Gardening in Pennsylvania: zones, frost dates and when to plant

Pennsylvania’s growing regions

Pennsylvania’s landscape divides it into several gardening regions. NOAA’s state climate summary describes the Appalachian Mountains cutting across the middle of the state. To the east lie the Piedmont and the Coastal Plain, and to the west the Allegheny Plateau and the Lake Erie lowlands. The climate shifts across these regions:

• The southeast, around Philadelphia and the lower Delaware Valley, sits nearest the Atlantic, which moderates coastal areas. NOAA singles out Philadelphia and other cities for extreme heat, where the urban heat island and high humidity raise summer heat index values.
• The central mountains separate the eastern lowlands from the plateau. NOAA notes that the area east of the Appalachians is the part of the state exposed to nor’easters.
• The northwest, along Lake Erie, is moderated by the lake. It also gets heavy lake-effect snow, sometimes more than 100 inches in a winter.
• Statewide, westerly winds bring air from the continent’s interior for much of the year, along with occasional bitter cold in winter.

The frost dates for your city above are the best guide to where your garden falls.

What limits Pennsylvania gardens

Rain is the main concern. NOAA’s summary calls Pennsylvania’s precipitation abundant and notes a large rise in extreme downpours. It also projects more winter and spring rain, which raises the risk of spring flooding that can delay planting. Even so, the state has had short-term droughts, including in 2002, 2016–2017 and 2020. Tropical storm remnants occasionally bring heavy rain and widespread flooding, and severe summer thunderstorms can do damage.

The season itself is the other limit. Penn State Extension says Pennsylvania’s growing season is too short to sow some warm-season crops directly in the garden. They are usually started from seed indoors and moved outside once soil and air have warmed. Warm-season crops get one cycle per year, from after the last frost into late summer. When nights turn too cold the season is over even if days are still warm, and cucumbers, for example, turn bitter.

Penn State Extension’s planting advice

Penn State’s Starting Your Summer Vegetable Garden: Seeds or Transplants? gives a sequence for most of the state:

• Peas go in first. They germinate once the soil is above 40°F and stop setting blossoms above 85°F, so sow them 50 to 70 days before hot weather.
• Spinach goes in during April so it can be harvested before it bolts in summer heat. Leaf lettuce can be sown from April in a succession until around August 1.
• Root crops such as beets, carrots, parsnips, radishes, rutabagas and turnips should be direct seeded, usually in April. Transplanting them can give misshapen roots.
• Early broccoli and cauliflower go out in early April, and cabbage transplants from mid-April to late May. Onion sets go in during early to late April.
• Sweet corn can be planted between May 1 and July 1. Later plantings mature faster.
• Tomatoes, peppers and eggplant wait for warm soil and nights around 60°F, which in much of Pennsylvania means late May to early June.
• Cucumbers, melons and squash can be direct seeded after the danger of frost. Transplants started indoors 3 to 4 weeks ahead may give a bigger harvest where the season is short.

Penn State’s article Cool-season vs. Warm-season Vegetables adds that many cool-season crops can be grown twice a year, in early spring and again in fall. Spring plantings must go in early enough to finish before summer heat, when radishes turn fibrous and sharp, and salad greens and spinach bolt. Look for bolt-resistant varieties. Covering beds with black plastic warms the soil faster in spring and can let warm-season crops go in a little earlier, as long as the air is warm enough. For a fuller reference, Penn State publishes Vegetable Gardening, a guide for home gardeners that covers site selection, mulching, irrigation, choosing cultivars and growing individual crops. County Master Gardener garden hotlines also answer questions.

Fall crops and soil testing

Penn State notes that cabbage, broccoli, cauliflower, onions, potatoes, spinach, squash and root vegetables, which are usually set out as transplants in spring, can be direct seeded in summer for a fall harvest. Kale and Brussels sprouts even benefit from a light frost, which turns starches to sugar and improves their flavor.

Before you fertilize, get a soil test. Penn State Extension county offices sell standard kits. You can also print a submission form from the Penn State Agricultural Analytical Services Lab and mail the sample yourself. For a vegetable garden, take cores 6 to 8 inches deep from 8 to 10 random spots and mix them in a plastic bucket. The report covers pH and lime needs along with fertilizer recommendations. Penn State suggests retesting sandy soils every 2 to 3 years and clay soils every 3 to 4 years, or sooner if problems show up.