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 14. 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 23, so we don't dare plant that early. We wait a few days and by May 1 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° | Mar 22 | Mar 14 | Mar 8 | Mar 3 | Feb 26 | Feb 22 | Feb 17 | Feb 11 | Feb 3 |
Last 20° | Apr 4 | Mar 28 | Mar 23 | Mar 19 | Mar 15 | Mar 12 | Mar 7 | Mar 3 | Feb 24 |
Last 24° | Apr 11 | Apr 5 | Apr 1 | Mar 29 | Mar 26 | Mar 22 | Mar 19 | Mar 15 | Mar 9 |
Last 28° | Apr 20 | Apr 15 | Apr 12 | Apr 9 | Apr 6 | Apr 3 | Mar 31 | Mar 28 | Mar 23 |
Last 32° | May 2 | Apr 27 | Apr 23 | Apr 20 | Apr 17 | Apr 14 | Apr 10 | Apr 7 | Apr 1 |
Last 36° | May 14 | May 10 | May 6 | May 4 | May 1 | Apr 29 | Apr 26 | Apr 23 | Apr 18 |
The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:
Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
---|---|---|---|---|---|---|---|---|---|
First 16° | Nov 9 | Nov 15 | Nov 20 | Nov 24 | Nov 28 | Dec 1 | Dec 5 | Dec 10 | Dec 17 |
First 20° | Nov 6 | Nov 10 | Nov 13 | Nov 16 | Nov 19 | Nov 22 | Nov 24 | Nov 28 | Dec 2 |
First 24° | Oct 24 | Oct 29 | Nov 2 | Nov 6 | Nov 9 | Nov 12 | Nov 16 | Nov 20 | Nov 25 |
First 28° | Oct 12 | Oct 17 | Oct 22 | Oct 25 | Oct 29 | Nov 1 | Nov 4 | Nov 9 | Nov 15 |
First 32° | Oct 5 | Oct 10 | Oct 14 | Oct 17 | Oct 21 | Oct 24 | Oct 27 | Oct 31 | Nov 6 |
First 36° | Sep 25 | Sep 29 | Oct 2 | Oct 5 | Oct 7 | Oct 9 | Oct 12 | Oct 14 | Oct 18 |
Data is provided by the NOAA National Centers for Environmental Information.