The “last frost date” is the date that, over the years, is the latest you will have frost in your area. I found a site that has pretty comprehensive data. You are bound to find your frost date here. 

I also created an Excel worksheet that helps you calculate your frost date (in Utah) and or calculate which SFG week you are currently in. Click here to download the Frost Date Worksheet. Click on the EASY tab. If you live in Utah, use the Location drop down to find the city closest to you. Keep in mind, you want the closest “weather-wise,” not necessarily closest distance.  Or, if you have found your frost date, use the Frost Date drop down menu.

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