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Freeze Data: Getting & Using

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What Is “Freeze”?

The freezing point of water is 32° Fahrenheit (or zero Centigrade): so much everyone knows. But the definition of a “freeze” for gardening purposes is not so simple; worse by far are the utterly (and literally) vital questions of what constitutes a “first-frost date” and a “last-frost date”.

As all informed sources agree, there is no precise definition of a “killing frost” because different plants react to cold in different ways. One can find a lot of somewhat similar yet differing interpretations of “frost”, “freeze”, and “killing freeze” in the literature. Here is one set:

Frost: 32°F. to 36°F. – little damage to most vegetation.
Light freeze: 29°F. to 32°F. – frost-tender plants are killed, but there is little damage to other vegetation.
Hard freeze: 25°F to 28°F – widely destructive to most vegetation.
Killing freeze: 24 ° F. or below– wide destructive effects on most vegetation, with heavy damage to fruit blossoms and frost-tender and semi-hardy plants.

There is an expanded explanation of all that available at the Garden City Harvest site.

You can obtain data specific to your general location from the U.S. government’s NOAA web site, as detailed a little farther below.

For our gardening purposes, we really need just two classes of information: which days are literally “frost-free”, meaning with air temparatures over 32°, and which “light-frost”, meaning with air temparatures from 29° to 32°. Generally, plants referred to as “frost-tender” need truly frost-free days, while most classed as “frost-hardy” can take light frosts, even repeated days (so long as they are not many continuous days). We say “most” because some of the really hardy ones can withstand deeper freezes than what is meant by “light”; but all that is best discussed in detail on the individual vegetable pages here.

Getting the Data

The best way is to start keeping your own records of daily high and low temperatures (and, if you like, of daily rainfall and snowfall). You need a decent “min-max” thermometer, ideally placed where it will always be in the shade (else take the daily high before direct sun hits it). You can't really rely much on those data till you have at least two or three year’s worth of them, but start now: the journey of a thousand miles begins with but a single step.

(We don’t emphasize rainfall much because we presume every serious gardener uses drip lines or soaker hoses in their beds, or at least waters by hand.)

If you don’t yet have a usable table for your own garden area, get one from the nearest official weather station. To do that, go to NOAA’s search page; once there, you will find that there are four things you need to enter. Do them like this:

Select Weather Observation Type/Dataset: this is a click-on “drop-down menu”; select Normals Annual/Seasonal.
Select Date Range: this is a click-on “drop-down menu”; you can leave it alone, as it defaults to the latest available data.
Search For: this is a click-on “drop-down menu”; select ZIP Codes.
Enter A Search Term: this is a free-form entry field; enter your ZIP Code.

When you have that done, click the Search button. That will take you to a page showing your ZIP Code area; at the left, click where it says View Full Details. That will take you to a page from which you can select what records you want to see for your local weather station. Have fun.

Using the Data

You can see our own 21-year complete, day-by-day historical weather data if you like, but that’s a 365-line table that can be tedious to comb through. So, from those data, here are the high points concerning freezes.

(The point here is not that you need to know our particular frost/freeze situation; rather, it is that the reckonings below demonstrate how to figure your own frost/freeze situation from historical records—your own, from your garden, or those from the nearest weather station.)

Average Spring Last-Freeze Dates:
- Hard Freeze: April 17th
- Light Freeze: May 9th
Average Fall First-Freeze Dates:
- Light Freeze: October 5th
- Hard Freeze: October 12th

Extreme Spring Last-Freeze Dates:
- Hard Freeze: July 4th
- Light Frost: July 9th
Extreme Fall First-Freeze Dates:
- Light Frost: August 16th
- Hard Freeze: August 29th

(In the listing above, we have used 29° to 31° to define “light frost” because we can get daily-low temperatures of 32° throughout the year—yes, even mid-July and August.)

So our average annual completely freeze-free days total 149, and our days with no frost or only light frost total 176. That, though, is based on average dates for the start and end of frost/freeze periods.

The extreme dates usually do not signify an absurdly cold year: most or all reflect some bizarre unseasonal cold snap of a day or two (a lot of people hereabouts lost whole gardens in 2012 to a July 4th overnight low of 28 degrees). If such a snap occurs after you have done most or all of your planting/transplanting, there’s not an awful lot you can do about it. In general, keep a close daily eye on weather forecasts, and if such a snap is foreseen, do what you can to protect your garden: mulch with a deep layer of straw, in the evening set plastic soda bottles full of hot water among the plants, put some row cover over everything, and whatever else occurs to you. And hope.

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