Frost Dates: First and last frost dates for Petaluma, CA
What's a safe temperature for tender plants? When the nightly temperature falls, frost can form, even above 32°, because the air is colder high above the ground and the frost can form up there and then fall down onto your garden and do some damage, even if it's 36° on the ground. So many factors come into play, including wind, concrete, houses, trees and other structures, etc etc etc. Because of all this, you might want to consider 36° as "the danger zone".
In your average springtime, you have a 90% chance that there will be no 36° nights by May 12. In other words, you can pretty much count on being safe from frost by that day. But we want to get those tomatoes in the ground as soon as possible, right? We see that there's still an 80% chance of 36° on March 13, so we don't dare plant that early. We wait a few days and by April 6 we are at the 50/50 point. At this point, we are close and we can start watching the weather forecast. If the upcoming week's forecast doesn't show below 40°, then it's probably okay to risk planting out your plants.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| Last 16° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 20° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 24° | Feb 23 | Jan 23 | Dec 23 | -- | -- | -- | -- | -- | -- |
| Last 28° | Mar 16 | Feb 27 | Feb 14 | Feb 3 | Jan 23 | Jan 12 | Dec 30 | Dec 13 | -- |
| Last 32° | Apr 15 | Mar 30 | Mar 19 | Mar 9 | Feb 28 | Feb 19 | Feb 9 | Jan 28 | Jan 13 |
| Last 36° | May 12 | Apr 29 | Apr 20 | Apr 13 | Apr 6 | Mar 30 | Mar 22 | Mar 13 | Feb 28 |
The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:
- You have a small 20% chance of getting 32° by November 8.
- There is a 50% chance of being hit by a 32° frost starting around November 27
- You have a 80% chance of seeing 32° by December 16
- Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| First 16° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| First 20° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| First 24° | Dec 14 | Jan 13 | Feb 14 | -- | -- | -- | -- | -- | -- |
| First 28° | Nov 21 | Dec 4 | Dec 13 | Dec 21 | Dec 29 | Jan 7 | Jan 17 | Feb 1 | -- |
| First 32° | Oct 29 | Nov 8 | Nov 15 | Nov 21 | Nov 27 | Dec 3 | Dec 9 | Dec 16 | Dec 26 |
| First 36° | Oct 10 | Oct 19 | Oct 26 | Nov 1 | Nov 6 | Nov 12 | Nov 17 | Nov 24 | Dec 4 |
Data is provided by the NOAA National Centers for Environmental Information.