Vegetable Garden - Tips on Growing Onion from Seed 

Sowing 

 Seed Depth: ¼-½" (.65-1.3cm)

• Germination soil temperature: 65-85º F (18-29ºC)
• Days to Germination: 5-8
• Sow indoors: 2 months before last frost.
• Sow outdoors: Spring
• Growing
• pH range: 6.0-7.5
• Growing soil temperature: 70-85º F (21-29ºC)
• Spacing in beds : 3-4 " if grown like scallions
• Watering: Moderate and even
• Light: Full sun for best yields, tolerates light shade.
• Nutrient requirements: N=moderate, P=moderate, K=moderate
• Rotation considerations: Follow squash or lettuce, do not follow any onion family crop or legume.
• Good Companions: Beet, cabbage family, carrot, pepper, spinach, strawberry, tomato.
• Bad Companions: Asparagus, bean, pea, sage.

START INDOORS:

• To sow indoors start 8 weeks before your last frost date.
• Sow seeds ¼" deep in seed starting formula, and cover lightly with soil
• Keep evenly moist. Seedlings emerge in 5-8 days at 70º F
• Do not let seedling become more than 6 weeks old because older seedlings do not mature well transplanted

DIRECT SOW:

• Place onion seeds outside 2-3 weeks before your last frost date, or when the soil can be worked easily.
• Germination will take place in about 21 days.
• They will grow best at 55º to 65º F outdoors.
• Set plants 2"apart in rows 12-18" apart.
• Be careful at this stage, and do not allow onions that are ready for transplanting to become to tall or with too many roots, as they are sensitive to shock by disturbing their mature roots.

USDA Hardiness Zone -First Frost Date- Last Frost Date

• Zone 1 -July 15th -June 15th
• Zone 2 -August 15th- May 15th
• Zone 3 -September 15th May 15th
• Zone 4 -September 15th May 15th
• Zone 5 -October 15th April 15th
• Zone 6 -October 15th April 15th
• Zone 7 -October 15th April 15th
• Zone 8 -November 15th March 15th
• Zone 9 -December 15th February 15th
• Zone 10 -December 15th January 31st (sometimes earlier)
• Zone 11 -No frost. No frost.

THE INFLUENCE OF DAY LENGTH

Bulb initiation in onions, leeks are affected by two environmental codes: day length and temperature. Onions are stimulated to bulb up and out under the lengthening days of spring that turn into summer. Generally, day lengths of 12–16 hours induce the bulb to expand.

 In addition, many of the onion varieties are classified in accordance to the photo-period (approximately) necessary to induce bulb growth.

• Short day onion varieties bulb up at day lengths greater than 12–13 hours.
• Intermediate day varieties, greater than 13 ½–14 ½ hours.
• Long day varieties greater than 14 ½– 15 hours.
• Very long day varieties greater than 16 hours.

THE EFFECTS OF TEMPERATURE

Temperature plays a secondary role in bulb initiation.

• Cooler temperatures (less than 70ºF average day temperature) retard the bulbing even when the desired day length is achieved.
• Temperatures that are greater than 75ºF will hasten the bulbing response.
• Temperature also affects flowering in onions— an undesirable scenario that will lead to small and tough bulbs.
• Temperatures that are under 50ºF for 10 days or longer (if seedlings are greater than ¼-inch in the stem diameter) followed by warm temperatures, in addition with lengthening days, will induce flowering.
• Thus while a bigger plant equals a bigger bulb, starting seedlings too early (especially if your area is experiencing back-and-forth spring weather) can doom your onion crop.

GARDEN HINTS:

• Water deeply once a week in dry weather.
• Mulch or Cultivate to control the weeds.
• Using fertilizer and an abundant supply of water are important throughout the growing season.
• Keeping the area around the onions well weeded with shallow cultivation is important for a full harvest.
• Harvest when tops fall over.

If you are like most gardeners, you want your onions sweet, the sweetness of an onion is determined by both nurture and by the growing environment. For the mildest in onions, start with a variety known to produce sweet, mild-flavored bulbs such as Walla Walla (at northern latitudes). To get the plants off to a strong start, fertilize by mixing an organic or timed-release fertilizer into the ground below the planting furrow (a raised row) before your plant your onions. This fertilization technique, is called "banding," which places the nutrients right along where the young onion roots will find them. They will use up the nutrient supply as they grow. This is a benefit because later as the onions form under lean conditions, they tend to taste sweeter.

Your plants will need abundant sun and good drainage, and they grow best when the soil pH ranges between 6.0 and 6.8. Raised beds or raised rows are called furrows and they are made by mounding up soil, especially if your soil is heavy clay. Mix a 2-inch layer of compost into the soil before placing an organic or timed-release fertilizer into planting furrows, be sure to follow the label rates. Set plants 1 to 2 inches deep, so that their roots will be well covered with soil but the top of the transplant’s neck is not buried too deeply. You do not want the part of the neck where the leaves grow away from the clear sheath to collect soil or water down between the young leaves or they will rot. Space transplants 6 inches apart in furrows 12 inches apart. Plants will appreciate a starter solution of liquid fertilizer after planting.

Onions roots are shallow and not very efficient at taking up moisture, so they need a steady supply of water to grow without interruption. Although they actually recover well from drought and start growing again when watered, it is best to keep the soil consistently moist until the bulbs enlarge.

You may mulch with a light layer of weed-free and herbicide-free grass clippings or fine mulch. Onions naturally push toward the surface as they form bulbs and its best if the tops of the bulbs bask in dry sun. Remove any mulch that might keep the expanding bulbs excessively moist.

An optional note about your seedling bundles: Seedlings that are about the diameter of a pencil produce the biggest, most beautiful bulbs. Very small seedlings set at close spacing can serve as a second crop of scallions. Use the pencil-sized plants to grow to full-sized onions so juicy that they spurt when you slice them.

Remember not to plant too deeply in order to avoid problems with rot, and make a hill, or furrow, if needed for better drainage in clay soil. You can harvest young onions just a few weeks after planting if you want to use them as “spring onions” or scallions. There is no perfect size or full-sized bulbs, let onions grow and mature. They are ready to harvest when the bulbs are big and the tops begin to turn yellow and fall over. Pull them up, shake off the soil, and lay them out to cure with the tops still attached.

Any warm, airy location is a good place to do this; you can even sling them over a fence as long as they are not rained upon. Bulbs must stay dry and have good air circulation. As the onions cure, the roots will shrivel and the necks above the bulbs will slowly dry – a natural process that helps to seal the top of the bulb, making the onions less likely to rot. After 7 to 10 days, clip off the tops of the onions and the roots with pruning shears, remove as much dry dirt as possible without taking off the papery outer skins, and store your onions in a cool place. Very sweet, juicy onions store best in the refrigerator.