Onion Seed Germination, Time, Temperature, Process

Introduction to onion seed germination

Onion (Allium cepa L) is one of the most important condiments widely used in all households all year-round. The green leaves and immature and mature bulbs are eaten raw or also used in the preparation of vegetables. Onions are a cool-season crop. The germination of onions is slow at 6 to 7°C, the optimum germination temperature range is 10 to 35°C, and the maximum temperature is about 40°C. They can be planted from onion sets, nursery-grown transplants, or onion seeds. Growing onions from seed mainly depend on shapes, flavors, sizes, and colors to grow.  Onion seed is small and can be difficult to handle; it can be easier to start seed indoors and transplant out seedlings. Though, onions started from seed in the garden grow larger and store longer. In this article we also discussed below topics;

  • Process of growing Onion from seed
  • Time to take Onion seeds take to germinate
  • How do you germinate Onion seeds
  • Onion seed germination temperature
  • Process for germinating Onion seeds
  • Onion seeds germination period
  • Tips for Onion seeds germination
  • Best time to plant Onion
  • Germinate Onion seeds indoors
  • Paper towel germination method for growing Onion
  • Soak onion seeds before planting or not
  • How do you direct sow onion seeds
  • Do onion seeds need light to germinate

Starting onion plants from seed takes the longest for onions to fully grow, but seeds give you the choices in variety. It can take up to 5 months for plants to mature enough to harvest. When starting onion plants from seed, allow the soil temperature to warm to at least 50F before sowing. Buying onion seeds, best using them within 2 years; the longer onion seeds sit, the less likely they will be to sprout. Once the onions sprout, then you can start using them as scallions and green onions, or you can wait until they mature and harvest them as bulbs.

A step by step guide to onion seed germination process

Onion seed starting is easy and grows best in fertile, well-draining soil. This should be worked with organic matter, such as compost. Seeds can be planted directly in the garden bed. Though, when growing onion seed, some people prefer to start them indoors. Place them about an inch (2.5 cm.) deep in the soil and about half an inch or more apart. If planting rows, space seeds at least one and a half to two feet (.45 to .61 m.) apart.

A guide to Growing Onions from Seed.
A guide to Growing Onions from Seed.

When it comes to the onion seed germination process, temperature plays an active role. While typically germination occurs within 7 to 10 days, soil temperature affects this process. For instance, the cooler the soil temperature, the longer it will take for onion seeds to germinate up to 2 weeks. Warm soil temperatures, on the other hand, can trigger onion seed germination in as little as 4 days.

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Planting onion seeds is better than planting sets

Onion sets are immature bulbs that grow from seed that was planted in mid-summer of the previous year. The partially-grown onion bulbs are pulled from the soil in the fall and stored in a dormant state through the winter to be replanted the following spring. Many gardeners plant onions from sets because they are widely available and it’s easy, but there are a few reasons why this may not be the best way to grow a good onion crop.

A step-by-step guide to growing onions from seed

Plant onion seeds indoors 8 to 10 weeks before transplanting them outside just before the average last frost date in your area. Seeds must be sown ¼ inch deep. Onions need an open and sunny site, fertile soil, and good drainage.

Start with fresh seed and seed starting mix

Onion seeds can be kept for 2 years in a dry place such as a properly monitored seed storage box, but fresh first-year onion seeds germinate best. Germination rates fall as the age of the seed, and three-year-old onion seeds that sprouted well, germination is always highest with fresh seeds. Onion seedlings stay in pots for up to 10 weeks, so use a soil medium that’s unlikely to host diseases.

Fill the containers with damp seed-starting mix, sprinkle the onion seeds evenly on top of the soil, mist them lightly with water, and then cover them with about 1/8 inch thick layer of seed-starting mix. Cover the seeds with the soil and then gently pat the soil down with hand. Don’t forget to mark your trays with the seed variety and the planting date.

Provide heat

The tops of my florescent plant lights are flat, and they give off just the right amount of bottom heat required to help onion seeds germinate quickly. Onion germination is fastest as 68 to 77°F (20-25°C), with slight temperature drops at night. Seeded containers in a plastic bag to keep moisture, provide them with bottom heat and the seeds germinate in about 8 days.

