Growing Onions in Zone 10: Plant Short-Day Varieties in Fall for a Full Spring Harvest
Zone 10 onion success comes down to timing and variety. Plant short-day types in fall, avoid spring planting, and follow this month-by-month guide for bulbs by spring.
Why Zone 10 Onion Growers Plant in Fall, Not Spring
Zone 10 gardeners face an onion problem that most seed packets don’t address: your warm climate makes year-round planting feel possible, yet spring onion plantings reliably fail. Beautiful green tops emerge — and then nothing worth eating forms underground. The fix isn’t complicated, but it requires understanding what actually triggers bulb formation, because the answer changes everything about when and what to plant.
Onion bulbing is controlled by day length, not calendar date. Once the number of daylight hours crosses a variety-specific threshold, the plant stops growing leaves and redirects all its energy into swelling the bulb. Our complete onion growing guide covers the full seasonal calendar, but zone 10 has its own distinct timing that deserves a closer look.
The Photoperiod Mechanism: Why Short-Day Varieties Are Non-Negotiable
Short-day onion varieties begin bulbing when day length reaches 10 to 12 hours of daylight. In zone 10, that threshold arrives in February or March — right in the middle of your mild, cool growing season. Long-day varieties need 14 to 16 hours of light before they bulb. In San Diego, for example, maximum day length peaks at roughly 14.4 hours in late June, when summer heat has already made the growing season impractical. Long-day onions simply don’t have a reliable window in zone 10.
Here’s the biological reason spring planting fails: if you plant onions in March in zone 10, day lengths have already exceeded 12.5 hours. Short-day varieties immediately hit their bulbing trigger before they’ve had time to build leaf mass. This matters because each leaf a plant grows before bulbing begins becomes one layer of the onion bulb. Fewer leaves going into the bulbing phase equals fewer layers — and flat, golf-ball-sized disappointments instead of full, round bulbs. Fall planting gives your onions an entire cool season to build leaf mass before spring day lengths cue the bulbing response.
Research published in Frontiers in Plant Science confirms this mechanism: bulbing is triggered when the ratio of maximum bulb diameter to neck diameter exceeds 2, driven by carbohydrate redistribution from the leaves into storage tissue once the photoperiod threshold is crossed. The onion’s FT gene family (specifically AcFT1) regulates this response in long-day conditions, while AcFT4 actively suppresses bulbing in short-day conditions — which is why matching variety to your latitude’s photoperiod is the single most important decision you’ll make.
Zone 10a vs. Zone 10b: Timing That Actually Matters
Zone 10a (minimum temperatures 30–35°F) and zone 10b (35–40°F) share the same short-day variety requirement but have a slightly different planting window. Zone 10a covers areas like the Imperial Valley in California, the Brownsville area of south Texas, and parts of Miami-Dade County in Florida. Zone 10b includes the low desert around Phoenix, Arizona, coastal San Diego, and the southern Florida Keys.
| Sub-zone | Typical locations | Seed start | Transplant out | Expected harvest |
|---|---|---|---|---|
| Zone 10a | Imperial Valley CA, Brownsville TX, south FL (Miami) | Late September–October | Mid-November | March–April |
| Zone 10b | Phoenix AZ, coastal San Diego, south FL Keys | October–mid-November | Late November–December | April–May |
Zone 10a gets an earlier start because its slightly cooler fall temperatures help seedlings establish before the risk of temperature-fluctuation bolting. Zone 10b can afford a later window because its winters rarely dip below 38°F, allowing slow steady growth through January and into February. In both sub-zones, a cold snap below 40°F followed quickly by warm weather can trigger premature bolting — the plant interprets the cold as “winter” and warm-up as “spring,” sending up a flower stalk. Fall-planted transplants raised from fresh seed are more bolt-resistant than sets that have already been stored.
