When to plant by counting back from the last frost date

For the area I live in:

Each winter, on average, the risk of frost is from September 24 through May 21.

Almost certainly, however, you will receive frost from October 8 through April 23.

You are almost guaranteed that you will not get frost from June 18 through September 11.

Your frost-free growing season is around 126 days. Zone 5b-6a

I am planting Yellow Pear Tomato, a heirloom variety grown since the late 1880's. Suggested to start indoors 6-8 weeks prior to last frost date and to plant outdoors after danger of frost has past. Grow in full sun (will do well in part shade.) Approximately 75 days to maturity. It is considered a vigorous vine type that is heat resistant and cold tolerant. It's growing habit is Indeterminate and needs support. Taking into consideration that the seeds may take 10 to 20 days to germinate and they won't be planted until June 1st, they should be started by about April 10th. To be honest, I usually start my tomatoes in March, but I am so behind this year that they didn't get started until the 19th of April. Still should be plenty of time to get some delicious little tomatoes.

Most seed packets will list the approximate maturity date for each plant. By counting backwards from the last frost date in your area, you can decide if you still have time to plant those bonus vegetables. For instance, if you would like to try growing bush beans, which take about 50-55 days to produce, count back 55 days from your first expected frost date. In our area, (according to farmer’s almanac) the first frost could be approximately October 20. Counting backwards would mean you should get those seeds in the ground well before August 26 to reap a harvest. Keep in mind that the timing stated on seed packets usually starts after the seeds have germinated, which can take a few days to a week. Most seeds will germinate more quickly by soaking them overnight, and you could buy yourself a couple of days.

Fortunately, some cool season crops such as spinach, kale, and lettuce can handle a light frost. You can count back to time these crops to mature before the first frost, but since they are “cut and come again” and will regrow once cut, you will be able to enjoy these vegetables into late fall and early winter. Kale is said to taste sweeter after a light frost. Reap a late harvest this season. Look for seeds with fast-growing rates such as lettuce, beets, and radishes

Hardiness Zone versus Last & First Frost dates

 

How to use your hardiness zone.

Some plants will adapt to a variety of conditions, including soil type, watering fluctuations, and humidity differences, but most plants will only tolerate a specific degree of chill in the winter. That’s where hardiness zones come in—they list the average lowest temp in every area of the country, so gardeners can select perennial plants that will survive the type of winters common to specific regions. 

Use a hardiness map to find the average lowest winter temperature in your community and then choose plants that will survive that temp. To simplify the process, most commercial greenhouses provide detailed plant labels that list suitable hardiness zones, the lowest temperatures the plants can survive, and other vital information, such as whether they will grow better in the sun or shade.

Use a zone map as one factor in choosing a plant. However, don’t forget to check off all the boxes on its other needs.

• Maximum temperature
• Precipitation
• Average frost dates

What are Frost Dates?

A frost date is the average date of the last light freeze in spring or the first light freeze in fall.

The classification of freeze temperatures is based on their effect on plants:

• Light freeze: 29° to 32°F (-1.7° to 0°C)—tender plants are killed.
• Moderate freeze: 25° to 28°F (-3.9° to -2.2°C)—widely destructive to most vegetation.
• Severe freeze: 24°F (-4.4°C) and colder—heavy damage to most garden plants.

Note that frost dates are only an estimate based on historical climate data and are not set in stone. The probability of a frost occurring after the spring frost date or before the fall frost date is 30%, which means that there is still a chance of frost occurring before or after the given dates!

Frost is predicted when air temperatures reach 32°F (0°C), but because it is colder closer to the ground, a frost may occur even when air temperatures are just above freezing. Always keep an eye on your local weather forecast and plan to protect tender plants accordingly. Weather, topography, and microclimates may also cause considerable variations in the occurrence of frost in your garden.

Last and first frost dates are 30% probability. Calculated using 1991-2020 Climate Normals from NOAA (National Oceanic and Atmospheric Administration).

On the reverse side of most seed packets will be a map of the suggested outdoor sowing months for areas, sowing instructions and information on the variety such as how deep to plant, how many days to harvest, light requirements (full sun, part shade, etc.) and plant size and how long till the seeds emerge (7-10 days from sowing). 

With your own knowledge and the information above you can decide what plants/seeds to start indoors and when to start plants/seeds outdoors. 

Happy Gardening!