Tips & Tools

How Do Last Frost Dates Work?

writerlauramckay

Even more than your Plant Hardiness Zone, your local last frost date is a critical piece of information you need to consider when planning and planting your garden. Vegetables and flowers are both impacted by this date, with some plants being more sensitive to low temperatures than others.

So what is a last frost date, how do you figure out yours, and how do you plant around it? Keep reading to find out!

What Is a Last Frost Date?

As you may have already guessed, a last frost date refers to the last time frost, or temperatures below 0°C, is expected in a particular region in the spring. Last frost dates is an average calculated with several decades’ worth of climate data, and depending on the resource you are looking at, it may be a single date or it may be a range of dates.

Why Do I Need to Know My Last Frost Date?

Some plants do not tolerate frost very well—if exposed to temperatures at or below 0°C, they may suffer stunted growth or damage, or even die. These plants include species that love heat, like tomatoes and peppers, but also hardier plants like onions.

To give your new seedlings the best chance of success, you want to avoid planting them in your garden until the risk of frost has passed unless you have other methods of keeping them from freezing. You will also need to know this date if you are starting your own seeds—most seed packets will tell you when to start the seeds indoors using the number of weeks before your last frost date.

It is worth noting that some plants can tolerate a small amount of frost, but we won’t be delving into that in this article—that’s more something to try after you’ve gotten a bit more experience with gardening and seed starting!

How Do I Find My Last Frost Date?

The easiest method for finding the Last Frost Date for your area is to simply Google the town or city you live in (or if you live in a rural area like me, a larger town nearby) with the words “last frost date.”

Provincial or state government websites are often a source of good information as long as it has been updated relatively recently. If not, other local gardeners, garden centres, and seed companies can often give you an idea of the last frost in your area.

However, if you poke around on a few different websites, you’ll often find many different answers, because each source is using a different level of risk. The Farmer’s Almanac, for example, provides a date with 33% risk of frost.

Here in Manitoba, you can check the maps linked at the bottom of the Agricultural Climate of Manitoba page on the Manitoba Agriculture website here. They have three options for maps of last frost dates: 50% risk, 25% risk, and 10% risk of frost.

For both my own calculations and the dates below, I have opted to go with a 25% risk of frost (i.e. the last frost date would be expected to be later in one out of every four years). However, you may want to use the dates for 10% risk instead for more sensitive plants, or if you don’t have a plan for protecting your plants from frost damage.

Below are the dates from the map 25% Risk* Date of the Last Spring Frost at 0°C for some of the larger communities in the province.

  • Altona – May 20 to May 23
  • Brandon – May 23 to May 26
  • Dauphin – May 26 to May 29
  • Morden – May 20 to May 23
  • Neepawa – May 23 to May 26
  • Niverville – May 23 to May 26
  • Oakbank – May 23 to May 26
  • Portage la Prairie – May 20 to May 23
  • Selkirk – May 23 to May 26
  • Steinbach – May 29 to June 1
  • Stonewall – May 23 to May 26
  • Winnipeg – May 23 to May 26
  • Winkler – May 20 to 23

Based on the above, I’ve settled on May 23 as my last frost date, but it’s something I plan to experiment with in future years to see how much I can push my luck earlier in the season.

So, Can I Start Planting Seedlings as Soon as the Last Frost Date Passes?

As mentioned above, your last frost date is calculated as an average based on historical climate data. The map from Manitoba Agriculture admits it’s likely wrong one out of every four years. Use your last frost date as a guideline, but when the time approaches, keep a close eye on your local weather forecast too.

Many gardeners will also have microclimates in their yards, meaning they may be able to plant a bit sooner (such as in a very sheltered garden with a lot of direct southern sun), or they may have to wait a bit later (for a garden that is more exposed, in a low spot, or without much shelter from the wind). Experience planting in the same area year after year will help you identify how your garden’s conditions may differ from your local area.

Whatever date you choose to go by, it’s best to have a plan for protecting your seedlings from frost for those years when Mother Nature decides to ignore historical climate data!

What is the last frost date you go by in your area? Which source do you follow? Let us know in the comments.

Sources

1 thought on “How Do Last Frost Dates Work?”

  1. Pingback: How to Plan Your First Vegetable Garden (A Complete Guide for Beginner Gardeners, Part 1) – Playing in the Dirt

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