When to Plant Onions in Vacaville, CA
Published: April 21, 2026
Onion Planting Dates for Vacaville, CA
| Start seeds indoors | January 11–18 |
| Last frost (average) | March 22 |
| Transplant outdoors | February 22–March 4 |
| Direct sow outdoors | February 22–March 4 |
| Minimum soil temperature | 35°F |
| Expect first harvest | May 23 – June 22 |
| First fall frost (average) | November 10 |
⚠ CRITICAL: Choose variety based on your latitude. Short-day varieties for south of 35°N (Texas, Florida, California). Long-day varieties for north of 35°N (Ohio, Minnesota, New York). Intermediate varieties work in the middle band.
Best Onion Varieties for Vacaville, CA
For Zone 6b Vacaville, the best-performing onion varieties are Candy, Super Star, and Yellow Sweet Spanish — all widely adapted, disease-resistant varieties proven across the transition zone. Walla Walla is a good alternative where disease resistance matters most, especially in humid summers. These varieties are typically stocked by local nurseries and are the safest bets for gardeners new to onion in Vacaville.
Growing Onions in Vacaville
Vacaville sits in Zone 6b, with an average last frost of March 22 and first fall frost around November 10 — giving a 233-day frost-free growing season. Cool-season crops like onion benefit from Vacaville's cooler spring and fall windows, when temperatures stay in the 55–75°F sweet spot that produces the best flavor and least bolting.
Vacaville's moderate climate supports onion on the standard transplant calendar with minimal special accommodation. Succession planting — sowing fresh seeds or setting new transplants every 2–3 weeks through spring — stretches the harvest window and gives you a backup crop if pests hit the first planting.
Vacaville's mixed soil conditions vary block by block — get a soil test if you have not in three years. Generally, a base of quality compost (2 to 3 inches annually) and consistent watering at 1 inches per week serves onion well across most soil types found in Vacaville.
Onion Calendar for Vacaville
| Month | Task |
|---|---|
| January | Start seeds indoors under grow lights |
| March | Last frost — harden off seedlings outdoors |
| February | Transplant outdoors into warm soil |
| May | Expect first harvest window to open |
| November | First fall frost — harvest remaining, end of outdoor season |
Onion Tips for Vacaville Gardeners
- •Match variety to latitude: short-day south of 35°N, long-day north of 35°N, intermediate in between.
- •Growing from sets (small bulbs) is fastest; growing from transplants gives the most variety options.
- •Succession plant onion in Vacaville every 2–3 weeks through spring to extend the harvest window into summer.
Common Onion Pests in Vacaville
- •Onion Thrips — peaks July in Zone 6b (active May–September); tiny insects causing silver streaks; strong water spray removes most.
- •Onion Maggot — peaks July in Zone 6b (active May–September); larvae feed on bulbs; crop rotation and row covers are best defense.
- •Downy Mildew — peaks July in Zone 6b (active May–September); yellow patches on leaves with fuzzy underside growth; improve airflow and apply copper.
Check plants every 2–3 days during peak season — early intervention prevents most infestations from becoming serious.
What to Plant with Onions in Vacaville
In Vacaville's moderate climate, Carrot and Tomato are the most beneficial plants to grow alongside onion. Carrot shares root-zone space without competing because carrot roots run deeper than most companions. Keep onion away from Bean — it inhibits garlic and onion bulb sizing when planted too close.
Frequently Asked Questions
When should I plant onion in Vacaville, CA?
In Vacaville (Zone 6b), start onion seeds indoors around January 11–18 and transplant outdoors around February 22–March 4. The city's average last frost of March 22 is the anchor date — count 10 weeks back for seed starting and 4 weeks back for transplanting.
What zone is Vacaville, CA for onion growing?
Vacaville is USDA Zone 6b. For onion, this means a frost-free growing season of roughly 33 weeks running from March 22 to November 10. Cool-season crops like onion thrive in this zone with both spring and fall planting windows available.
When is onion harvest season in Vacaville?
Expect the first onion harvest in Vacaville around May 23, with harvest continuing through June 22. This is based on 90–120 days from transplant or direct sow.
How long does it take to grow onion in Vacaville, CA?
From transplant or direct sow to first harvest, onion takes 90–120 days in Vacaville's climate. Based on a typical planting date of February 22–March 4, expect your first harvest around May 23. Vacaville's Zone 6b transition-zone climate produces maturity times right in the middle of the stated range.
What soil does onion need in Vacaville?
Vacaville's soil conditions vary block by block. Before planting onion, do a soil test (most state extension offices run them for $15–30). The test reveals pH, key nutrient levels, and organic matter content, so you can amend appropriately rather than guessing. Generally, 2–3 inches of compost annually improves most soils for vegetable production.