How To Determine Last Frost Date

December, 2023
Darren Spalding

Knowing when the last frost date is can have a major impact on your garden. For example, if you plant your vegetables before their appropriate planting time, they may be killed from a hard freeze that comes during what scientists call "frost danger season."

Frost danger season is a term used by scientists to define the average number of days between the last frost in spring and the first frost in fall.

The term "last frost" is important because it's the first day that all leaves on trees, shrubs, well-established perennial plants and other aboveground foliage have been killed due to freezing temperatures.

When is the last frost date?

When considering frost dates, it is important to note that the last frost date varies from place to place. This is because they are calculated based on historical meteorological reports, which differ by region. Frost data can be tracked as far back as a century through reports that are no longer current. The most recent frost date is the latest recorded version of light or hard frost 90% of the time.

Putting out plants isn't an absolute either. There's always a chance of frost even after the date which is predicted for the last frost to hit your area.

What this means is that while the last frost date can be a good indicator of when it is safe to put out plants.

The effects of frost on your garden

In many ways, the weather impacts how your plants will do. One of the most obvious is when frost hits tender plants overnight - it can kill them right away. The weather in your area impacts the health of the plants you grow. This article examines the ways that frost forms, where it mainly occurs and steps to lessen its effects.

Understanding the different effects of outdoor weather can be difficult. The last frost date refers to when you can plant annuals, in contrast with the lowest average temperature which is more important for hardy perennials. The kind of plants that you can garden with depends on the lowest average temperature in your area. For example, if the last frost in your garden is late spring, then you will be able to plant out tomato plants a week or so later.

If the garden has the lowest average temperature of -15°C / 32°F, then some shrubs will be killed by this low temperature. Although these shrubs are "frost hardy", they are not fully hardy. Some shrubs are only partially frost-hardy.

Be aware that the last frost date is different for each location, and it can vary depending on factors like your area's terrain and nearby bodies of water. There are six months in the year when hail can occur (May, June, July, September and October). Each town listed above has an accurate frost date the majority of the time. Exceptionally late frosts sometimes catch even experts off-guard. Although the UK has a national last frost date of September 15th, different regions within the country have significantly different last frost dates.

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