On any seed packet, you will read instructions about how and when to plant your seeds. They will give information about spacing, watering, and so on. Most instructions will also reference a planting date related to the last frost date in your area.
So, what does last frost date mean? The last frost date in an area is the day in the spring when the last frost of the season occurs. This means that there will be no more frost in that area until the fall or winter. A frost means a freezing temperature of 32 degrees Fahrenheit (0 degrees Celsius) or below.
Of course, the last frost date that you see online or in almanacs is often an average. This means it is likely (but not guaranteed) that there will not be a frost after that date. This is due to variations in weather patterns by year and by location.
Let’s take a closer look at what last frost dates mean, how to find them, and how to use them for planning and planting your garden. That way, you can avoid losing plants to frost before they get a chance to grow!
What Does Last Frost Date Mean?
The last frost date is the day in the spring when the last frost of the season occurs (meaning that there will be no more frost until the fall or winter). A frost means freezing temperatures of 32 degrees Fahrenheit (0 degrees Celsius) or below.
The average last frost date for a location is the day that the last frost occurs on average. For example, if we look back and see that the last frost date in a city was on April 2 in 2018 and on April 4 in 2019, then the average frost date for those two years is April 3 (since the average of 2 and 4 is 3).
If we take the average over many years, we can get a good idea of the average frost date for a location. Looking at all of the last frost dates from past years, we can also estimate a likely last frost date for the current year.
This is helpful when you are planning your garden and trying to decide when to plant seeds or transplant seedlings outdoors. You want to minimize the risk of losing plants to cold and frost, while also extending your growing season if possible.
The last frost dates that you see for a given year mean that there is a 30% chance of frost after that date (or a 70% chance that it is actually the last frost date for that season).
What Is Considered A Frost or Freeze?
According to the Old Farmer’s Almanac, there are a few different types of cold, based on temperatures and how they affect plants:
- A light frost occurs when nighttime temperatures drop to 32 degrees Fahrenheit (0 degrees Celsius). This causes ice crystals to form (you may have seen this as frozen morning dew on grass after a cold night).
- A light freeze has temperatures from 29 to 32 degrees Fahrenheit (-1.7 to 0 degrees Celsius). A light freeze will kill plants that are tender (frost-sensitive).
- A moderate freeze has temperatures from 25 to 28 degrees Fahrenheit (-3.9 to -2.2 degrees Celsius). A moderate freeze will kill most plants.
- A severe freeze has temperatures of 24 degrees Fahrenheit (-4.4 degrees Celsius) and below. A severe freeze will damage or kill almost all plants.
Be sure to watch the weather forecasts carefully and protect your plants if necessary! (More on this later).
What Month Is The Last Frost?
In most of the United States, the last frost date is somewhere between February and June. This is a very wide range, and it can vary from year to year even in a specific area.
Generally, as you go further north, the last frost dates come later in the season. However, there are exceptions.
For example, according to the NOAA, in Boston, Massachusetts, there is a 10% chance of frost after April 21 in any given year. However, if you go 25 miles south to Brockton, Massachusetts, there is a 10% chance of frost after May 16 in any given year!
For this reason, it is a good idea to keep a gardening journal over the years. That way, you can record variations in frost dates for your area, and compare them to what frost date calculators are telling you.
To get started now, you can get a better idea of the last frost date in your area from the Old Farmer’s Almanac website. Just enter your zip code into the frost date calculator.
What Is Considered A Hard Frost?
According to the Old Farmer’s Almanac, a hard frost (or hard freeze) occurs when air temperatures are below 28 degrees Fahrenheit (-2.2 degrees Celsius) for at least four hours straight. This often happens overnight, when temperatures drop and the sun is not out to warm the air and soil.
A hard frost is also called a killing frost because it is often the end of the season for most plants, whether they are annual or perennial. When temperatures fall below 28 degrees Fahrenheit for several hours, the result is that most vegetation dies back.
If you plant seeds or put transplants in the garden too early, then a hard frost will kill them if they are too young or tender (frost-sensitive).
How to Protect Plants from Spring Frost
If you have already started seeds or transplanted seedlings into the garden, it is too late to take them back indoors. So what should you do if a late spring frost or freeze threatens your plants?
One option is to use a cloche for each individual plant. A cloche is a container, usually made of plastic or glass, which covers a plant and protects it from cold and wind.
You can make a cloche from a plastic bottle by cutting out the bottom and placing it over your plant. You can offer extra cold protection to your plants by draping a cloth over the cloches at night.
One cool thing about cloches is that you can take off the cap of the bottle on a hot day to allow ventilation. Then, you can simply replace the bottle cap to offer frost protection at night.
Another option is to use row covers for your plants. This is a good option if you have lots of plants and not much time.
A row cover is a long piece of fabric that protects plants from cold and pests while still allowing some sunlight through so that plants can grow. You can use a cloche on more mature plants that will not fit inside cloches.
Of course, the best way to protect your plants from cold is to keep them inside until danger of frost has passed. Starting seeds indoors and then transplanting established seedlings outdoors later is one way to do this.
However, certain seedlings (such as carrots) do not transplant well, and your growing season may be short. In those cases, you have no choice but to start seeds outdoors, hope for good weather, and get your cloches and row covers ready in case of unseasonably cold weather.
What Temperature Should You Cover Plants?
If the weather forecast calls for temperatures of 32 degrees Fahrenheit (0 degrees Celsius) or below, you should definitely cover your plants. For one thing, freezing temperatures will kill many tender plants, and can end the season early for many others.
Also, a late spring frost can kill unprotected plants, meaning that you will have to plant again. This can even be a season-ender if you lose established seedlings that are several weeks old!
Remember that temperatures can be even colder close to the ground. So, the bottoms of plants can be killed even if the top parts are unaffected.
If temperatures are forecasted to be anywhere close to freezing, take the necessary steps to protect your plants with cloches or row covers.
What To Plant before the Last Frost Date
These plants can be planted a little later in mid spring, about two weeks before the average last frost date:
- Swiss Chard
You can plant beans and corn about one week before the average last frost date. For the rest of the plants in your garden, you may have to wait a little bit longer until temperatures warm up a bit.
What Does First Frost Mean?
The first frost date refers to the first day in the fall when a frost occurs (temperatures below 32 degrees Fahrenheit, or 0 degrees Celsius). As with last frost dates, the first frost date that you see is often an average, indicating a date after which you will likely have seen a frost.
By now, you know what the last frost date means for a given location. You also know how to find the last frost date in your area, and how to apply the information when planning your garden.
I hope you found this article helpful – if so, please share it with someone who can use the information.