Gardening in New Hampshire: zones, frost dates and when to plant

Seacoast, south and North Country

NOAA’s state climate summary describes New Hampshire as a place of cold, snowy winters and mild summers. A jet stream that often sits overhead makes the weather highly changeable. The Gulf of Maine moderates temperatures in the southeast, where the land is also lower. The Portsmouth area on the Seacoast has the mildest winters, and Concord in the south-central part of the state is next. The far north and west get the least ocean influence and the most severe cold. Higher elevations such as the White Mountains are colder still. In July, average highs run in the upper 70s °F in the north and low 80s in the south.

The state averages close to 49 inches of precipitation a year. More falls in the south and along the eastern border than in the west and north. NOAA notes that annual and summer totals have been above average since 2005. Water shortage is rarely the main problem. Cold soil and a short season are.

These differences are large for a small state. Check the frost dates for your city above, and expect frost pockets and hilltop gardens to run behind the nearest town.

Timing with UNH Extension’s planting chart

UNH Extension’s fact sheet Growing Vegetables: When to Plant Your Vegetable Garden is built around your local last frost date. For each crop it gives the weeks needed from sowing to transplant size and when it is safe to set out, counted in weeks before or after that date. Mark the planting date on a calendar and count back to find when to start seeds. Some examples from the chart:

• Peas can be direct-seeded as soon as the soil thaws.
• Cabbage, kale, kohlrabi, leeks and onions can go out about four weeks before the last frost.
• Tomatoes need six to eight weeks from seed and go out one to two weeks after the last frost.
• Peppers and eggplant need eight to ten weeks from seed. Peppers go out two weeks after the last frost, eggplant two to three weeks after.
• Rutabaga is direct-seeded from mid-June to mid-July for a fall crop.
• Lettuce, beans, broccoli and corn have short harvest windows. Sow several small plantings in sequence to stretch them out.

UNH’s tomato fact sheet adds a soil-temperature rule: wait until the soil is about 60°F. Across much of the state, Memorial Day weekend is a safe transplanting time. Southern gardeners can often go a week or two earlier, and northern frost pockets may need to wait until mid-June. Choose varieties bred for a short season, ideally ones tested under local conditions. Black plastic mulch warms the soil early in the season. Straw, leaves or pine needles also work but slow soil warming, so lay them only after the soil is thoroughly warm. UNH notes that early blight and Septoria leaf spot are very common. Gardeners who keep losing plants to them may want a simple temporary tunnel, because covered plants have drier leaves.

Stretching a short season

UNH Extension treats row cover as a main tool for New Hampshire gardens. Lightweight fabric gives about 2°F of frost protection. Heavyweight cover can give 6 to 10°F and lets gardeners plant several weeks earlier in May. Temperatures under the cover can run 15°F above the outside air, and tomato, pepper and bean flowers may drop above 90°F. For those reasons, take heavy covers off by early June. Uncover squash, cucumbers, melons and tomato-family crops when female flowers open so bees can pollinate them.

Heat-loving crops are possible with help. UNH’s sweetpotato fact sheet says the crop needs 90 to 120 frost-free days and soil above 65°F at planting. At Durham, soil under black plastic usually reaches 65°F by about the start of June. Cooler sites or gardens without plastic should wait another week or two. Of the varieties UNH evaluated in New Hampshire, Beauregard and Covington were highly recommended.

Fix the acidity first

Acidic soil is the most common soil problem in New Hampshire gardens. UNH’s Fertilizing Vegetable Gardens fact sheet says vegetables do best at pH 6.5 to 6.8, while native New Hampshire soils typically sit around 4.5 to 4.8. Lime raises pH and supplies calcium and magnesium, but it works slowly, so fall is the best time to apply it. Wood ash works faster and can go on in spring. It is also easy to overdo, so don’t add it every year unless a test shows the need. Without a soil test, UNH’s general guideline is 75 pounds of dolomitic limestone per 1,000 square feet every three years.

UNH Cooperative Extension offers soil testing services, and a test tells you exactly how much lime and fertilizer to use. The fact sheet also recommends using only composted manure on food crops. Heavy use of manure builds up phosphorus, which can wash into lakes and ponds.