Veggie Garden 101: Last Spring Frost Date and When to Plant

Cabbage transplant in April.  Photo: Liz Bosak.

One of the first obstacles for new gardeners is determining when your frost dates are for the spring and fall. They are the boundaries to a typical growing season without using any season extension techniques like row cover, unheated low tunnels, and high tunnels. Pennsylvania has a wide range of USDA hardiness zones ranging from zone 5a to zone 7b. USDA has a website with a plant hardiness by zip code tool, available here. The hardiness zones are based on the extreme minimum temperature recorded each year between 1976 and 2005.
The last spring frost and first fall frost dates are a moving target. Many people have a relative or friend that use either phenology (when certain plants bloom in the spring) or a holiday to guide when they plant certain vegetables. My Grandfather was fond of saying tomatoes were safe to plant by Mother's Day. Seed packets have a variety of instructions on when to direct seed or transplant.
Seed catalogs and packet backs are a good guide for determining when to direct seed or transplant but these usually depend on knowing when your last spring frost date is. There is general confusion about when this is because different plants can tolerate different minimum temperatures. For a comprehensive collection of first frost dates and other important weather information, visit the Pennsylvania State Climatologist's Agricultural Interests page. For central and southern Dauphin county, the average departure in spring from 36 degrees Fahrenheit is 119 to 129 Julian days, putting a conservative last frost date between April 28 and May 8. For the average departure in spring from 32 degrees, the last frost date would be between April 13 and April 22. Confusing? I like to think it is nuanced.
Gardening is both an art and a science. As soon as March hits, I start to closely track the day and night time temperatures. Every season is different. In 2020, the months flip flopped and we saw warmer weather in March than April. For cold hardy crops the instruction "as soon as the ground can be worked" applies and direct seeding and transplanting can occur before the last spring frost date. It is important to remember that seed germination and early plant growth is dependent upon the soil and air temperature. Tomatoes prefer night-time temperatures around 50 degrees Fahrenheit, too much below 50 F and the plants will suffer. A low tunnel with a layer of Agribon 19 row cover and greenhouse plastic provides about 9 degrees of warming and row cover alone about 2 degrees. For tomato transplants into a low tunnel with row cover and plastic, I typically wait until the night time temperatures are at 40 degrees or more.
Any questions? Just type them in the comments section.