Frost Dates: First and last frost dates for Napa State Hospital, 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 5. 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 16, so we don't dare plant that early. We wait a few days and by April 5 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° | Dec 20 | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 28° | Feb 8 | Jan 27 | Jan 18 | Jan 10 | Jan 2 | Dec 23 | Dec 9 | -- | -- |
| Last 32° | Apr 6 | Mar 22 | Mar 12 | Mar 2 | Feb 22 | Feb 13 | Feb 4 | Jan 25 | Jan 10 |
| Last 36° | May 5 | Apr 25 | Apr 17 | Apr 11 | Apr 5 | Mar 30 | Mar 23 | Mar 16 | Mar 5 |
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 15.
- There is a 50% chance of being hit by a 32° frost starting around December 5
- You have a 80% chance of seeing 32° by December 25
- 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 28 | -- | -- | -- | -- | -- | -- | -- | -- |
| First 28° | Nov 27 | Dec 8 | Dec 15 | Dec 23 | Dec 30 | Jan 8 | Jan 24 | -- | -- |
| First 32° | Nov 5 | Nov 15 | Nov 23 | Nov 29 | Dec 5 | Dec 11 | Dec 18 | Dec 25 | Jan 5 |
| First 36° | Oct 30 | Nov 4 | Nov 8 | Nov 11 | Nov 15 | Nov 18 | Nov 21 | Nov 25 | Nov 30 |
Data is provided by the NOAA National Centers for Environmental Information.