Using first and last frost dates to plan your vegetable garden

When I first got serious about vegetable gardening, I heard a lot of talk about first and last frost dates. However, it wasn’t until I lost a crop of tomatoes and cucumbers to frost that I took an interest. Frost dates are calculated by the National Centers for Environmental Information using climate data from 1981-2010. Frost dates are estimates using these climate data. As a result, your plans must be adjusted based upon actual weather forecasts observed in your area.

Why are frost dates important and what are the benefits you might ask? Well, frost dates provide guidance for when to plant in a geographic area. So, two of the biggest benefits are garden planning and risk mitigation. Using frost dates, you can plan when to start seeds and plant warm-season crops in the spring and when to start seeds and plant cool-season crops in the fall. Thus, you reduce the risk of damage or loss to crops. What vegetable gardener doesn’t want that!

Let’s take a closer look at using first and last frost dates.

First, to understand the dates for your location, launch this link from the National Gardening Association and enter your zip code. Beware, there are a few exceptions to frost date calculations if you live in one of the following areas:

• Too cold to compute: Most of Alaska falls into this category
• To warm to compute: Puerto Rico and most of Hawaii fall into this category

Last Frost Date in Spring (USDA Gardening Zone 6b example)

Frost dates are displayed via charts and are estimates based upon probabilities. Therefore, multiple dates are given based upon the probability of frost occurring on various dates. Let’s look at the chart below. There is a 50% probability frost (32°F) will occur on May 5. However, there is a 90% probability that frost will occur on April 20. Gardeners can use this information to set their planting schedule. This is especially important for warm-season crops like tomatoes, peppers, cucumbers and eggplant. If a warm-season crop is planted and frost hits, then damage will likely occur to these vegetables. When the frost is long enough to delay the start of the gardening season, it’s called a ‘killing frost’.

Temperature	10%	20%	30%	40%	50%	60%	70%	80%	90%
32°F	May 20	May 15	May 11	May 8	May 5	May 2	Apr 29	Apr 25	Apr 20

Now, let’s examine how a gardener can use these dates and which date should be used. I believe it is important to be conservative. Thus, I use May 20 as the date to plant warm-season vegetables in my gardening zone 6b. Waiting until this date gives the ground time to warm up and there is a relatively low probability (10%) that frost will occur on this date (risk mitigation). However, if the local weather forecast indicates frost on or after this date, it is important to be prepared and use a tool such as a floating row cover to protect crops. To mitigate risk even further, add two weeks to the 10% probability date.

Last Frost Date and Seed Starting in Spring

Using the May 20 date also provides guidance on when seeds should be started indoors. Seeds need time to germinate, and the resulting plants need to be hardened off before planting in the ground. For a warm-season crop like cucumbers, I start the seeds one month in advance of the frost date or by April 20. I then monitor the weather situation. If it is still too cold to plant on May 20, I wait for an additional 1-2 weeks. Prior to using frost dates, I started seeds much too early and then had seedlings that were outgrowing their containers.

Cool-season crops can generally be planted as soon as the soil can be worked in spring. The chart does not provide specific guidance on when to plant cool-season crops such as broccoli, cabbage, lettuce and spinach.

First Frost Date in Fall (USDA Gardening Zone 6b example)

While it is important to understand the last frost date for spring planting, it is equally important to understand when the first frost date will occur in the fall. Gardening in the fall is a great way to extend the season. While cabbage, lettuce and spinach are cool-season spring crops, we can get a second harvest of these vegetables in the fall with good planning.

Temperature	10%	20%	30%	40%	50%	60%	70%	80%	90%
32°F	Sept 22	Sept 27	Oct 1	Oct 4	Oct 7	Oct 10	Oct 13	Oct 16	Oct 21

The chart above can be read in the following manner. There is a 10% probability that the first frost will occur by September 22. There is a 90% probability that the first frost will occur by October 21.

Let’s look at an example of how the first frost date can be used. Seed packages indicate how long it takes from germination to harvest. For cabbage (cool-season crop), the package below indicates 87 days to harvest. If I use the most conservative date from the table above for planning purposes (Sept 22), I would need to plant these seeds by July 2 (87 days earlier) to ensure a harvest before the first frost date of Sept 22. Again, to further extend the season beyond Sept 22, row cover can be placed over the cabbage. To mitigate risk further, subtract two weeks from the 10% probability date.

Additional Helpful Information

The last piece of interesting information from examining frost dates is season length. For example, in garden zone 6b, the season length is ~125 days if we use the most conservative first/last frost dates (May 20 thru Sept 22) from the tables above. That’s a relatively short season.

I hope a better understanding of first and last frost dates helps you plan your garden, mitigate your risk and have a great harvest! For more information on garden planning and seed starting, please reference the following posts:

• How to create a simple crop management plan
• The complete guide to starting seeds indoors

References

University of Maryland Extension: Spring Frost/Freeze Dates in Maryland