

Onion seed or sets spacing should be 3″ to 4″ apart and your onion seed depth should be 0.5″ to 1″ deep into the soil. Ensure that you water the soil after planting.

Onion growing stages

Knowing the growing stages of the vegetables you plant can help aid in your care of them. That’s why we want to share with you the five stages of growth onions go through.

Seed Germination

For any plant to begin growing, their seeds need to germinate. During this phase, the onion seed’s layers will start to peel with the help of moisture and heat from the soil. Germination can take six to twelve days.


Following germination, we have the seedling stage. This stage occurs once the onion sprouts a couple of mature leaves. Also, when this stage occurs, the plant is beginning to develop its root system. By the end of this stage, more mature leaves will appear along with additional roots.

Vegetative Growth

Next, we have the vegetative growth stage. In this stage, you will see the plant grow its leaves rapidly. The leaves before this are often small and very few, in this stage, you can have eight to ten that grow quickly. Although, before entering the next stage, these leaves will stop growing.

Bulb Initiation

Now we have bulb initiation, also known as maturation. In this stage, the plant will use all the energy stored in its leaves to grow the bulb. As it grows, you will notice the onion bulb peeking out from the soil and the leaves will fall to the sides.


Now we arrive at the final and most exciting stage, harvesting. In this stage, you can remove your onions from the soil once they reach maturity in 85 to 150 days. You can do this by digging out the onions and pulling them from the soil. After that, place them on a rack to dry in the sun for a week. They are ready for storing once the outer layers of the onions are fully dried out.

Onion Companion Plants

Companion planting can help protect from pests, optimize space, and provide vital nutrients. That’s why we recommend planting these other valuable plants with onions!

Summer Savory & Chamomile

When it comes to onions summer savory plants and chamomile share one thing in common. Both of these plants can alter and improve the flavor of your onions when planted nearby. Summer savory is particularly known for its ability to increase an onion’s sweetness.


Marigold companion plant for onions

Marigold flowers are a powerhouse when it comes to insects. The stunning blooms scent can attract beneficial insects to your onions. But the best part is that the same scent can also help keep pests away.

Swiss Chard

Onions require a certain level of moisture that may be hard to maintain for some gardeners. To prevent drying, consider planting swiss chard. The plant’s large leaves help cover the soil and trap moisture.

Other Onions & Lettuce

If you want to optimize your gardening, plant other onion varieties and lettuce. Lettuce has shallower roots than onions and does not compete with them for nutrients. Other onions simply grow well together; leeks are one of the best options.

Onion varieties

Allium cepa or common onions come in several varieties. The plant’s many cultivators fall into three main categories. Those include red/purple, brown/yellow, and white. As you can see this is based on their coloring. An onion’s color usually indicates the type of flavor it will have. Let’s dive deeper into those three onion types.

  • Brown/Yellow

Yellow onion variety
Spanish Onion

A globe-shaped onion with yellow peels. Their bulbs have a mildly sweet flavor and mature in 110 days.

Walla Walla

Walla wallas have a noticeable sweet taste and yellowish-brown peels. They mature in 150 days.

Vidalia Onion

A slightly sweet variety with a noticeable flat shape compared to other varieties. It has pale yellow to brown peels and matures in six to nine weeks.

  • Red/Purple

Red onion variety
Red Brunswick

An onion with burgundy peels and a semi-flat shape. They mature in 100 days and have a strong flavor.

Red Beauty

A rounder variety that has purplish red peels. They mature in 100 to 120 days and have a sweeter flavor than other red varieties.

  • White

White onion variety
Crystal White Wax

This variety has a mild flavor with white peels. They are slightly smaller in size and mature in 90 days.

White King

A globe-shaped cultivator that has a strong flavor with a slight sweetness. They mature in 90 to 100 days.

How to store onions

To prevent molding and sprouting, store your onions in a dark location that is cool and dry. You should also put the onions in a basket or bin before placing them in the optimal location.


[1] USDA, Agricultural Research Service, National Plant Germplasm System. 2023. Germplasm Resources Information Network (GRIN Taxonomy). National Germplasm Resources Laboratory, Beltsville, Maryland. URL: http://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomydetail?id=2244. Accessed 22 January 2023.

[2] Onion. PlantVillage. (n.d.). Retrieved January 22, 2023, from https://plantvillage.psu.edu/topics/onion/infos

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