Month-by-Month Growing Calendar for Zone 10
| Month | Zone 10a tasks | Zone 10b tasks |
|---|---|---|
| September | Sow seed indoors or direct-sow in prepared beds | Prepare beds; too warm to sow outdoors |
| October | Thin seedlings; sow late-season batch if needed | Sow seed now; temperatures settling into range |
| November | Set out transplants 3 inches apart; water in well | Set out transplants or direct-sow; apply starter fertilizer |
| December | Side-dress nitrogen at 5-leaf stage; mulch rows | Monitor establishment; side-dress at 5-leaf stage |
| January | Continue nitrogen side-dressing every 3–4 weeks; maintain moisture | Side-dress nitrogen; watch for cold snaps and protect if needed |
| February | Watch for bulbing to begin; reduce then stop all nitrogen | Continue nitrogen if plants have fewer than 13 leaves; taper off |
| March | Harvest when one-quarter of tops have fallen over | Bulbing underway; maintain water; no nitrogen |
| April | Cure bulbs in shaded ventilated spot for 10–14 days | Harvest as tops fall; cure promptly |
| May | Store or use fresh; sweet varieties last 4–6 weeks | Cure and store; begin planning next fall’s planting |
Best Short-Day Varieties for Zone 10
Not all short-day varieties perform equally across zone 10’s varied microclimates. For a broader look at onion classifications, see our guide to onion types and varieties. Here’s how the main short-day options compare on the criteria that matter most in warm climates:
| Variety | Color | Flavor | Storage | Bolt resistance | Best for |
|---|---|---|---|---|---|
| Texas 1015Y (SuperSweet) | Yellow | Very sweet, very mild | 4–6 weeks | Good | Both sub-zones; widest adaptability |
| Yellow Granex | Yellow | Sweet, mild (Vidalia type) | 3–4 weeks | Moderate | Both; fresh use, not storage |
| Texas Grano 502 | Yellow | Mild, slightly pungent | 4–6 weeks | Strong | Zone 10a; bolt-prone areas |
| Crystal Wax (White Bermuda) | White | Very mild, clean | 2–3 months | Good | Both; best for longer storage |
| Red Burgundy | Red | Mildly pungent, slightly sweet | 2–3 months | Moderate | Both; good all-rounder |
| Red Creole | Red | Pungent, firm flesh | 3–4 months | Excellent | Zone 10a; hotter summer areas needing long storage |
Texas 1015Y earns its “SuperSweet” name through exceptionally low pyruvic acid content — the compound responsible for the sharp, tear-inducing bite in standard onions. It’s the most widely adaptable short-day variety and performs reliably from south Texas to the California low desert. For the best combination of flavor and shelf life, grow Texas 1015Y for fresh eating alongside Red Creole for any onions you want to keep more than a month.
I’ve had the best luck in Southern California conditions with Yellow Granex transplants set out in late November — they build leaf mass through December and January, then bulb up beautifully when day lengths start stretching in February. The key is resisting the urge to push them along with extra nitrogen in February; at that point the plant is switching modes and nitrogen just pushes top growth at the expense of the bulb.
Soil Prep and Planting Methods
Zone 10 soils vary significantly — San Diego’s well-drained sandy loams differ from Miami’s heavy clay and Phoenix’s caliche-laden desert soil — but onions need the same baseline: pH 6.0 to 6.5, good drainage, and at least 8 inches of workable, stone-free depth.
Common zone 10 soil corrections:
- Heavy clay (common in south Florida): Work in 3–4 inches of compost before planting. Raised beds 8–10 inches high solve persistent drainage problems immediately.
- Sandy desert soil (Arizona low desert): Compost plus a 2-inch layer of straw mulch between rows reduces moisture loss dramatically.
- Caliche hardpan (AZ and south TX): Break through to at least 10 inches with a digging bar, or grow in raised beds entirely.
Seeds, transplants, or sets? Transplants give the most reliable results in zone 10 because you control the start date precisely. Start seeds 8–10 weeks before your transplant date, harden off for one week, then set out at 3/4 inch deep and 3 inches apart (4–6 inches for maximum bulb size). Seeds direct-sown in the garden work well in zone 10a’s slightly cooler fall; sow at 1/4 inch depth and thin to 2–3 inches once seedlings reach 3 inches tall. Sets are the fastest method but limit you to whichever short-day varieties your local nursery stocks — confirm the label says short-day before buying.
Space rows 12 inches apart. Onions are shallow-rooted, with most activity in the top 6 inches of soil — this affects both watering and mulching decisions more than it does spacing.
Fertilizing and Watering Through the Season
Onions are heavy nitrogen feeders through the vegetative phase, then need nitrogen cut off entirely once bulbing begins. The two-phase approach recommended by Texas A&M AgriLife Extension works well across zone 10 climates.
