While most perennials aren’t affected by freeze and frost dates, when planting annuals and vegetables, these are dates you MUST know.
Most vegetables, such as tomatoes, cucumbers, and pumpkins won’t tolerate even a light frost.
Annual flowers like petunias, nasturtiums and morning glories won’t fare well either. If you plant them too early in the season, they will likely be stunted or even die.
Moreover, these crops are usually the hardest hit when the first frost sets-in come fall.
What Are Frost Dates?
These refer to the average dates on which the first frost or last freeze occurs either in spring or fall. The actual days differ from place to place as subtle environmental differences can cause huge variations.
The National Centers for Environmental Information uses historical weather data to calculate frost dates.
The classification of frost temperatures based on their effects on vegetation is as follows:
- Light freeze: 29 to 32 degrees Fahrenheit – kills tender plants.
- Moderate freeze: 25 to 28 degrees Fahrenheit – destroys most vegetation.
- Severe freeze: 24 degrees Fahrenheit or colder – causes substantial damage to most plants.
However, leafy vegetables like peas, broccoli, and leeks can tolerate moderate freezing temperatures. This allows you to plant them before the final expected freeze. It also means that they can grow for longer in the fall.
How to Determine Your First and Last Freeze Dates
At the start of spring, we may be tempted by the warmer days to rush and start planting in our gardens once more. But don’t let a few sunny days in early spring fool you into breaking out your gardening tools just yet.
If you send your newly germinated seedlings outside too soon, they could wither overnight from exposure to near-freezing temperatures.
This is why you are better off waiting until after the final freeze to add new crops to your garden.
But how exactly do you determine whether the final freeze has passed?
Well, there are a few sources you can turn to. One option is to consult your local nursery. Most nursery workers know these dates for the area you live in.
You can also ask that green thumb neighbor, especially one that’s lived in the area for years. But, your best bet option is to seek professional help.
How to Determine Your Frost Dates by Zip Code
Most people believe that working out the first and last frost date by postal code gives the most accurate results. This stems from the fact that subtle nuances in your area can cause dramatic variations.
The go-to websites to help determine your freeze dates by zip code are:
You can also sign up using your email address to get regular updates about the weather conditions in your area.
However, remember that these dates are average approximations. They don’t take into account any microclimates in the area. And nor do they account for the effects of climate change in the area.
Microclimates are pockets that are warmer, colder, or windier than the surrounding area. They can either present planting opportunities or limit what you can plant and when.
Some of the factors that aid the formation of a microclimate include:
- Windbreaks and other interruptions to the breeze like buildings, fences, and hedges.
- Changes in elevation. Cold air will roll down a slope and settle in lower spots. Thus vegetation on the side of the hill experiences warmer temperatures than those at the bottom.
- Concrete structures absorb the sun’s heat during the day and radiate it at night. Eaves and overhangs also tend to be warmer at night as they retain heat underneath.
- Areas of the yard with southern and western exposures are warmer than those with eastern and northern exposures.
- Water bodies like streams, lakes, ponds, and pools provide cooling in hot weather and warming in cold weather.
Pay close attention to areas with any of these features. Make notes from year to year about when to sow, based on your observations.
A great resource to work around this challenge is Mother Earth News Garden Planner.
This website allows you to input your zip code and it will track your frost-free dates. It also sends you alerts through your email address for when to plant.
You can even manually adjust the dates to accommodate microclimates in your area.
The “Garden Planner” feature simplifies laying out your garden as well. It offers layout ideas, automatically spacing out your vegetables using the appropriate measurements. This way, you end up with a blueprint that shows you exactly how many to plant, when, and where.
How to Work Out Frost Dates Using Plant Hardiness Zone
Use the Plant Hardiness Zone Map (PHZM) to determine your appropriate zone. This will help you pick out what crops to grow in your area based on what will survive in that temperature range.
The PHZM provides useful information on when to sow your seeds as well as how late in the season you can plant. It uses GIS-based data to give you specific information on your precise location.
This new GIS-based weather tracking is a great leap forward from the old model. The old model merely showed the average maximum extreme temperatures for large areas.
