Portsmouth, NH Frost Dates
Average frost dates, USDA hardiness zone, and growing season length for Portsmouth, New Hampshire.
Gardening in Portsmouth
The Seacoast's historic port city has maritime-moderated conditions that extend the growing season beyond what most New Hampshire gardeners enjoy.
Zone 6a with 173 frost-free days — significantly longer than inland New Hampshire. The Atlantic moderation provides milder winters and a more gradual spring. Coastal soil is sandy.
Portsmouth's nationally recognized restaurant scene is powered by the Seacoast's small farms and home gardens. The maritime heritage extends to a food culture that values what's local and seasonal.
What This Means for Portsmouth Gardeners
The average last spring frost in Portsmouth is around April 22, and the average first fall frost arrives around October 12. That gives you approximately 173 frost-free days to work with.
A solid, workable season. Most standard vegetable varieties have enough time to mature, though the longest-season crops (like sweet potatoes at 90+ days or large watermelons at 85+ days) need to be started early and chosen carefully. Indoor seed starting isn't optional — it's how you buy the extra weeks that make the difference between a good harvest and a great one.
These dates are based on NOAA 30-year Climate Normal data for the Portsmouth area. Your actual frost dates could shift 2-3 weeks in either direction in any given year. Learn more about our data sources.
What to Grow in Portsmouth
Portsmouth's 173-day season gives you plenty of time for most vegetables with good planning. Start warm-season crops indoors to maximize your window. Cool-season crops thrive in your spring and fall shoulder seasons. Recommended starting points: tomatoes, peppers, lettuce, carrots, beans, broccoli, garlic, and basil.
See the full New Hampshire planting guide for all 40 plants: New Hampshire Planting Calendar. Or enter your zip code for exact planting dates personalized to Portsmouth.
More About Zone 6A
Portsmouth is in USDA Hardiness Zone 6A, which means average annual extreme minimum temperatures between -10°F to -5°F. View the full Zone 6A planting guide.
See the complete planting calendar for New Hampshire: New Hampshire Planting Calendar.
Other Cities in New Hampshire
Frequently Asked Questions
These dates are based on NOAA's 30-year Climate Normal data for the Portsmouth area. They represent historical averages, not predictions. In any given year, the actual last frost could be 2-3 weeks earlier or later. Microclimates within Portsmouth (urban heat islands, hilltops, low-lying valleys) can also shift your local frost dates by a week or more.
Start warm-season seeds indoors 6-8 weeks before your last frost (April 22) to maximize your 173-day window. Direct sow cold-hardy crops like peas and lettuce 3-4 weeks before last frost. Every week of early indoor starting matters at this season length. Enter your zip code for exact dates.