Use plastic bags to cover the seedling trays to conserve moisture, and put them in a warm location, such as near a wood stove or on the top of the fridge (where the temperature ranges from 21 to 24°C) for germination stimulating. You can use a seed-starting heat mat if your home is too cold.

Spacing requirements of onion seedlings

When transplanting your seedlings, space them at least about 6 inches apart. Plant onion seedlings in the least weedy part of the garden; onion seedlings are small and do not compete well with weeds. When the onion seeds have sprouted, take off the humidity dome and move the trays to a cooler place under the lights.

Keep the soil moist and fertilized. The popular fertilizers are fish emulsion, compost tea, and manure tea. Trim the baby onion leaves to about 3 inches long to keep the plants from getting too top-heavy and to give the roots more nutrients.

Starting seeds

Sow onion seeds indoors about 8 to 12 weeks before the last average frost date. Sow onion seeds in pots, flats, or trays. Thin seedlings to about 1 inch apart when they are four inches tall. Sow seed outdoors 2 weeks before the last average frost date in spring or four weeks before the first expected frost in autumn. Avoid sowing onion seed directly in the garden until the soil temperature range has reached 50°F. Outdoors sow onion about ½ inch deep and 1 inch apart. Later, thin to four inches apart. Growing onions from seed will give you the widest choice of seed varieties.

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Onion seedlings in pots.
Onion seedlings in pots.

Planting onion seeds under lights

Onions are cool-season crops that need 90 days or more to reach maturity. Planting onion seeds directly into the garden in the spring makes it difficult for the bulbs to reach an excellent size before warm temperatures arrive. This means the seeds have to be started many weeks in advance of moving the onion plants outside into the garden. Thus, if you want to grow onion seeds indoors under grow lights, you should start 10 to 12 weeks before it’s time to plant them into the garden in early spring.

However, planting onion seeds indoors under grow lights is a bit more nuanced than growing other vegetables from seed. When growing the seeds of tomatoes, eggplants, and other veggies indoors under grow lights, the lights must be on for 16 to 18 hours per day. If you grow onion seeds indoors under grow lights and leave the lights on for that long, it will initiate an early bulb set and result in puny onions. That means that if you want to start onion seeds indoors under grow lights start early and only leave the lights on for 10 to 12 hours per day.

Transplant onion seedlings

It takes between 35 and 45 days (depending on variety) after sowing to transplant seedlings. Over matured seedlings can lead to small bulb size and late or no bulbing. Once seedlings have sufficient leaf growth, thin them down to around 3 to 4 inches (7.6 to 10 cm.) apart. Transplant onion seedlings that were started indoors about 4 to 6 weeks before the last expected frost or freeze date, provided the ground is not frozen. Onion plants have shallow roots and need frequent irrigation throughout the growing season. Though, once the tops begin to layover, usually by late summer, watering should be stopped.

You can sow seeds directly into the garden. Spread seeds in the prepared garden bed and cover & pat lightly with soil. Once the seedlings have begun to push through the soil, they can be thinned about 1 and 2 inches apart. After a couple of weeks, these can be further thinned out to every 4 to 5 inches. Another process, instead of thinning by removing plants, is to transplant the bulbs to another row. Be sure to plant the rows about 9 inches apart and bulbs between 4 to 5 inches apart.

Reasons for onion seeds didn’t germinate

It wasn’t warm enough

Keeping your onion seeds in temps 60°F and above is important. They want warmth to germinate. If your onion seeds got too cold, they won’t come up. If seedlings started sprouting and it got cold. Seeds are smart, if they think it is too chilly, they won’t sprout.

The seeds were started too early

Timing is just about everything when it comes to gardening, particularly concerning seed starting. Start onion seeds too early for your zone, they won’t germinate, or they will die off before they ever hit the ground. Same with direct sowing that means sow them too early, or too late, your germination rate is either going to be low or you’ll have incredibly weak plants.

