The right way to Discover Your Final Frost Date

If you’re confused about your last frost date and how to find it, you are not alone. This simple tool was designed to make garden planning easier, but it can initially seem confusing. 

A spring frost date is the average last day you’ll experience freezing weather in your region. It is calculated based on historical weather data. Knowing your estimated date helps you anticipate when to start sowing seeds and preparing plants for transplanting.

Let’s dig into the most straightforward way to find this important date and use it to plan a successful garden.

How Accurate Are Frost Dates?

Use a data-based calculator to guide your spring seed-starting.

While last frost dates are fairly accurate historically, there is still a 10% chance that a light frost can occur after the spring date. The dates are calculated using averages of national weather data from the NOAA National Centers for Environmental Information. There can be large temperature fluctuations between different microclimates and weather years, but overall, this date is the most accurate way to plan your garden. Experiencing a hard-killing frost after your last estimated date is fairly rare.

If you are concerned about erratic spring weather, you can always move your planting forward another week or use cold protective strategies like cold frames, low tunnels, row covers, and mulch.

Alternatively, you could try winter sowing. This method uses DIY mini greenhouses that allow you to seed hardy plants outdoors in winter, no matter the weather. You will, however, still have to be aware of your frost dates to ensure you transplant your winter-sown seedlings at the right time in spring.

6 Steps to Find and Use Your Last Frost Date 

Gardening includes many tools; these dates are yet another tool to keep in your figurative tool shed. Finding your date is very simple, but we also want to help you use it to plan a successful garden. Here are six easy steps to determine the important dates for your region and use your calendar to plan your seeding and transplanting.

1. Access the National Gardening Association Calculator

Input your zip code or city location in the calculator to get your first and last frost dates.

Your last frost date is the approximate day when your area will stop experiencing temperatures below 32°F in the spring. To find it, you only need to type in your zip code in this Frost Date Calculator from the National Gardening Association. The calculator will create a simple chart with average dates of freezing and sub-freezing temperatures in the spring. 

The lowest left date on the chart is the absolute safest time for planting warm-weather crops with 90% certainty that your garden won’t experience any more freezing weather. You can use this date to count backward or forward on your calendar based on different seed requirements. 

For example, if the date given is April 14 and your ‘Dazzling Blue’ kale seed packet instructs to start seeds indoors 4 to 6 weeks before your average last frost date, then you would sow kale seeds in cell trays between March 3 and March 17. This timing ensures that the plants will be healthy and ready for transplanting by the time the weather has warmed.

2. Examine Your Date Chart

Numerous online date calculators exist, offering varying degrees of detail.

The tool will generate a date chart for spring and fall. While a frost is technically considered 32°F, this calculator explains how temperatures as low as 36°F can still damage certain plants. It provides average probabilities of different temperature fluctuations based on past seasons. The bottom left dates on the chart are the most reliable “safe zones” for planting tender crops.

If you search “frost date calculator,” you will find there are many other calculators available online, and some will just tell you a single date without a chart. A single date is helpful for beginners seeking the simplest garden plan.

However, we like this tool because it provides an in-depth chart with probabilities for different weather scenarios. No matter what calculator you choose, almost all of them use the same government-collected weather data from the NOAA National Centers for Environmental Information. 

Example

For example, a standard calculator may tell you that your estimated last frost is on June 7. But the National Gardening Association’s calculator will generate a chart that tells you the probability of experiencing a 28°F low around May 14 versus a 36°F low on June 14.

If your area historically experiences temperatures below 36°F about 50% of the time in late May, you can get a more accurate idea of when you reach the “safe zone” for planting tender crops. Personally, I would feel fine seeding kale or lettuce in mid-May for this example, but I would wait until the third week of June to plant out melons or cucumbers.

3. Document Dates from the Lower Left Corner

Look to the lower left corner of the chart for your official spring dates.

To use your date chart in the simplest way, take the lower left dates on the chart as your official last spring frost dates. Find where the left column indicates the Last 32° and the top column lists 10%.

This day marks when there is a 90% chance you won’t experience any more temperatures below freezing (32°F) for the duration of the growing season until the estimated first fall frost date. The date below this (next to Last 36°) indicates the ultimate “safe date” for tender crops. You are highly unlikely to experience any frosts after that date.

