When to plant Onion in Knox County, IL
Spring Onion in Knox County goes in April 3–April 24, once nighttime temps stop dipping near freezing. A second sowing from August 6 to August 20 extends the harvest into fall.
When to Plant Onion in Knox County, IL
Onions are a fundamental kitchen staple available in yellow, white, and red varieties. Choose long-day, short-day, or intermediate types based on your latitude.
Knox County, Illinois is in USDA Zone 5b. The average last spring frost is April 17 and the first fall frost is October 15, giving you a growing season of approximately 181 days.
At an elevation of 1,301 feet, Knox County receives approximately 33.2 inches of rainfall annually with predominantly silt loam soil. Summer highs average 84°F, so choose short-season varieties of Onion to ensure they mature before fall.
Knox County Soil Profile
Soil Type
Silt Loam
Soil pH
6.0-7.0
Drainage
Well Drained
Onion Planting Timeline — Knox County, IL
Onion Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Start Indoors | March 13 | Mar 13 – Mar 27 |
| Transplant Outdoors | April 17 | Apr 17 – May 1 |
| Direct Sow | April 3 | Apr 3 – Apr 24 |
| Harvest | July 17 | Jul 17 – Sep 4 |
| Fall Sowing | August 6 | Aug 6 – Aug 20 |
Plant 1" deep · 6" apart · Rows 12" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | — |
| March | Start Indoors |
| April | Transplant Outdoors Direct Sow |
| May | Transplant Outdoors |
| June | — |
| July | Harvest |
| August | Fall Sowing Harvest |
| September | Harvest |
| October | — |
| November | — |
| December | — |
Growing Conditions
☀️ Sun
Full Sun (6-8+ hours)
💧 Water
Moderate — regular watering
📅 Days to Maturity
90–120 days
🧪 Soil pH
Needs 6–7 · Your soil: N/A
🗺️ USDA Zone
Zone 5b
📆 Growing Season
181 days in Knox County
Growing Tips for Knox County
Start seeds indoors 8-10 weeks before transplanting. Match day-length type to your latitude. Stop watering when tops begin to fall over and cure bulbs for 2-3 weeks before storage.
Companion Planting
Good Companions
Avoid Planting Near
Check more plant combinations with our Companion Planting Checker →
Onion in Other Locations
Frequently Asked Questions
When should I plant Onion in Knox County, IL?
Knox County is in Zone 5b with an average last frost of April 17. Plan your Onion planting based on this frost date — see the calendar above for exact timing.
What planting zone is Knox County, IL?
Knox County, Illinois is in USDA Hardiness Zone 5b. The average last spring frost is April 17 and first fall frost is October 15.
When should I plant Onion in Knox County, IL?
In Knox County, IL, plant Onion after the last frost (around April 17) and before the first frost (around October 15). Best results when soil holds above 50°F.
What growing zone is Knox County, IL for Onion?
Knox County sits in USDA Zone 5b. Onion grows reliably in zones 3a through 10b, so it's a good fit here.
Can Onion grow in Knox County's climate?
Yes — Onion grows well in Knox County's temperate climate. Knox County averages a 181-day frost-free season, with last frost around April 17 and first frost around October 15.
Your Knox County Garden Planner — Free
A 22-page printable planner built for Knox County (Zone 5b). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.