Too Much Water

Onion seeds need water, but they don’t need a flood. Seeds need to be moderately moist to sprout and seed germination is highly dependent on watering. Too dry and they won’t get to sprout, too wet and they will rot in the dirt.

Too high or too low temperature

The temperature of the soil is of the utmost importance in getting good seed germination. Temperatures that are too high or too low can cause problems germinating the seeds. Given all else is equal, even the tray germination process requires even temperatures. If temperatures plunge at night or peak over 100 for a prolonged period, seeds will remain dormant or die.

Common pests and diseases of onion plants

Several bacterial, fungal, and viral diseases mainly affect onion growth. Plant your onions in a well-drained space. Diseases and fungi such as Fusarium basal rot, white rot, and Botrytis neck rot can affect onion crops in storage. Crop rotation can help prevent these kinds of diseases.

Harvesting onions

Onions harvesting will depend on proper planting and care throughout the growing season. Rich soil, consistent moisture, and cool temperature ranges help bulb development. And you can harvest onions at any stage. The onion plants you thin from a row can be used as green onions. Harvest tops for green onions as soon as they reach about 6 inches in height.

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Onion harvest time is generally 100 to 120 days after planting, depending on the cultivar. Onion harvest time must be early in the morning when temperatures are not too hot.

Paper towel seed germination of onions

  • First, moisten a paper towel and squeeze out excess water. The paper towel must be damp, but not dripping wet.
  • Then spread the damp paper towel out on a clean surface and fold in half.
  • Select at least 10 seeds from the package you are testing and spread the seeds out on one-half of the towel, so they are not touching.
  • Fold the towel in half sandwiching the seeds between the 2 layers. Press down gently to make sure onion seeds come in contact with the damp paper towel.
  • Put the paper towel and seeds in a zipper bag and seal it to keep it from drying out.
  • Mark the bag with the date and variety of onion seed. Put the zipper bag in a warm place away from direct sunlight.
  • Every 3 days, check to see if the seeds have sprouted. Then re-moisten the paper towel if it dries out.

Frequently asked questions about growing onion from seed

Questions about growing onions from seed.
Questions about growing onions from seed.
Where do onion seeds come from?

You’ll know it’s time for harvesting onion seeds when the flowering heads begin turning brown. Carefully clip the stalks a few inches below the head and put them in a paper bag. And set the bag in a cool, dry place for several weeks.

Can you start onion seeds indoors?

If you want to grow onion seeds indoors under grow lights, you should start about 10 to 12 weeks before it’s time to plant them into the garden in early spring. But, planting onion seeds indoors under grow lights is more nuanced than growing other vegetables from seed.

How many onion seeds are in a hole?

In general, two to three seeds must be planted per hole. Onion seeds do not have 100% germination rates, so not every seed planted will sprout.

How many onions do you get from one plant?

To grow an onion to get more, you want to plant it. As a bulb, it will grow, produce a flower head. From that flower head, you can collect seed, plant and then grow more onions. If your onion sets seed, and collect and sow it carefully, you can get several hundred from one.

How long soak onion seeds before planting?

Onion seed benefits from pre-soaking before onion planting. And they can be soaked for as little as 15 minutes or as long as several hours. You can use plain water, compost tea as you prefer. Drain and pat dry just before planting to make it easier.

Do onions need a lot of water to grow?

Onion plants need adequate water to produce high yields, but it doesn’t take much overwatering for your onions to become diseased and rot in the ground. In a typical about 12 weeks growing season, we recommend irrigating with one inch of water once or twice a week depending on the amount of rainfall received.

What are the best growing conditions for onion seeds?

Onion seed germination is most fast and reliable in the low 70s Fahrenheit. A commonly accepted range is 68 to 75°F (20-24°C). Use a heat mat to keep the bottom of the propagator but discontinue use once the majority of the viable seeds appear to have sprouted.

How to get onions to grow bigger?

Onion sets and onion plants need loose soil and should be planted early (end of February or March). Dig a shallow trench, working in compost or fertilizer for getting big onions. Also, raised beds can be implemented. Plant the onions about an inch deep and 4 to 5 inches apart.

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