But it did not take into account variations due to microclimates. As a result, many dates were inaccurate
How to Determine Your Last Frost Dates Using a Chart
Although the PHZM updated its zone maps, the charts still don’t show the new zone subsets. Thus, rendering some of the charts redundant.
Instead, you should visit the Nations Oceanic and Atmospheric Administration (NOAA) website. Here you can search for weather information from research stations within your state.
Once you’ve found one near you, you’ll see that each station offers data for either 28, 32, or 36 degrees Fahrenheit. For most crops, the one you will want to pay attention to is 32 degrees.
The above image shows data pulled from Colorabdo’s NOAA’s Frost Occurrence Data. You’ll notice three dates listed under spring and fall by probability (10, 50, and 90%).
If you want to play it safe, use the 10% probability date. This means there’s only a 10 percent chance that the first fall frost or last spring frost will be later than that date.
But, if you are willing to cover your vegetables in the event of sub-freezing temperatures, you can use the 50 or 90% dates.
How to Grow Your Garden Beyond Frost Dates
Let’s say you know your last spring frost is on May 30, and your first fall frost is on September 15. That doesn’t leave a large window for growing long-season crops like tomatoes.
Thankfully, there are several things you can do to extend the season. These include:
Start Seeds Indoors
You can extend your growing season by up to three-months by starting your seeds indoors.
Often, when you start your seedlings indoors, they take off once transplanted. Unlike store-bought seedlings which sometimes weaken and even die.
Plus, it’s easier on the wallet!
Set up your nursery about three months before the expected last freeze in your area. If it’s your first time starting seeds indoors, it’s best to start small. Pick one or two plants that are easy to start from seeds.
You can check the seed package for the number of weeks you should start before the final expected freeze. Usually, it takes about six to eight weeks.
Seed flats are an excellent option for planting indoors. You can also dig through your recycling bin for egg cartons, yogurt containers, and plastic cups. These will work just as well but don’t forget to poke a drainage hole in each container before planting.
Transplant your seedlings into slightly larger containers when they are 3-4 inches tall. And again when they hit 6-8 inches.
Come spring, when your garden soil has warmed up enough, you will have sturdy seedlings with strong roots.
The following table shows the germination and growing temperatures for popular vegetables:
Build a Cold Frame
A cold frame is a shallow rectangular box with no bottom and a glass, plastic, or fiberglass roof. The sides can be made from planks of wood or straw bales and should slope southwards to capture as much sun as possible.
Fill the cold frame with rich garden soil. It should be suitable for planting your seeds before they’re ready for transplanting.
You can use your cold frame to get a head start on the planting season and even extend the growing season.
When your seedlings are ready for transplanting, leave a few behind in the cold frame. These will mature earlier than the transplanted crop. After the risk of freezing passes, remove the cover and use the cold frame as a raised bed.
Make Use of Plastic Mulch
Covering the cold soil with plastic mulch can boost soil temperatures by as much as 10 degrees.
Simply lay down plastic strips of between 4-6 feet wide and as long as necessary over the soil. Bury the edges to secure them against the wind.
Plastic mulch has four advantages that make it desirable as a commercial much. Installing a plastic mulch will:
- Retain moisture.
- Keep the soil warm.
- Prevent the growth of weeds.
Additionally, you can choose to either retain the plastic mulch all-season or remove it before planting.
Keep in mind, however, that using plastic mulch on a small scale can be a little cumbersome.
Block Spring and Fall Frosts Using Floating Covers
In most situations, one night with temperatures of 32 degrees Fahrenheit will destroy all but the hardiest of plants.
Although blankets and cardboard boxes provide reasonable emergency solutions, floating covers are much more effective.
Floating cover fabrics come in a variety of thicknesses. Some of which can protect your plants to temperatures as low as 25 degrees Fahrenheit!
These agricultural fabrics allow water and sunlight through while keeping the plants warm.
Additionally, floating covers are simple to use and highly flexible. They are easy to erect and move to cover specific crops as necessary.
You can also pair floating covers with other techniques like plastic mulching for even more protection.
Gardening in near-freezing temps can be a challenge, even for experienced gardeners. But with the right tools and information, you can maintain a healthy crop and boost your yields. No matter the season.