Onions are a popular cool-season vegetable requiring careful timing to produce large, healthy bulbs. Successful growth in California depends on matching the onion variety to the local climate and planting schedule. Bulb formation is triggered by a biological response to the number of daylight hours, not simply plant age. Understanding this light requirement, known as photoperiodism, is the foundation for a successful harvest across the state’s diverse microclimates.
Understanding Onion Varieties and Day Length Needs
The process of bulbing in onions is triggered by a specific duration of sunlight, which categorizes them into three distinct types. Short-day varieties require 10 to 12 hours of daylight and are suited for the mild winter growing season in Southern California and desert regions. Intermediate-day varieties need 12 to 14 hours of sunlight and perform well in the Central Valley and mid-California.
Long-day onions require 14 to 16 hours of daylight and are adapted to northern latitudes. Planting the wrong variety for a region’s photoperiod will result in small bulbs or cause the plant to “neck” prematurely. For most of California, the sweet, mild short-day and intermediate-day types are the most reliable choices for bulb production.
Regional Planting Timetables Across California
The vast geographical differences within California require planting schedules to be segmented into three major zones to ensure optimal results. In Southern California and the Imperial Valley, the best time to plant short-day onions is in the fall, typically mid-October through December. This timing allows plants to establish robust foliage during mild winters and ensures bulbing occurs when daylight reaches the 10-to-12-hour threshold in late winter and early spring.
The Central Valley and inland transitional areas experience slightly colder winters and longer summer days, making them best suited for intermediate-day varieties. Planting should occur from late November through January. Establishing the seedlings during this cool period allows them to develop necessary leaf growth before longer days initiate the bulbing process in late spring.
In the cooler, high-latitude regions of Northern California and along the coastlines, intermediate-day and some long-day varieties can be planted. The window shifts to late winter and early spring, specifically February through early April. This timing capitalizes on workable soil after winter chill and primes plants to respond to the 13-to-15-hour daylight periods leading up to the summer solstice. Using transplants, which are small, pre-grown seedlings, allows for earlier planting than starting from seed, providing an advantage in shorter growing seasons.
Preparing the Soil and Initial Planting Steps
Successful growth relies on proper soil preparation once the optimal planting date is determined. Onions thrive in loose, well-draining soil that is rich in organic matter, as their shallow root systems are easily inhibited by compaction. The ideal soil pH is slightly acidic to neutral, targeting a range between 6.0 and 7.0.
Before planting, incorporate well-rotted compost or manure to improve both drainage and nutrient retention. Onions are heavy feeders, particularly requiring a steady supply of nitrogen for healthy leaf development, which correlates directly to final bulb size. A soil test can confirm the need for phosphorus and potassium, but nitrogen should be applied monthly until bulbing begins.
Gardeners generally have three options for starting their crop: seeds, sets, or transplants.
Starting Options
- Starting from seed is the most economical method and offers the widest variety selection, but it requires the longest growing time.
- Onion sets, which are small, immature bulbs, are the easiest and fastest to plant but come with a limited variety selection and a higher risk of premature flowering.
- Transplants, often sold as small, dormant seedlings, offer a quick start with a high success rate, as they bypass the slow germination phase.
When planting sets or transplants, place them about one inch deep in the soil to avoid inhibiting bulb expansion. Space individual plants approximately three to four inches apart in rows to allow for adequate bulb development and air circulation.