Last Frost – What Does That Mean?

When planning your vegetable garden and other gardening tasks – you need to look at “Last Frost” date for your area. And as Autumn comes along you will need to start looking at “First Frost” data – but that is another article.

Cover image is frost on a sage plant.

Which Garden Tasks go by Last Frost Date?

Last frost date tells you when you can safely put your tender young seedlings into the ground. You can’t just walk away though – gotta cover them in case of cold snap.  Note that these dates are only average.

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Vegetable seeds. You need to wait until the soil is warm enough for the species. Last frost is only a guide. Sunflowers don’t mind cool soils but Malabar spinach does. I traditionally plant my sunflower seeds on Last Frost day. It’s a kind of celebration.

Prune Frost Damage. Last frost date means you can usually (I stress usually) prune any frost damage off plants like yellow bells that may have been nipped by the cold.

Other Pruning. Last frost date can also help with other pruning. Count back 8 to 10 weeks from the last frost date and that’s generally the ideal time to prune rose bushes, and winter dormant fruit trees like apples, peaches, and plums. In Low Desert – you can’t do that because there isn’t 8 weeks in some of our region!  So just pick the middle of the cold period to prune.

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Succulents. If you have cold-tender succulents like the desert rose (Adenium) or elephants food (Portulacaria), you need to wait longer past this last frost date because they can be get damaged by cold below 45 degrees.

Do you find this useful?

The article continues but first a quick note.

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How Can You Predict the Future?

We can’t. All we can do is look at the past history (up to 200 years of it in some areas) and give an average.  Now despite all these years of historic data, farmers only use the last 30 years of data when calculating the last frost date – because the world has been getting warmer (for whatever reason).  Best to use the average of the most relevant data.

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Compiling Last Frost Data

Your donation dollars plus tax dollars at work! The numbers in the chart below are courtesy of the National Climatic Data Center (NCDC), National Oceanic and Atmospheric Administration (NOAA).  You can also find this plus your local area chill hours (critical for selecting fruit trees) and rainfall data on the United States Geologic Survey (USGS site) – but you gotta dig.

Best site for gardening frost data – AND in an easy to use format (just enter your zipcode) – the nonprofit National Gardening Bureau (NGB) has compiled them into a very useful frost database you can use to look up your area.  Feel free to join this lovely non-profit, and don’t forget to tell ’em I sent you.

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So When is Last Frost?

The dates listed are normal averages for a light freeze (keep reading for definition). The possibility of a freeze occurring after the spring dates and before the fall dates is 50 percent. That’s a fifty/fifty chance folks.  Are you the betting sort?

These freeze/frost designations are categorized by their effect on plants that we typically grow in out gardens.

Light freeze – 32 F down to 29 F
Tender plants killed. Many tropicals damaged. Citrus ripening may get damaged inside the skin. Little destructive effect on temperate plants.

Moderate freeze – 28 F down to 24 F
Widely destructive effect on most leafy vegetation, with heavy damage to fruit blossoms. Many plants native to the Low Desert may freeze to the ground. Baby saguaros and other cactus seedlings killed unless sheltered under nurse plants.

Severe freeze – below 24 F
Many Middle Desert plants damaged. Some will die entirely. Much depends on the following day and how soon it warms up, and or gets sunny. Some Middle Desert native plants can stand these temperatures if it warms up early in the morning. Upper Elevation native plants are entirely genetically used to this.  Think pine trees in ski areas.

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Frost Data Charts:

These are based include information based on where tthe trackers of Google say you are located.  If I missed a place where you live, please let me know.

arizona-frost-chart-desert-soule

california-frost-chart-desert-soule

colorado-frost-chart-desert-soule

newmexico-frost-chart-desert-soule

nevada-frost-chart-desert-soule

texas-frost-chart-desert-soule

utah-frost-chart-desert-soule

Thanks for Reading

Jacqueline

 

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