Spanish onions are favored by home gardeners for their large size and sweet, mild flavor. Achieving these impressive, globe-shaped bulbs requires attention to specific environmental and cultural practices. Success relies on careful preparation of the planting site, precise timing, and consistent care after planting.
Preparing the Planting Site
Spanish onions require full sun exposure, meaning the site must receive a minimum of six to eight hours of direct sunlight daily. This high light requirement is necessary for growing large bulbs. The soil structure must be loose and highly friable, allowing the bulb to expand without physical restriction.
The ideal soil is a sandy or silty loam that drains quickly, as onions cannot tolerate soggy conditions that lead to rot. Before planting, work the bed deeply (at least ten inches) to remove rocks, clods, or debris that could deform the developing bulb. Incorporate a one-inch layer of well-rotted compost or manure to enrich the soil and slowly release nutrients.
Onions require consistent feeding, so incorporate a granular fertilizer with a lower nitrogen ratio during preparation, such as a 10-20-10 blend. Excessive nitrogen early on encourages leaf growth at the expense of bulb development. The soil should be fertile and near-neutral, ideally with a pH between 6.0 and 6.8. Creating raised beds is beneficial, especially in heavy clay areas, as this ensures superior drainage and a loose environment for bulb expansion.
Determining Optimal Planting Timing
Spanish onions are a “long-day” variety, requiring 14 to 16 hours of daylight to trigger bulbing. This requirement makes them best suited for northern latitudes where summer days are long. Planting too late results in small bulbs because the plants lack sufficient time to grow large tops before daylight hours decrease.
The best time to plant is in early spring, four to six weeks before the last expected frost date, when the soil is workable. This early start allows plants to develop substantial foliage before the summer solstice triggers bulb formation. Transplants or sets can be placed in the ground once the soil temperature reaches at least 50 degrees Fahrenheit.
If starting Spanish onions from seed indoors for maximum variety selection, sow them eight to twelve weeks before the planned outdoor transplanting date. This ensures the seedlings are pencil-thick and robust enough to establish quickly. For direct seeding outdoors, planting should occur once the soil temperature is consistently above 55 degrees Fahrenheit.
Step-by-Step Planting Instructions
Planting Spanish onions must be done carefully to avoid burying the developing bulb too deeply. When using sets or transplants, plant them shallowly so the bulb base rests just beneath the soil surface. Dig a narrow trench or use a trowel to create a shallow hole about one inch deep, wide enough to accommodate the roots.
Place the set or transplant into the hole, ensuring the roots point downward and spread slightly. The top of the bulb or the neck of the transplant should remain exposed, or barely covered with soil. Planting too deep restricts the bulb’s ability to enlarge and may result in a misshapen onion.
Spacing is paramount for achieving large bulb size. Plants should be spaced four to six inches apart within the row. Closer spacing can be used if some plants are harvested early as green onions, but the final spacing must be maintained for full bulb expansion. Rows should be spaced 12 to 18 inches apart for weeding, feeding, and air circulation.
For direct seeding, sow seeds one-quarter to one-half inch deep, placing two to three seeds per inch. Once seedlings emerge and reach about five inches in height, they must be aggressively thinned. Thinning is necessary to ensure remaining plants have the resources and space to form large bulbs.
Initial Care After Planting
After planting, provide a thorough, deep watering to settle the soil and alleviate transplant shock. Onions have a shallow root system, making them highly dependent on consistent moisture, especially during establishment. Keep the soil uniformly moist but never waterlogged, as excessive water promotes fungal diseases.
If the area was direct-seeded, thinning must be completed promptly once seedlings are established, removing all but the most vigorous plants to maintain the four-to-six-inch spacing. The pulled seedlings can be used as scallions or green onions. Maintaining a weed-free environment is important in the early stages, as young onion plants are poor competitors for light, water, and nutrients.
Applying a light, organic mulch, such as straw or grass clippings, between the rows helps suppress weeds and regulate soil moisture and temperature. This initial care sets the stage for a healthy growing season, allowing the plant to focus energy on developing a strong root system and robust foliage.