How to Plant & Grow Walla Walla Onions

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Small immature onion bulbs called "sets" are planted rather than seeds.
Small immature onion bulbs called "sets" are planted rather than seeds. (Image: Jupiterimages/Creatas/Getty Images)

Typically grown from small-sized immature bulbs called "sets," onions (Allium cepa) are shallow-rooted vegetable crops that are best grown in moist soil where temperatures are not too hot. Walla Walla onions develop a yellowish flesh with a pungent flavor and are considered by horticulturists as a "long day" onion, which means the plants need exposure to day lengths between 14 and 16 hours to develop well. Walla Walla onions are therefore best suited for vegetable gardens in the northern U.S. and southern Canada where summers are cool to warm and the days are long.

Things You'll Need

  • Garden shovel
  • Organic matter (compost or well-rotted manure)
  • Rake
  • Hoe

Cultivate the garden soil in early spring as soon as it is workable -- when frost has left the ground and the soil isn't too soggy. Dig the soil with a shovel or rototiller to a depth of 6 to 10 inches. Onions grow best in a crumbly, not-too-dense soil. Avoid heavy clay or compacted loam soil types.

Scatter 1 to 3 inches of organic matter onto the soil and mix it with the shovel to a depth of 2 to 4 inches. Compost and well-rotted manure improves the soil's fertility and improves soil texture and drainage. Adding organic matter to sandy soils is particularly important for growing lush vegetables.

Rake the planting area smooth and let the soil naturally settle for three to five days.

Create a planting furrow in the soil that is 1 to 1 1/2 inches deep with the corner of the hoe blade. If the sets you have seem rather small, perhaps only dime-sized, make the furrow depth 3/4 to 1 inch deep at most.

Place Walla Walla onion sets into the furrow, pointed side upwards. Space them 2 to 4 inches apart in the row. Additional rows need to be spaced 12 to 18 inches apart, according to the University of Illinois Extension.

Backfill soil into the furrows to cover the Walla Walla onion sets. Gently tamp the soil down with your hand or the back of the hoe blade. Apply 2 inches of water to the soil to compact the dirt further to bring soil particles into direct contact with the bulbs and eliminate air pockets.

Hand-pull tiny weeds that sprout around the onions over the spring and summer. The shallow roots of the onions are easily cut by the hoe, so don't scrape the soil to kill weeds any closer than 5 or 6 inches from the growing onion.

Pull up the onions in summer or early fall once the upright leaves begin to naturally wither and yellow. Brush off soil from the Walla Walla onion skins and allow the bulbs to air-dry. Store them in a cool, dry place before using them.

Tips & Warnings

  • Do not mound up soil, called "hilling," over the growing Walla Walla onions' stem bases. This can lead to rot of the bulb and plant death.

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