Portsmouth, NH Frost Dates
Published: February 1, 2026 · Updated: April 23, 2026
USDA Zone 5a
Last Spring Frost
April 15
Safe to transplant after this date
First Fall Frost
October 15
Protect tender plants before this date
Frost-Free Season
183 days
Mid-April to Mid-November
320 days until last frost in Portsmouth — April 15
Portsmouth’s location in USDA zone 5a creates one of the shorter growing seasons in the continental US. With a last spring frost of April 15 and first fall frost of October 15, the frost-free window is just 183 days — every gardening and lawn care decision must be precisely timed.
Complete Planting Calendar for Portsmouth
Lawn Care Timing for Portsmouth
Based on Portsmouth's USDA zone 5a frost dates: The last frost of April 15 marks when warm-season grasses begin recovering. Apply spring fertilizer in Mid-April when soil reaches 55°F. The first fall frost of October 15 sets your deadlines — complete overseeding by August 25–September 20 and apply winterizer by Mid-November.
USDA Zone 5a Summary
Six months frost-free. Excellent cool-season lawn climate. Chicago, Detroit, Boston area.
Frequently Asked Questions
What is the last frost date in Portsmouth, NH?
The average last spring frost in Portsmouth is April 15. This is based on USDA zone 5a historical averages. Plan transplanting and spring lawn care around this date — wait 2 weeks after for frost-sensitive plantings.
When is the first fall frost in Portsmouth?
Portsmouth’s average first fall frost is October 15. This sets deadlines for fall overseeding (must be done 6 weeks before) and winterizer application (by Mid-November).
When should I plant tomatoes in Portsmouth?
Transplant tomato seedlings outdoors after April 15. Start seeds indoors Early February (10 weeks before last frost). Direct sow warm-season crops after april 22.
When should I overseed my lawn in Portsmouth?
The prime overseeding window in Portsmouth is August 25–September 20. This ensures warm soil for germination before the first fall frost on October 15.
When should I apply winterizer in Portsmouth?
Apply winterizer fertilizer in Portsmouth by Mid-November. This high-potassium application hardens grass cell walls before freeze damage. Your first fall frost of October 15 is the hard deadline.