What is "heavy frost"?
April 9, 2020 11:23 AM   Subscribe

I think I understand about the "last frost" date and how to look that up, but I have a bunch of plants that say they can go outside, or that their seeds should be sown outside, or that they need to be pruned, "in early spring after the danger of heavy frost has passed." How am I supposed to know when that is? Is it now? Last year I waited until the "last frost" date for everything and it felt too late: mid-May was practically summer. This is in Madison, WI/zone 5a if anyone has a specific date in mind, but I'm also wondering in general what the concept means.
posted by teremala to Home & Garden (15 answers total)
If your plants get frost on them they likely will die. It's the date of the last frost of the year. Does your area get frost into May? I would research when the last frost typically is and put them soon outside after that date
posted by Amy93 at 11:36 AM on April 9, 2020

From The Farmers Almanac:
Light freeze: 29° to 32°F—tender plants are killed.
Moderate freeze: 25° to 28°F—widely destructive to most vegetation.
Severe freeze: 24°F and colder—heavy damage to most plants.

I like the way this information is phrased from Dave's Garden (I put in a random Madison zip):
  • Each winter, on average, your risk of frost is from October 4 through April 28. Almost certainly, however, you will receive frost from October 17 through April 17.
  • You are almost guaranteed that you will not get frost from May 10 through September 20.
  • Your frost-free growing season is around 159 days.
So basically, it's not perfect. The frost date is based on the probability of frost. In my experience down here in Chicago I'm experiencing the same thing—it feels like it goes from frost risk to summer in a week. It also feels like the seasons are shifting and we can grow later into the fall. My neighbors (who have never gardened) planted seeds against my advice this week when we had a random 70 degree day. It might snow this week. I usually wait until Mother's Day for anything that likes warmth, and maybe even Memorial Day for tomatoes and cukes. My mom is closer toward you and doesn't plant anything until after Memorial Day.

I think seasons and zones are changing with global warming and I don't know that advice has been adjusted for this. I think it will be up to you to understand what works in your yard and to plant when you personally feel the risk from frost is okay for you. Keep in mind that seeds need a certain soil temperature to germinate, which may make them harder to put out "early" than plants. Are there any local gardening groups or resources you can bounce this off of?
posted by Bunglegirl at 11:49 AM on April 9, 2020 [4 favorites]

I am not a climatologist, but this definition is pretty close "In gardening terms, a "light freeze" or "light frost" refers to temperatures that fall just a few degrees below freezing for a few hours. Some hardy plants may not be damaged. A "hard frost" or "killing frost" comes when the temperature drops further, below 28 degrees, for a longer time."

So the average low temp in Madison is already above 32F, but you only have to go back as far as 2016 to find temps below 28F for April 9.

For April 20, you only have to go back to 2018 to get 3 hours of heavy frost.

Hate to say it, but if you plant any time soon, you are rolling the dice.
posted by The_Vegetables at 11:50 AM on April 9, 2020

I have some experience over in Sheboygan and I got in a lot of trouble pushing the seasons up there. I'd plant peas in late March and they'd work... sometimes... but May is really the cutoff for frost up there if you don't want to take chances. I'll play around with vegetables that are easy to replant, but nothing else. I learned my lessons.
posted by Tchad at 11:54 AM on April 9, 2020

It's easier to see the difference in the fall, when there are more things growing outside. Sometimes the temperature gets just barely below freezing during the night and the next day you'll see that some plants were damaged and some were unaffected. Tomato plants, for instance, are likely to have at least some damaged foliage, but your brussels sprouts and pansies will be fine. A bit of cover over the tomato plants, just a blanket or tarp, can be enough to protect them at that temperature. That's a light frost. Sometimes it gets colder than that and you look out the next morning and the grass is white with frost and lots of plants have turned black. That's a heavy frost.

A lot of garden plants can stand temperatures a bit below freezing and those are the ones you can put outside "after the danger of heavy frost has passed." That probably is now or soon where you are. Looking at the forecast for Madison, I see nighttime temperatures in the low 20's predicted in the next week, so it's probably a bit early to set out plants, but you certainly could plant something like spinach seeds. And if in another week you look at the 10-day forecast and don't see nighttime temperatures lower than 30 or so predicted, you probably could set out hardy plants.
posted by Redstart at 11:56 AM on April 9, 2020

For April 20, you only have to go back to 2018 to get 3 hours of heavy frost.
I checked April 25 and May 5, and 0 days below freezing in the past 10 years for either. So I think if you wait until the 25th, then you should be safe minus a rare historical freak cold snap.