At planting: Work 2–3 pounds of a balanced fertilizer (10-10-10) into every 100 square feet of bed. This provides the phosphorus and potassium roots need to establish before the nitrogen push begins.
Vegetative phase (November through January): Side-dress with 1/2 cup of ammonium sulfate (21-0-0) per 10 feet of row every 3–4 weeks, or apply 0.5–1 pound of actual nitrogen per 100 linear feet as your plants approach 3–4 inches tall. Stop all nitrogen once bulbing begins — applying nitrogen after the photoperiod trigger pushes top growth at the direct expense of bulb size and storage quality.
Watering: Keep the top 6 inches consistently moist but not waterlogged. In zone 10’s drier climates (Arizona, inland California), drip irrigation set to run 2–3 times per week works well. In south Florida’s heavy winter rains, drainage matters more than irrigation — raised beds and well-amended soil prevent the root rot that kills more zone 10 onions than drought does. A 2-inch straw or shredded-leaf mulch between rows stabilizes soil moisture and moderates the temperature swings that trigger premature bolting.
Zone 10 companion planting can help maximize bed space — see our onion companion planting guide for crops that share the fall-to-spring window well. If you’re growing garlic alongside your onions, our zone 10 garlic guide covers the same fall planting principles with garlic-specific variety and timing details.
Reading the Harvest Signs and Curing Properly
In zone 10, most fall-planted short-day onions are ready between March and May, depending on your sub-zone and exact planting date. The signal to watch isn’t on a calendar — it’s in the tops themselves.
When roughly one-quarter of the green tops have softened and fallen over on their own, start checking bulbs. Once half have fallen, harvest promptly. Leaving onions in zone 10’s warming spring soil risks disease and accelerates deterioration, particularly in the sweet Granex-type varieties whose thin skins offer little protection.
To harvest: pull by hand or pry gently with a garden fork. Brush off loose soil and lay bulbs in a single layer in a shaded, well-ventilated location for 10–14 days. Do not wash them. The necks must dry completely before you remove tops and roots — any remaining moisture creates an entry point for storage rot. In zone 10’s spring warmth, curing happens faster than in northern climates; check necks at 10 days and remove tops only when they’re paper-dry and the neck has shriveled closed.
Sweet varieties (Texas 1015Y, Yellow Granex) store only 4–6 weeks even when properly cured — plan to use them fresh, give them away, or refrigerate them. Crystal Wax and Red Creole last considerably longer, up to 3–4 months with good air circulation in a cool, dry spot.
Frequently Asked Questions
Can I grow onions year-round in zone 10?
Green onions (scallions) grow year-round, including through zone 10 summers. Bulbing onions need the specific combination of cool temperatures and increasing spring day length that only your fall-through-spring season provides. Summer plantings produce green tops but won’t form bulbs because the heat prevents proper leaf development and the already-long days trigger premature bulbing before plants are large enough.
Stop missing your zone's planting windows.
Select your US zone and month — get a complete checklist of what to plant, prune, feed, and protect right now.
→ View My Garden CalendarWhy does my seed packet say to plant in spring?
Most seed packets are written for zone 5–7 conditions, where spring soil temps are too cold for fall planting and the entire summer season is available for growth. In zone 10, ignore those instructions entirely. Fall is your onion season.
Can I grow long-day varieties in zone 10?
In most of zone 10, long-day varieties won’t receive enough daylight hours (14–16 hours) to bulb reliably. Maximum day length in San Diego tops out around 14.4 hours in late June — barely enough for borderline long-day cultivars — but by then summer heat has already ended the practical growing season. Stick with short-day types unless you’re in the coolest edge of zone 10b coastal California and want to experiment with a small trial planting of an intermediate-day variety.
My onion sent up a flower stalk — what do I do?
Bolting is usually triggered by a cold snap (below 40°F for several days) followed by a quick warm-up — the plant interprets this as winter-then-spring and initiates flowering. If a stalk appears, snap or cut it off at the base immediately. The bulb will continue developing but won’t store as well as unbolted onions. To reduce bolt risk going forward, use fresh transplants rather than stored sets, plant within the recommended timing windows, and cover plants with row cloth if temperatures threaten to drop below 35°F for more than two or three nights.
