Knowing your area’s last frost date is something that every gardener and homeowner should know. It can keep you from inadvertently damaging or killing your plants. But most “last frost dates” on the internet are wrong. Discover how to find an accurate and precise last frost date.
Regardless of what kind of gardening you do — growing edibles, container annuals, or typical landscape plants — knowing your area’s last frost date is critical. If you plant or prune too early, you’re setting yourself up for failure with cold-damaged plants or seeds that won’t germinate.
Why Most Last Frost Dates Aren’t Helpful
If you do an online search for “last frost date” in your area, you’ll come across sites that list the last average frost date. For example, here are some of the top search results I found for Tucson.
- FarmersAlmanac.com – Feb. 17
- Plantmaps.com – Jan. 21-31
- National Weather Service – Feb. 7
You’ll notice that these dates vary by nearly a month.
But when I checked in with the local experts, their answers were substantially different. The University of Arizona Cooperative Extension considers March 15 the last frost date. So does Tucson’s renowned rain harvesting authority, Brad Lancaster.
So why the big difference? What’s going on here?
Big national weather sites use historical data and come up with an average last frost date. But the problem with an average date is that it will be too early nearly half of the time!
The National Weather Service lists Tucson’s earliest and latest frost dates on record as Dec. 27 and May 3, respectively. Their average takes into account these extremes plus all the dates in between.
This is why you should ignore the big national sites and always stick with expert local advice.
How to Find Expert Local Advice
It wasn’t hard to find expert advice here in Tucson, but it might not be as easy where you live. So I would suggest the following.
Check with your local extension service. They should be able to provide you with an accurate last frost date for your area. Nearly every county in the US has an Extension Master Gardener Program. In Arizona, you can find contact information by county here. If you live outside of Arizona, you can find contact info for your county’s Master Gardener program here.
Talk to experienced gardeners in your area. You’ll find them at local nurseries, community gardens, garden clubs, or educational gardening organizations. They will have educated opinions on the last frost date.
A final approach is to use Garden.org’s frost date probability chart. Enter your zip code here and you’ll find a chart that tells you the likelihood of freezing temperatures as spring proceeds. For example, in my zip code, there’s a 50% chance that there will be another hard freeze after Jan. 19 but there’s only a 10% chance of one after Feb. 24. Then you can decide how lucky you feel. 😉
Microclimates Matter
Even the generally accepted last frost date isn’t the last word. Most yards have microclimates that can vary significantly. Here are some factors that affect the microclimates of your yard:
- Areas near your house or a block wall will be warmer.
- Areas that get full sun will be warmer than areas shaded by a tree, shade sail, or porch roof.
- Gravel mulch holds the heat and radiates it back. This can make your ground hotter on summer nights but also warmer on winter nights.
- Elevation matters. A low area in your yard will be cooler.
- Living near a wash will make your yard cooler.
So I recommend setting up a few outdoor thermometers in the coldest parts of your yard and figure out your own last frost dates. No one knows your yard as well as you do!
I have two backyard thermometers, and there is usually a 5 to 10 degree difference between them, even on winter’s coldest nights. That’s enough of a difference to change that area’s Hardiness Zone and last frost date.
A Final Word: Know Your Zone and Plant Accordingly
I’m all for making gardening a joy rather than a source of disappointment and stress. You can alleviate a lot of stress around frost dates and freeze damage by sticking with plants suitable for your area.
Look for plants rated to grow in your USDA Plant Hardiness Zone. Growing a plant in the wrong zone can significantly reduce its chances of thriving and surviving.
If you’re new to the desert southwest or new to gardening in general, learn your USDA Plant Hardiness Zone. Once you know your zone, you can look for plants rated to grow in your zone when you shop.
You can find your zone by entering your zip code here. This information is surprisingly precise — it can vary from neighborhood to neighborhood.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for 15 years.
