Frost Dates: First and last frost dates for San Luis Obispo Poly U, 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 10. 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 February 13, so we don't dare plant that early. We wait a few days and by March 19 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 16 | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 28° | Jan 16 | Jan 2 | Dec 17 | -- | -- | -- | -- | -- | -- |
| Last 32° | Mar 29 | Mar 8 | Feb 21 | Feb 7 | Jan 24 | Jan 8 | Dec 18 | -- | -- |
| Last 36° | May 10 | Apr 22 | Apr 9 | Mar 29 | Mar 19 | Mar 8 | Feb 25 | Feb 13 | Jan 26 |
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 26.
- There is a 50% chance of being hit by a 32° frost starting around December 30
- You have a 80% chance of seeing 32° by --
- 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 23 | -- | -- | -- | -- | -- | -- | -- | -- |
| First 28° | Dec 23 | Jan 8 | Jan 30 | -- | -- | -- | -- | -- | -- |
| First 32° | Nov 10 | Nov 26 | Dec 8 | Dec 19 | Dec 30 | Jan 11 | Jan 30 | -- | -- |
| First 36° | Oct 26 | Nov 8 | Nov 17 | Nov 24 | Dec 1 | Dec 8 | Dec 16 | Dec 25 | Jan 6 |
Data is provided by the NOAA National Centers for Environmental Information.