Today is Palo Alto’s First Frost Date…

….and I haven’t seen frost in the garden yet.

According to weather.com, our lowest temperature so far this season has been 34 degrees on November 9th. That was the morning I woke up to frost on our neighbor’s roof, but the garden seemed fine with no significant frost damage.

The average low temps in December and January are 38 and 39° F. I wonder if it’s possible that we’ll never get below freezing at night at all? I think I underestimated just how mild these winters would be. Could I have planted a winter garden here?

I have to remember that just because the plants won’t freeze, it doesn’t mean they will grow. Not only do plants need a certain amount of heat, they need sunlight too. In these winter months, we have less than 10 hours of bright sunlight, and my garden plot – which is already on the shadier side – gets even less light. I was out in the garden just before 1 PM the other day, and it was already in shade. The growth of my kale and mustard greens has, not surprisingly, essentially stalled.

So, what I need to do next year, is A) get the plants in the ground in early fall so they have enough time to mature before the dark winter days set in and B) pick the sunniest spot for my fall plants. That way, they’ll be guaranteed to get as much sunlight as possible in winter. Hopefully, since I don’t have to worry so much about a freeze, I can leave the plants in the ground and slowly harvest from them all winter!

Was Monday Our First Frost?

Our first frost date, according to Farmer’s Almanac, is November 29th. I woke up on the morning of November 9th to frost on the roof of the house next door.

I went out to check the garden, and everything looks pretty much the same. There wasn’t any obvious frost on any of the plants. Even the basil is alive! There may have been a tiny bit of damage on one of the zucchini plants, but it’s hard to know if this is from the frost or just my own benign neglect. (Since they’ve stopped producing any fruit, I’ve hardly watered them. I should pull them out, but they keep flowering, and I don’t have anything else planned for that space, so I let them live.) Hmmm…false alarm?

The Farmer’s Almanac’s first frost date is the date in the fall when (on average) there is a 30% chance of the temperature dropping below 32° F before that date. (This was news to me. I had assumed that the first frost was just the average first frost date, meaning there’s a 50% chance of the first frost happening before and a 50% chance of the first frost happening after that date. Wrong!)

A “light frost” is when the air temperature drops just below 32° F for only a few hours at most. A “hard frost” is when the temperature drops below 28° F for at least four hours. Some plants can tolerate a light frost but not a hard frost. Zucchini and basil shouldn’t be able to tolerate even a light frost, but my kale should be just fine.

The weather reports say the temperature in Palo Alto got down to 37° F that night. Although it’s possible we are in a colder microclimate, given the state of my vegetables and herbs, I don’t think we had our first frost Monday night. The roof was, indeed, a false alarm.

As a side note, how does frost form on roofs if it doesn’t get below freezing?Frost forms from water vapor touching a freezing surface and forming a layer of ice crystals (this is called deposition, when a gas turns to a solid without becoming a liquid first). Although the measured air temperature was higher than 32° F, the roof temperature could have been less than 32° F. This has to do with radiative cooling and frost/dew points, which you can read more about here if you are curious.