Frost Dates: First and last frost dates for San Gregorio, 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 June 3. 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 April 15, so we don't dare plant that early. We wait a few days and by May 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° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 28° | Feb 27 | Feb 4 | Jan 14 | Dec 19 | -- | -- | -- | -- | -- |
| Last 32° | Apr 23 | Apr 5 | Mar 24 | Mar 13 | Mar 3 | Feb 21 | Feb 11 | Jan 29 | Jan 12 |
| Last 36° | Jun 3 | May 24 | May 17 | May 11 | May 5 | Apr 29 | Apr 23 | Apr 15 | Apr 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 4.
- There is a 50% chance of being hit by a 32° frost starting around November 30
- You have a 80% chance of seeing 32° by December 27
- 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° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| First 28° | Dec 5 | Dec 29 | Jan 19 | Feb 14 | -- | -- | -- | -- | -- |
| First 32° | Oct 21 | Nov 4 | Nov 14 | Nov 22 | Nov 30 | Dec 9 | Dec 17 | Dec 27 | Jan 10 |
| First 36° | Oct 4 | Oct 12 | Oct 17 | Oct 22 | Oct 26 | Oct 30 | Nov 4 | Nov 10 | Nov 17 |
Data is provided by the NOAA National Centers for Environmental Information.