New Hampshire Frost Dates & Growing Season Guide
New Hampshire (NH) spans USDA hardiness zones 5a, 5b, 6a, with growing seasons ranging from 141 days in Concord to 167 days in Portsmouth. The average growing season across the state is approximately 153 days.
New Hampshire City Frost Dates
The table below shows the average last spring frost date, first fall frost date, growing season length, and USDA hardiness zone for each city. Click a city name for detailed planting calendars and zone information.
| City | Last Spring Frost | First Fall Frost | Growing Season | Zone | Min Temp |
|---|---|---|---|---|---|
| Concord | May 10 | September 28 | 141 days | 5a | -20°F |
| Manchester | May 5 | October 2 | 150 days | 5b | -15°F |
| Nashua | May 3 | October 5 | 155 days | 5b | -15°F |
| Portsmouth | April 28 | October 12 | 167 days | 6a | -10°F |
Planting Windows for New Hampshire
Based on average frost dates, here are the recommended planting windows for each city. The indoor seed start date is approximately seven weeks before the last spring frost. Transplanting should occur about two weeks after the last frost. The last direct sow date for fall crops is ten weeks before the first fall frost.
| City | Start Seeds Indoors | Transplant After | Last Direct Sow |
|---|---|---|---|
| Concord | March 22 | May 24 | July 20 |
| Manchester | March 17 | May 19 | July 24 |
| Nashua | March 15 | May 17 | July 27 |
| Portsmouth | March 10 | May 12 | August 3 |
Gardening in New Hampshire
New Hampshire's rugged landscape and northern latitude create a challenging but rewarding gardening environment. The seacoast around Portsmouth in zone 6a has the mildest climate and longest growing season, while the interior valleys and White Mountains region can see frost into early June. Cold-hardy crops are the backbone of New Hampshire gardens: peas, brassicas, root vegetables, and potatoes perform reliably. Season extension with cold frames and hoop houses is widely practiced. Starting seeds indoors in March and April is essential for warm-season crops. The state's acidic, rocky soils need lime for most vegetables but suit blueberries and potatoes perfectly. Maple syrup production and apple growing are signature agricultural activities.
Understanding New Hampshire's Hardiness Zones
New Hampshire includes USDA hardiness zones 5a, 5b, 6a. These zones indicate the average annual extreme minimum winter temperature for each area, which determines which perennial plants, trees, and shrubs can survive winter without protection. When purchasing trees, shrubs, or perennial flowers, always check that the plant is rated for your hardiness zone or a lower (colder) zone number.
Tips for Using New Hampshire Frost Dates
These frost dates represent long-term averages and should be treated as guidelines rather than guarantees. In any given year, the actual last spring frost or first fall frost may arrive one to three weeks earlier or later than the average. Factors that affect your specific location include elevation above the city center, proximity to water, slope direction, and whether you are in an urban or rural area. South-facing slopes and areas near large pavement or building masses tend to be warmer than surrounding open land. Low-lying valleys and exposed hilltops are often colder than mid-slope positions.
To protect against late spring frosts, keep row cover fabric, old bed sheets, or frost blankets on hand. Monitor your local weather forecast daily as the average frost date approaches. When frost is predicted after you have transplanted, covering plants in the late afternoon traps ground heat and can protect against temperatures down to about 28 degrees Fahrenheit. For fall season extension, the same covers protect mature plants from early frosts, often buying several additional weeks of harvest.