Weather Underground Historical Data
posted by The_Vegetables at 11:58 AM on April 9, 2020

I'm a transplant to southern WI and what everyone told me upon arriving is to wait until Mother's Day if you don't want to risk anything.
posted by catdapperling at 1:26 PM on April 9, 2020

If you are direct seeding then soil temperature is also important. Most won't germinate if it's too cool. I have planted peas and even without frost they rotted in the too cold soil. Poke an instant read thermometer down a few inches in a few places. Here is a chart but don't believe the minimums. Use the optimum. You can get the soil to warm a bit faster by raking off mulch and dead leaves and maybe by temporarily laying black plastic.
posted by Botanizer at 2:03 PM on April 9, 2020

Heavy frost is one where it's so cold covering your seedlings won't save them. Light frost is one where some of your plants get damaged but some of them survive anyway, even if you don't do anything.
posted by Jane the Brown at 2:08 PM on April 9, 2020

One thing to keep in mind is the location of your garden - if your neighbours get a frost and you get a frost their plants might all die and your might all survive because of the immediate micro-conditions. Get a thermometer and measure the ground level temperature of your garden, and compare that with the ground level temperature in some other locations. If your garden's temperature is consistently colder than your neighbours garden wait longer. If your garden's temperature is consistently a little warmer then you can start earlier than she can. Measure at different times of day, and if possible also at night.

The other way you can make a guess about this is by how soon things in your garden come out compared to the neighbours. If your crocuses are the last ones on the block and your forsythia is only in bloom after every one else's have turned green you'll need to wait longer to start than if you lead the pack.

Check with a local gardening group such as on fb what they are doing, or what they suggest.
posted by Jane the Brown at 2:14 PM on April 9, 2020 [1 favorite]

Maine, same zone, and we had several very warm sunny days where it got into the low 60s. I put down black plastic to get the soil warming a couple weeks ago, and planted peas and lettuces last weekend. I also used tent poles to make some hoops, and am glad I did; currently 32F, snowing hard, will probably get colder overnight. I went out as the rain was turning to snow and covered my raised beds with plastic. Yesterday I was able to sit on the deck in shirtsleeves. Weather is quite the trickster, but I'm still happy to have gotten started in the garden - it feels hopeful.

For any site with frost predictions, check the date; climate change is rapidly affecting these dates.
posted by theora55 at 2:57 PM on April 9, 2020

"in early spring after the danger of heavy frost has passed." How am I supposed to know when that is? Is it now?

Now is the time for peas, spinach, arugula, carrots, lettuce, rutabagas, beets, turnips, scallions and transplanting onions/leeks. They can survive a little frost. Mine got snowed on last night. I'm putting nasturtiums in today. Cilantro doesn't mind a little cold.

Last frost date in zone 5b northeast is May 15 so close to yours. A lot of people treat Memorial Day as the last frost day. That's the right window for tomatoes, cucumbers -- those last two weeks in May. Squash and cucumbers can even go in later -- they like the warmth.
posted by A Terrible Llama at 6:40 AM on April 10, 2020

I guess I should note: it's not perfect. Last year an arctic blast showed up in April and I lost all my peach blossoms. Some years there is no frost after the first of May. One year I got away with putting tomato starts out the first week in May.

The best thing is to separate the veg categories into cold-friendly versus not, and if a heavy frost shows up--or if you're just worried - you can throw a bedsheet or bucket or tupperware or whatever over the vegetables that have been planted out. Usually I do an early April planting and a mid-May planting of vegetables that like it warmer.

As to how to know -- they usually mention it on weather reports and I think there are services that will send you a text -accuweather, darksky, or weather.gov might....I'm squinting at the gov site but wearing my bad glasses so it's hard to read. I get Accuweather email alerts, which are surprisingly useful.
posted by A Terrible Llama at 7:26 AM on April 10, 2020

Response by poster: Thanks, all! It was delightful that just after I asked this, we got a couple hours of some kind of puffy snowy hail in the middle of a patch of sun, just to underscore everyone's point that no really, it's still plenty cold. Does it seem though like maybe a heavy frost is the same as what's labeled a "killing frost" here? April 26-May 2 feels pretty in line with what folks are saying in terms of temperatures and plant outcomes. I promise to keep the baby tomato seedlings cozy inside for longer than that, but I really do want to finally get my overgrown arborvitae trimmed for once!
posted by teremala at 6:25 PM on April 10, 2020

Yeah, I would say a heavy frost is the same thing as a killing frost.
posted by Redstart at 7:21 PM on April 10, 2020

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