Here are a few examples of last frost dates (last 32°F day with 90% probability) from around the U.S.:

• San Diego, CA: January 22
• Phoenix, AZ: March 7
• Frisco, TX: April 8
• Portland, OR: April 22
• Nashville, TN: April 28
• Denver, CO: May 13
• Amherst, MA: May 24

4. Mark Your Date on the Calendar

Starting a garden calendar is highly recommended for both beginners and experienced gardeners.

Whether you’re a beginner or an experienced gardener, I highly recommend starting a garden journal ASAP. It’s helpful to mark dates here and track local temperatures as you document your planting. 

If you have a super successful crop of spring carrots this season, you’ll want to check back next year and see exactly when you planted them. You may think you’ll remember, but getting things down with pen and paper makes it much easier to improve next spring.

5. Determine the “Safe Zone” for Tender Crops

Protective measures like cold frames or row covers can mitigate potential cold damage.

Nobody can predict the weather, but we can determine the safest windows of time to plant different species based on their needs. Cool-weather crops like brassicas, kale, lettuce, onions, and many perennial plants are fairly resilient in early spring weather. However, tender crops can be finicky about exposure to cold, especially if those plants are native to tropical regions.

The “danger zone” is when temperatures are technically above freezing but can still cause damage to tender plants. For most warm-weather garden crops, consider the “danger zone” between 35 and 45°F. You can technically plant tender crops outside on your last frost date because nighttime temperatures are no longer freezing, but there is more risk. Random winds, rains, or microclimate fluctuations in different areas of your yard could still pose issues. A cold frame or row cover can help raise the temperature a few degrees to buffer against cold damage.

For example, planting out a tender plant like a celery seedling or a young petunia on your last frost date may still cause damage to the plant. For many species, exposure to temperatures under 40 or 50°F increases the risk of premature bolting (for example, celery), cold-damaged leaves (petunias), or loss of early flowers (tomatoes).

The “safe zone” is the most predictable time period where you won’t experience any more frigid weather. You’ve entered the “safe zone” of spring about 1 to 2 weeks after your last frost date when nighttime temperatures are reliably above 40°F. This is important when planting crops like peppers, melons, cucumbers, and squash because they suffer if exposed to temperatures below 40°F.

6. Determine Seeding and Transplant Dates for Your Crops

Use a simple chart to determine seeding and transplanting dates.

Once you’ve determined the “danger” or “safe” zone planting windows, you can easily calculate what days are best for seeding and planting. The three most important dates are:

• Last Frost Date (you’ve already got that!)
• Transplanting Date: When you will move indoor seedlings outside
• Seeding Date: When you will start seeds indoors or direct sow in the garden

Don’t worry. No complicated math is needed. You can either count weeks on a desk calendar or simply type into your web search engine “X days before (date)” to find specific dates. Use the instructions in your grow guide or your seed packet to get the numbers you need. 

For example, if your last frost date is April 14 and you want to start ‘Glacier Bush’ tomato seeds indoors, the seed packet instructs you to start the seeds indoors 4 to 6 weeks before transplanting. It recommends transplanting when air temperatures are above 45°F, about 1 to 2 weeks after the average last frost. This information can be sorted into three dates:

• Last Frost Date: April 14
• Transplanting Date: 2 weeks after April 14 is April 28
• Seeding Date: 6 weeks before April 28 is March 17

So you can start your tomato seeds in cell trays around March 17, and you should have perfectly sized seedlings to transplant in the garden around April 28 when the nights are sufficiently warm! This process gets really simple when you create a chart like this one to track all your planting dates:

Final Thoughts

Finding your last frost date is as simple as typing your zip code into a calculator. If using the National Gardening Association tool, the most reliable “safe zone” date will be in the bottom left-hand corner of the chart. Then, you can use this information to determine when the best time is to seed your crops. Check the recommended seeding window on your seed packet and count backward from the last frost date.

When in doubt, just wait a little longer! If it seems like an extra cold spring, it’s better to wait another week for warm weather than to risk losing all your hard work to an unexpected cold night!