When to Plant Onions and Potatoes: 8 Key Steps for Big Yields
Plant onions and potatoes in early spring, 2 to 4 weeks before the last frost, once soil warms to 40°F for onions and 45°F for potatoes. This guide covers timing by region, soil temperature, variety choice, step-by-step planting, and common mistakes so both crops start strong
Onions plant 2 to 4 weeks before the last spring frost when soil hits 40°F. Potatoes plant 2 to 4 weeks before the last frost when soil reaches 45°F to 50°F. In USDA zones 7 to 10, short-day onions also plant in fall from September to November.
Contents
What Are Onions and Potatoes?
Onions (Allium cepa) are cool-season bulb crops grown for their layered edible bulbs. Potatoes (Solanum tuberosum) are cool-season tuber crops that form underground. Both tolerate light frost, prefer 55°F to 75°F air temperatures, and stop forming storage organs above 85°F. That is why I plant both early on my Topeka farm, so bulbs and tubers size up before summer heat slows them down.
When to Plant Onions
Onions plant 2 to 4 weeks before your last spring frost, once soil reaches 40°F at a 4-inch depth. Timing then shifts based on onion type and latitude.
Onion Type Controls Timing
- Long-day onions (Walla Walla, Candy): plant March to April in USDA zones 3 to 6, north of 36° latitude.
- Short-day onions (Texas Grano, Vidalia): plant September to November in USDA zones 7 to 10, south of 35° latitude.
- Day-neutral onions (Superstar, Red Candy Apple): plant March to May across zones 5 to 8.
Day length triggers bulbing. Wrong-type onions stay small, so match the variety to your latitude before buying sets or seed. Fall planting also works for short-day types in the South, and I covered that window in more detail in my post on fall onion planting.
Onion Schedule by Region
- Northern states (MN, WI, ND): plant sets late March to early May.
- Midwest (KS, MO, IA): plant sets mid-March to mid-April.
- South (TX, GA, FL): plant short-day transplants October to December.
- West Coast (CA, OR): plant January to March.
Seed-grown onions start indoors 8 to 10 weeks before transplanting. I start mine in early January for an April transplant, and I outlined the full process in growing onions from seed. For gardeners timing both alliums together, my guide on garlic and onion planting windows lines up the dates.
When to Plant Potatoes
Potatoes plant 2 to 4 weeks before the last spring frost, once soil reaches 45°F to 50°F at 4 inches deep. Cold wet soil below 40°F rots seed pieces before they sprout. I shared the full breakdown in potato planting dates by zone.
Potato Schedule by Region
- Zones 3 to 4 (ND, MT): plant mid-May to early June.
- Zones 5 to 6 (KS, NE, OH): plant late March to late April.
- Zones 7 to 8 (TN, NC, AR): plant February to March.
- Zones 9 to 10 (FL, AZ): plant January to February, or fall for a second crop.
The old St. Patrick’s Day rule works in zones 6 to 7. North of zone 5, that date runs too early in most years. Check your zone on the USDA Plant Hardiness Zone Map before setting a planting week.
Soil Temperature Beats the Calendar
Soil temperature predicts sprouting better than a date. Push a soil thermometer 4 inches deep at 9 a.m. for three days. If the average reads 45°F for potatoes or 40°F for onions, planting goes forward. University of Minnesota Extension publishes vegetable planting guidance that confirms these thresholds for cool-season crops.
Where to Plant Onions and Potatoes
Both crops need full sun, loose soil, and steady drainage. Aim for 6 to 8 hours of direct sun, pH 6.0 to 6.8, and sandy loam or loam. Heavy clay stunts onion bulbs and deforms potato tubers. Raised beds help in wet ground. Avoid fields where tomatoes, peppers, or eggplant grew last year because they share blight and verticillium wilt pathogens with potatoes.
How to Plant Onions and Potatoes Step by Step
- Test soil temperature. Use a soil thermometer 4 inches deep. Onions at 40°F, potatoes at 45°F.
- Prep the bed. Loosen soil to 10 inches. Mix in 2 inches of compost per 100 square feet.
- Cut and cure seed potatoes. Cut pieces with 2 eyes each, then cure 2 to 3 days before planting. I walked through this in curing seed potatoes to reduce rot.
- Plant onion sets. Set bulbs 1 inch deep, 4 to 6 inches apart in rows 12 inches apart.
- Plant seed potatoes. Place pieces 4 inches deep, 12 inches apart in rows 30 inches apart.
- Water once. Give 1 inch after planting, then hold back until sprouts appear.
- Hill potatoes when shoots reach 6 inches. My post on how hilling works covers the technique.
- Mulch onions with 2 inches of straw to hold moisture and block weeds.
Mistakes to Avoid
- Planting in wet, cold soil below 40°F causes seed rot.
- Picking long-day onions in Georgia gives marble-sized bulbs.
- Planting potatoes in last year’s potato bed spreads scab and blight.
- Skipping the curing step on cut seed potatoes raises rot risk.
- Hilling potatoes too late exposes green, bitter tubers.
Troubleshooting Common Issues
- Onions bolting early: cold snaps after planting trigger flower stalks. Pick bolt-resistant varieties for spring risk zones.
- Potato seed rot: cold wet soil kills pieces. Replant after soil warms 10°F above the planting threshold.
- Slow onion growth: low nitrogen. Side-dress with 1 tablespoon of blood meal per plant at 4 weeks.
- Hollow potato centers: uneven water during tuber bulking. Keep soil moisture steady from flower stage forward.
Safety Notes
Green potato tubers contain solanine, a natural toxin. Cut off green parts or toss the tuber. Sprouted storage potatoes also carry solanine, so those go to seed use, not the kitchen. Wear gloves when handling treated seed potatoes because some carry fungicide coatings. Keep children and pets away from treated seed during planting.
FAQs about Plant Onions and Potatoes Together
Can I plant onions and potatoes together?
Yes, onions and potatoes grow well side by side. Onions repel some beetles, and their shallow roots avoid competing with deeper potato tubers. Keep 12 inches between the two rows.
What is the earliest I can plant onions?
Plant onion sets once soil hits 40°F at 4 inches deep, usually 4 weeks before the last frost. Seed starts indoors 8 to 10 weeks earlier in seed trays under lights.
Do potatoes need frost protection?
Potato shoots freeze at 30°F and die back after a hard frost. Cover emerged plants with row cover or straw if a late frost hits. Tubers stay safe underground below 4 inches.
How deep should I plant seed potatoes?
Plant seed potato pieces 4 inches deep, eye-side up, in loosened soil. Deeper planting delays emergence by 5 to 7 days and raises rot risk in cold ground.
Can onions survive a freeze after planting?
Yes, onion sets handle light freezes down to 20°F without damage. Transplants need a few days to harden first, or cover them with row cover until the roots set in.
Final Thoughts
Timing decides onion and potato yield more than fertilizer or variety. Watch soil temperature, not the calendar. Plant onions at 40°F, plant potatoes at 45°F, and stay 2 to 4 weeks ahead of your last frost. Match onion type to your latitude, cure cut potatoes, and keep soil loose and well drained. Get those two numbers right, and the rest of the season takes care of itself.
