While evidence of onions grown in gardens in India and China can be found as far back as 5000 years ago, it’s believed that the wild version of this plant may have been incorporated into the human diet way before the advent of farming. Grown either from seed or small bulbs known as sets, most onion varieties grow best in cool to cold seasons and are hardy enough to plant in fall for overwintering in mild climates. Interestingly, different varieties of bulb onions fall into different categories in terms of how much light they prefer (e.g., short-day, long-day, and intermediate-day), so some onions do better in one region of the country than others. On top of that, onions come in many shapes, sizes, and colors with varying levels of pungency, so be sure to check out all aspects of different varieties available before planting the best type for your garden.

The popular Red Amposta heirloom onion variety produces medium-sized red onions that are just slightly sweet in flavor, making them an excellent option for eating raw on sandwiches and in salads. These onions are an intermediate-day variety, meaning they need an average amount of daylight to start forming their bulbs—around 12–14 hours—and generally wont grow well in the regions of southern Texas or Florida. We suggest planting this variety in the early Spring to allow the plants to grow to a decent size before the bulb formation begins, as bigger plants will mean bigger bulbs!

  • Botanical Name: Alium cepa
  • Plant Type: Vegetable
  • Variety: Red Amposta
  • Growth Cycle: Annual Biennial
  • Season(s): Spring Summer Fall Winter
  • Climate Zone(s): 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b 8a 8b 9a 9b 10a 10b 11a 11b
  • Light: Full Sun Partial Shade
  • Soil Type(s): Clay Loamy Sandy
  • Yield: 1 lb per plant
  • Garden Dimensions: 1 plant per square foot
  • Germination: 5–15 days
  • Maturity: 115–125 days
  • Harvest: 115–150 days



Seed Depth: 1/4–1/2″
Space Between Plants: Start seeds at 2″ apart, thin to 6–10″
Space Between Rows: 6–18″
Germination Soil Temperature: 65–85°F
Days for Germination: 5–8
Sow Indoors: 8 weeks before average last frost date.
Sow Outdoors: 2–3 weeks before average last frost date.

Vegetative: Onion sets are small, immature bulbs, which can be planted in either spring or fall. Growing from sets will allow faster production of full size bulbs than the planting of seeds but are also more likely to bolt.


Grows best when started in cool weather and will begin to create bulbs when temperatures start to rise in the early summer. Because of its frost tolerance, this onion variety can be planted very early in the spring as soon as soil is workable. If you live in a mild climate zone, plant in the fall and overwinter under a layer of mulch.


Natural: Full sun. Tolerates partial shade.

Artificial: Will grow well under standard 40 watt fluorescent bulbs. Keep lights at least 4 to 6″ above the tops of plants to prevent overheating or burning.

Growing Media

Soil: Prefers well drained sandy, loamy, or clay soils. A pH of between 6.0 and 7.5 will keep plants healthy and nourished. Soil must be loose for proper bulb development.

Soilless: Germinate seeds in a soilless mix of either well-rotted manure or vermiculite for optimum growth.

Hydroponics: Thrives in hydroponic systems, including media-based ebb and flow systems. Use rockwool, perlite, or gravel as your growing medium.


Water: Requires moderate levels of water. Onions are drought tolerant and will survive dry periods, but consistent moisture is required for the best quality, sweetest bulbs.

Nutrients: Requires moderate levels of nutrients. Add compost before planting, and side-dress every 3 weeks with compost or a balanced organic fertilizer during vegetative growth period. Needs more potassium and phosphorus than nitrogen. Reduce fertilization once plant tops have reached full size.

Foliar: Will benefit from foliar feeding containing a balance of nitrogen, phosphorus, and potassium during the vegetative grown period. Once bulb formation has begun, foliar feeding of calcium will help with the structural development of the bulbs.

Pruning: Once a plant has sent up its flowering stalk, remove it unless you are growing your own seeds. At this time, the bulbs are past their prime.

Mulching: Use a light layer of mulch to maintain soil moisture and suppress weeds. Don’t mound the mulch around the base of the plant where it emerges from the soil. This area is sensitive to rot if kept too moist.



  • Onion maggots
  • Thrips


  • Botrytis leaf blight
  • Downy mildew

Deficiency(s): A nitrogen deficiency will cause stunted growth and lightening of older leaves. A phosphorus deficiency will cause stunted growth and yellowing of lower leaves, and bulbs will not develop. A potassium deficiency will result in droopy plants, with leaves dying, starting from the tip and moving downwards. If the younger, inner leaves begin to turn pale yellow, this could be caused by a manganese or zinc deficiency.

Rotation and Companion Plants

Rotation: Plant onions before a squash or lettuce crop. Avoid following it in the next year with another onion family crop or with beans or other legumes.

Companions: Grows well with beets, carrots, peppers, spinach, strawberries, cabbages, kale, and tomatoes. Avoid asparagus, beans, peas, and sage.

Harvest and Storage

Harvest: This variety of onion is ready to harvest when about half of the leaves have fallen over and turned brown. You can bend the tops over to speed drying once they have begun to yellow. To cure onions, first loosen the soil around the plants. After a few days, use a garden fork to turn the soil and bring onions to the surface of the ground. Leave them to dry in the sun, or move to a place where you know they will remain dry for 7–10 days. Onions can cure either hanging or on an elevated screen. Harvest time should be in late summer before temperature begin to drop as cool weather may cause rotting.

If you want to harvest onions as scallions, you can pull them up a few weeks after you plant them.

Storage: Onions that have been cured, either outside on the soil surface or indoors in a warm, dry area, will store for longer. Keep them at 40–50°F in a dark, well-ventilated place for up to 2 months. If your onions begin to sprout, don’t worry, they should still be safe to eat. You can even use the green part as you would use a scallion. Don’t store near apples or potatoes. Keep green onions in the refrigerator for 1–2 weeks.

Other Info

Seed Saving: To save onion seeds, plant a few extra bulbs and leave them in the ground for at least two seasons. By the second season, your plants will start producing lovely flowers in the late summer. Once you notice the seeds starting to drop, cut off the entire head of the plant and bring indoors to complete the drying process. Shake the seeds out of the flower heads and store in an airtight container.


Preserve and Prepare

Preserve: Onions can be frozen, pickled, or dehydrated for later use.

Prepare: Fresh onions can be eaten raw, sautéed, grilled, or roasted and are found in a huge variety of dishes and cuisines. The papery outer skin and roots are removed before consumption. Chopping onions can cause you to cry like crazy; the best solution to this problem is to use your sharpest kitchen knife to expedite the process or place onions in the refrigerator for 10 to 15 minutes before cutting.


Nutritional: Provides vitamin C, potassium, and calcium. Leaves are rich in vitamin A. Also a good source of antioxidants and sulfur-containing compounds.

Medicinal: Historically prescribed for infertility, constipation, headaches, coughs, snakebites, and hair loss. Onions have anti-inflammatory and anti-cancer properties and can be beneficial for cardiovascular health, bone density, and joint health.

Warnings: Onions are toxic to dogs, cats, guinea pigs, and many other animals,  so keep dishes with this ingredient away from your furry friends!


Unless they are being added to a burger or salad, onions often get hidden among other ingredients to soften their intense and sometimes biting flavor. Not so in this Roasted Red Onion recipe! With a little honey and balsamic vinegar, these Red Amposta onions finally get their time to shine, and we guarantee that they will not disappoint. Instead of butter, try using coconut oil to up the sweet flavor and reduce the number of calories.

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