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How Frosts and Freezes Affect Your Flowers

by | Updated: Feb 26, 2021

How frosts and freezes affect annuals, like this flower with frost on it

When I was first learning how to garden…

I used to think that 32 degrees was the temperature I needed to keep an eye on for my flower plants.

32 degrees is when water freezes, so that must be the point when my flowers will freeze too, right?

But over time, I’ve learned:

There are levels of frost and freezing that affect plants in different ways

32 degrees isn’t the magic number for all plants.

(Because Mama Nature makes her own rules, y’all!)

So, in this article, I want to share tips I wish I had understood sooner, so you’re a step ahead as a gardener.

As our temperatures dip in states like Colorado and Utah, I hope this article helps you understand:

  • What’s going on with the annuals and perennials in your garden
  • Which flowers you may want to cover and protect (if you want them to last longer)
  • Why the heck some of your plants still look good — while others may be a mushy mess!

Annuals are the flowers that live for a single year, but typically don’t return. Perennials are the flowers that come back for multiple years.

Here’s a quick summary

Between 36 and 29 degrees, your most vulnerable flowers are your “tender annuals.”

Tender annuals are your one-season flowers that like warm air and warm soil to grow. They like having “warm feet,” meaning they want their roots to be comfortably warm. These are the guys you definitely want to cover if you want them to last longer. (You can see examples further down this post.)

Below 29 degrees, more of your flowers will be affected.

Infographic showing how frost, light freezes, moderate freezes and hard freezes affect annuals and perennials in states like Colorado and Utah

Keep in mind, there are always exceptions with Mother Nature. I’ve shared some of them down at the end of this post.

Nonetheless, this will give you a good overview.

Let’s start with frost because it often shows up first.

Frost

When temperatures reach 36 degrees or below — and the moisture conditions are right

Frost is a thin layer of water vapor that turns into ice on the surface of your plants. When ice forms on the surface of your plants, the water inside your plants can freeze too, causing the cells to rupture and lose all their water.

And this can damage or kill your plants.

If the moisture conditions are right for frost, you can get it when temperatures reach 36 degrees or below.

You can get frost on your flowers when the temperature drops to 36 degrees and below

As I mentioned earlier, between 36 and 29 degrees, frost often has the biggest effect on your tender annuals.

Have you ever had a friend visit from a city like San Diego or Sarasota? When our temperatures are 50 degrees, you’re thinking about wearing shorts… while your friend wants to put on a parka. Your out-of-town guest is like your tender flowers. These plants do not adjust well to the cold.

These flowers are happiest when temperatures when it’s warmer, and they’re VERY sensitive to frosts and cold temperatures.

Light freeze

Between 32 and 29 degrees

You know the tender annuals we’ve been talking about? The temperatures in a light freeze will likely kill them.

A light freeze takes place between 32 and 29 degrees.

What are examples of tender flowers? You may recognize some of the annuals below.

Examples of tender flowers in Colorado that prefer shade, like impatiens, new guinea impatiens, coleus and begonias

Examples of tender flowers in Colorado that prefer sun, like sweet potato vine, salvia and zinnias

In my garden, sweet potato vine (pictured above) is ALWAYS the first plant to get nipped.

A light freeze also can damage how your hardier flowers look. Usually, though, it’s just cosmetic damage.

If you’re catching your local weather report, you’ll hear your weather forecasters issue a “freeze warning” for these temperatures.

Hard freeze (National Weather Service language)

Between 28 and 25 degrees

A hard freeze should severely damage or kill most of your annual flowers — even your hardier flowers that are happier in chillier temps.

Gardeners call this a “moderate freeze.”

However, I’ve used the language from the National Weather Service here because it’s what you’ll see on a weather app or hear on a forecast.

When the weather forecasters say, “There’s a hard freeze warning tonight,” they’re talking about the threshold of 28 degrees or below.

It’s going to be destructive to your annuals if you don’t protect them.

This is the temperature zone where you may start noticing some damage to your perennials too.

For example, you may notice their leaves turning a dark color or becoming gooey.

Severe or killing freeze — known as a “hard freeze” in gardening lingo

24 degrees (or colder)

And the freeze that rules them all is known as a “severe” or “killing freeze.” You’ll hear gardeners refer to this as a hard freeze.

It takes place at 24 degrees or colder.

It will likely cause widespread damage to the exposed flowers, leaves and stems on your perennials. (It can depend on how protected your flowers are in your yard.)

This is also the freeze that kills most remaining annuals.

… with a few exceptions. Pansies and violets, for example, are tenacious little flowers that can survive these freezing temperatures. At lower (warmer) elevations in Colorado, you may see them planted in the fall for spring blooming.

Pansies and violets are cold-tolerant flowers that are resistant to freezing temperatures

Some exceptions to these guidelines!

Remember, Mama Nature makes her own rules.

Here are some exceptions that are relevant if you’re gardening in Colorado, Utah or similar states:

  • If you have young or newly planted flowers, they’re more vulnerable to freezing (and dying) than your plants that have well-established roots.
  • If we experience a really big temperature swing, your plants are more vulnerable to damage or death from freezes. In the Front Range of Colorado, we can get 50- to 60-degree temperature swings in the fall and spring. In the fall, this is confusing for our plants because they don’t have time to naturally prepare for hibernation mode, known as “dormancy.” And in the spring, they’ve often started to wake up from dormancy when we get these cold temperature swings.
  • If you have flower buds that are opening in the spring, I’ve found they’re more vulnerable to getting dinged by freezes than plant leaves. The freezes likely won’t kill your plants. You just may not get flowers that year.
  • If you’re growing flowers in a protected area of your yard (like along a south-facing fence that’s shielded from the wind), your plants may stay warmer when temperatures drop. I often have flowers that freeze in the front yard, and those same flowers are wide awake and happy in the backyard, which is more protected!

The dates for the first fall frost

We have such varied elevations and conditions in Colorado and Utah, it depends on where you live.

The Front Range of Colorado usually gets its first frost in early- to mid-October.

But hey, it’s Colorado.

We also can get it in early September. (I’m looking at you, 2020.)

And if you live at a higher elevation, your garden may be at risk for frost year-round.

How to find the spring and fall frost dates in your area

In the fall, if you’d like to find the “average first fall frost date” for your area, I suggest googling:

  • Average first fall frost in {your city}

Because this is the “average” date, you’re 50% likely to get frost sooner than this date, and you’re 50% likely to get your first frost after this date.

In the spring, you may want to search for the “average last spring frost date”:

  • Average last spring frost in {your city}

Again, this means you’re 50% likely to have the last spring frost before this date, and you’re 50% likely to have it after this date.

Whew! We covered a lot!

If these tips were helpful to you, please share this article.

Related topics that may interest you:

© 2020-2021, Go West Gardener
You’re welcome to share a link to this article on social media sites, but no other re-use in any form without written permission.

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Ann from Go West Gardener with her flowerpots and garden

Hey there, I'm Ann

I’m a Certified Colorado Gardener, dog mama and Midwesterner-turned-Colorado girl. I help budding gardeners in the intermountain west get more confident with flower gardening, so you can create an outdoor space you love. More about Ann >>

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