Learn to use your first and last frost dates and plant hardiness zone to know when it’s safe to plant.
Every spring, I hear from new gardeners or read comments online from beginners asking the same question: When can I plant my garden?
The short answer is: whenever the weather is warm enough and we won’t get a frost again for several months.
But if you want to grow cool-season crops like onions or cabbage, you can and should plant those out before your warm-season crops like tomatoes.
This post will dive into the long answer of “When is it safe to plant my garden?” and give you the confidence to put those plants in the ground!
We will discuss first and last frost dates, plant hardiness zones, and the temperature needs of common cool- and warm-season crops.
This information is so important if you are planning to start a vegetable garden or cut-flower garden.
Let’s plan your best garden season yet!
Find Your First and Last Frost Date
Have you ever read the back of a seed packet and wondered what the words “start seeds 6-8 weeks before your last frost date” meant?
Here’s what that means.
Your last frost date is the last time after a cold season (a.k.a. winter) you might get a frost.
The first frost date is the first time your area will get a frost after a warm season (summer), or as I think of it when your tomatoes and zinnias will die.
Your two frost dates are essential for successful seed starting, succession planting, and growing annuals.
Research these dates by typing your zip code or your city name and country along with the words “first and last frost date” and doing a Google search.
For US growers, you can also check the Farmer’s Almanac here.
Record these dates in your garden planner or keep it on a sheet of paper wherever you store your seeds for future reference.
For me, in central Indiana Zone 6a, my last frost date is April 25, and my first frost date is October 10.
Why Frost Dates Matter
Our frost dates tell us when it is relatively safe to plant things that are frost-tender, or warm-season crops.
“Relatively safe” because there is still a chance of getting frost after your last frost date has passed. Remember, these dates are estimates, not exact.
We had snow once very close to our last frost date of April 25.
Predicting the weather is not an exact science. Anything can happen!
If you decide to plant too early, or try to grow a warm-season crop too late in the season, there’s a higher risk that those plants will experience a frost and die.
That’s why it’s good to know your frost dates!
Find Your Hardiness Zone (Optional)
The US Department of Agriculture has divided the US into 13 different hardiness zones.
Each numbered zone stands for 10 degrees of temperature difference. Each letter within the zone stands for 5 degrees of temperature difference.
Your hardiness zone tells you what kinds of plants will be perennial in your garden.
For example, oregano is hardy from USDA zones 4 to 10, so I can assume that it will grow back year after year for me in zone 6. However rosemary is listed as hardy to zone 7, and it has not once made it to spring for me yet.
The easiest way to find your hardiness zone is to google your zip code followed by the words “hardiness zone.”
You can also check the USDA Plant Hardiness Zone map and find where you live.
Canadian friends can check their zone on this map.
Side note: my zone actually changed in 2023 from 5b to 6a. If you think you know your zone, it might be worth rechecking to see if it’s the same!
If You Don’t Live in the US or Canada
If you live in an area that does not classify regions using hardiness zones, don’t worry about it.
Most vegetables are grown as annuals, so we can expect to plant them fresh each year anyway.
However if you’re curious, you can research your average monthly temperatures and compare them with the USDA or Canadian charts linked above.
Keep your first and last frost dates in front of you and move on.
So, When Is It Safe to Plant?
Ok, you should now have two important dates, and also a number and possibly a letter.
The chunk of time between your last frost date and your first frost date will be your main growing season.
This should be the warmest time of the year for you. This is when it is safe to plant your annual vegetables.
Once you have passed your last frost date, you can plant any vegetable or flower you want.
If you live somewhere where you don’t go below freezing, you can search for your average monthly temps and divide the year into warm and hot seasons.
Many vegetables and annual flowers thrive between 55°F and 85°F.
Any plants that can survive in your hardiness zone will be perennial, meaning the plant will come back year after year.
These can be planted whenever the ground is soft enough to dig into.
So both early spring and fall are still good times to plant perennials.
Cool vs. Warm Season Vegetables
Some vegetables thrive in warmer temperatures, but others like cooler temperatures.
This is where it can get a little tricky.
Plants that thrive in cooler temperatures can be planted before your last frost date, or late into your main growing season to mature in cooler weather.
For cool-season plants, temperatures will need to stay between 35°F (1°C) and 65°F (18°C).
Some common cool-loving plants include:
- onions
- eedspinach
- lettuce
- chamomile
- calendula
- carrots
- cilantro
- parsley
- arugula
- broccoli
- cabbage
- kale
- cauliflower
- sugar snap peas
- sweet pea flowers
- fava beans
Common heat-loving plants include:
- cucumbers
- squash
- green beans
- basil
- rosemary
- tomatoes
- peppers
- tomatillos
- eggplants
- sweet potatoes
- swiss chard
These plants thrive once temperatures are 55°F (13°C) or higher.
I hope this information gives you the confidence to plant out your garden this year!