Question:

I have been hearing different dates for the last frost date in Crescent City. Some places say the end of March; some places say the end of May. Are you able to help select a last frost date for seeding planning? The garden is located on the College of the Redwoods campus at 833 W Washington Blvd. 

Answer:

Master Gardener Terry A. shares her experiences and information about frost dates in your community:

About That “May 15” Last Frost Date

The standard last frost date of May 15 gets repeated a lot, mostly because it’s historical. In real life, at least at the coast, it doesn’t match what many of us actually experience.

I’ve lived and gardened here for 33 years and have run an accurate personal weather station that whole time. We have not had a frost on or after May 15 in all those years. That lines up with the airport data, too.

Last frost dates aren’t guarantees anyway. They’re based on probability—usually meaning there’s less than about a 5% chance of frost after that date. Even if frost hasn’t happened for decades, the date sticks because it happened at some point farther back in the record.

And that’s before you get into all the reasons those dates don’t translate well to real gardens. Read on to learn why frost dates don't tell the whole story here.

Where the data comes from matters.

Most frost data come from the airport, which is about as different from a backyard garden as you can get. It’s flat, open, and exposed. Most people garden in sheltered areas, near buildings, trees, or on slopes.

Climate has changed.

Over the last 30+ years, we’re seeing more and more seasons where coastal temperatures never actually hit 32°F. That doesn’t mean frost never happens—but when it does, it’s very location-specific.

Topography matters a lot.

Cold air drains downhill. I’m on a slope near the ocean, and the valley below me gets frost far more often than I do. Even on my own property, frost shows up in swales, on roofs, or in open low spots. It almost never forms under trees, even bare ones.

Frost is picky.

Whether frost forms depends on ground warmth, cloud cover, wind, soil moisture, and surface exposure. It also means frost risk can be reduced—or sometimes created—by site design.

So, how do we deal with it?

There are basically three approaches:

• Use season extenders for frost-tender plants
• Start plants early in a frost-free space (indoors, heat mats, protected areas)
• Or simply wait until frost risk has clearly passed—which is the least work and lowest stress option

All of these can work, depending on your set-up and goals.

The bigger issue isn't frost. 

Around here, cold soil and rain are usually the real limiting factors, not frost.

Yes, you can plant things like corn, beans, or tomatoes outside in March and they might survive. But surviving isn’t the same as thriving. In most years, a May planting of the same crops will easily outperform plants that struggled through March and April in cold, wet soil.

Protected growing makes a big difference. For example, SeaBreeze Farms in Klamath starts tomatoes around February 1 on heat mats in a hoop house, then grows them on in pots under protection. That’s how they end up with big, healthy plants for sale earlier than anyone else locally.

So, while frost management matters, it’s only one piece of the puzzle—and is often not the most important one.

Additional information related to frost dates in specific zip codes can be found at The National Gardening Association’s website, https://garden.org/apps/frost-dates/ 

Resources:

Frost Dates: First and last frost dates by zipcode, The National Gardening Association

Frost Protection and Extending the Growing Season, Colorado State University Extension