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.

Is It Fall Here Now?

It’s October, and October should mean the beginning of rainy, cozy days with sweaters and books and tea, but instead we’re still having 80 degree weather (plus a 90-degree heat wave last week), and it definitely doesn’t feel like fall. And yet…. the leaves on some of the trees seem to be turning to orange and red around here….hmmmm… It feels so anachronistic.

I assumed that leaves wouldn’t change color until the days get cooler. Apparently, this is a common misconception. Leaves changing color actually has more to do with shortening day length than the temperature dropping.

Leaves typically change colors around the time of the autumn equinox. The equinox is when the daylight hours equal the nighttime hours: equi– means equal, nox means night. (This isn’t strictly true. It’s currently a couple of weeks past the equinox and our daylight hours are still a bit over 12 hours, but this is because the sun still provides some sunlight even after it dips below the horizon. Technically, the equinox is when the sun crosses the celestial equator, and is therefore above the horizon for roughly 12 hours. Or so I gather…. It’s a lot of celestial mumbo jumbo that I don’t really understand. Details details…)

As the days shorten, the plants are triggered to stop making chlorophyll. Thus, they lose their green color. The leaves become shades of red, orange, and yellow because of carotenoids, xanthophylls, and anthocyanins in the leaves. Carotenoids and xanthophylls are always present in leaves. They make leaves yellow, but when there’s also a lot of chlorophyll in the leaf, the green color of the chlorophyll overpowers the color of the carotenoids or xanthphylls and we perceive the leaves to be green rather than yellow. As the chlorophyll disappears, the underlying yellow color is revealed. Anthocyanins are produced only in the fall. They make leaves red. Trees produce more anthocyanins when when days are warm and sunny, and nights are cold. In this situation, leaves can photosynthesize and produce fuel for the plant during the day, but the fuel (sugars) can’t travel down the leaf veins at night because it is too cold, so excess sugars build up. The excess sugars are turned into anthocyanins (the reasons for and mechanisms of this are “complex” according to multiple sources, so I am probably oversimplifying things here). Years that have an abundance of warm early fall days with cold nights result in more anthocyanin production and thus a better fall foliage display.

According to Farmer’s Almanac, the Big Sur Coast Highway is prime fall foliage location in California. Who knew? Peak season is expected to be Oct 15 – 31, so I might have to make a trip out there one of these weekends. If only it would get cool enough so I could wear a sweater and drink some tea while admiring the leaves too…

What’s the Difference between Zone 8b and Zone 9b?

As I’ve said before, the scenery in California is very different from that of Seattle. I moved from zone 8b to zone 9b. Doesn’t seem like it should be that big of a difference, right?

The current USDA hardiness zones were determined by the annual lowest winter temperature, averaged over the past 30 years. (Actually, the current USDA zones are based on the average from 1976-2005.) Zone 8b has an average minimum temp of 10-15 deg F, whereas zone 9b has an average minimum temp of 25-30 deg F. Those are both below freezing, but 9b is just barely below freezing.

First and last frost dates mirror USDA hardiness zones pretty closely. The average first frost date for my area (according to the Farmer’s Almanac) is Nov 29th, and the last frost date is Feb 22nd. Palo Alto has an essentially year-round growing season. In Seattle, our first frost was Nov 16th, and our first frost was Mar 17th. That’s only a few weeks of difference.

These changes are minor, but they do have a significant impact on the plants that can survive the winter. Here in zone 9b, citrus trees and certain other tropical fruits are grown outdoors, whereas in Seattle, they would need to be taken indoors in the winter, or otherwise protected.

The Meyer lemon

Interestingly, though, some fruit trees actually need colder weather in order to produce fruit. Some apples and pears need a freeze in order to set fruit. Here, in California, we have “low chill” varieties, which don’t need to be cold in the winter in order to set fruit. “Chill hours” are the the cumulative number hours during the winter when the temperature is below 40 degrees, but above 32. Low chill varieties need fewer chill hours than normal trees, often under 500 hours (less than 20 days below 40 degrees).

Temperature aside, I think the main reason zone 9b looks so different from zone 8b is rainfall. The data below is from bestplaces.net (I can’t vouch for it’s accuracy; the average July high seems a little suspect).


In the above chart, “Rainfall” is the total number of inches of annual rain fall and “Precipitation” is the number of days with measurable rainfall. Seattle gets 50% more rain than SF, and has over twice as many rainy days. All that rain means lush green plants, something I sorely miss in the dry dusty desert of the Bay Area.