When to Plant Onions in Portsmouth, NH
Published: April 21, 2026
Onion Planting Dates for Portsmouth, NH
| Start seeds indoors | February 4–11 |
| Last frost (average) | April 15 |
| Transplant outdoors | March 18–28 |
| Direct sow outdoors | March 18–28 |
| Minimum soil temperature | 35°F |
| Expect first harvest | June 16 – July 16 |
| First fall frost (average) | October 15 |
⚠ 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 Portsmouth, NH
For Zone 5a Portsmouth, the best-performing onion varieties are Copra, Stuttgarter, and Yellow Sweet Spanish — all long-day varieties that bulb under the 14+ hour daylight periods of northern latitudes. These varieties are typically stocked by local nurseries and are the safest bets for gardeners new to onion in Portsmouth.
Growing Onions in Portsmouth
Portsmouth sits in Zone 5a, with an average last frost of April 15 and first fall frost around October 15 — giving a 183-day frost-free growing season. Cool-season crops like onion benefit from Portsmouth's cooler spring and fall windows, when temperatures stay in the 55–75°F sweet spot that produces the best flavor and least bolting.
Portsmouth's cooler summers are close to ideal for onion. The same conditions that limit tomato and pepper yields benefit cool-season crops — slower bolting, sweeter flavor, and longer harvest windows. The fall onion crop in Portsmouth is often more productive than the spring crop.
Portsmouth's well-draining loam soils are among the best for onion growing — focus on annual organic matter additions (2 to 3 inches of compost) and consistent moisture during drier months. Water onion at 1 inches per week; loam holds moisture evenly without the drought-crack cycle of heavy clay or the rapid-drain losses of pure sand.
Onion Calendar for Portsmouth
| Month | Task |
|---|---|
| February | Start seeds indoors under grow lights |
| April | Last frost — harden off seedlings outdoors |
| March | Transplant outdoors into warm soil |
| June | Expect first harvest window to open |
| October | First fall frost — harvest remaining, end of outdoor season |
Onion Tips for Portsmouth 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.
- •Fall plantings in Portsmouth often out-produce spring plantings — cooler temperatures slow bolting and concentrate flavor. Light frost can actually improve the taste of onion.
Common Onion Pests in Portsmouth
- •Onion Thrips — peaks July in Zone 5a (active June–August); tiny insects causing silver streaks; strong water spray removes most.
- •Onion Maggot — peaks July in Zone 5a (active June–August); larvae feed on bulbs; crop rotation and row covers are best defense.
- •Downy Mildew — peaks July in Zone 5a (active June–August); 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 Portsmouth
In Portsmouth's cold 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. In short-season gardens, interplanting companions at transplant time maximizes each bed's productive weeks.
Frequently Asked Questions
When should I plant onion in Portsmouth, NH?
In Portsmouth (Zone 5a), start onion seeds indoors around February 4–11 and transplant outdoors around March 18–28. The city's average last frost of April 15 is the anchor date — count 10 weeks back for seed starting and 4 weeks back for transplanting.
What zone is Portsmouth, NH for onion growing?
Portsmouth is USDA Zone 5a. For onion, this means a frost-free growing season of roughly 26 weeks running from April 15 to October 15. Cool-season crops like onion thrive in this zone with both spring and fall planting windows available.
When is onion harvest season in Portsmouth?
Expect the first onion harvest in Portsmouth around June 16, with harvest continuing through July 16. This is based on 90–120 days from transplant or direct sow.
How long does it take to grow onion in Portsmouth, NH?
From transplant or direct sow to first harvest, onion takes 90–120 days in Portsmouth's climate. Based on a typical planting date of March 18–28, expect your first harvest around June 16. Portsmouth's cooler Zone 5a climate often lands at the slower end of this range — cooler nights slow fruit development.
What soil does onion need in Portsmouth?
Portsmouth's loam soil is near-ideal for onion. Work in 2–3 inches of compost before planting to boost organic matter and nutrient content. Minimal amendment is needed beyond that — loam holds moisture evenly without the drought-crack cycle of clay or the nutrient-loss issues of sandy soil.