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 29. 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 May 21, so we don't dare plant that early. We wait a few days and by June 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. If conditions change and a surprise frost does threaten, there's always things you can do to protect plants from frost.
Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
---|---|---|---|---|---|---|---|---|---|
Last 16° | Mar 22 | Mar 13 | Mar 6 | Feb 28 | Feb 22 | Feb 17 | Feb 11 | Feb 4 | Jan 26 |
Last 20° | Apr 9 | Mar 31 | Mar 24 | Mar 19 | Mar 13 | Mar 8 | Mar 2 | Feb 23 | Feb 14 |
Last 24° | Apr 23 | Apr 17 | Apr 12 | Apr 8 | Apr 4 | Mar 31 | Mar 27 | Mar 22 | Mar 16 |
Last 28° | May 9 | May 3 | Apr 29 | Apr 25 | Apr 22 | Apr 18 | Apr 15 | Apr 10 | Apr 4 |
Last 32° | Jun 10 | Jun 2 | May 28 | May 23 | May 18 | May 14 | May 9 | May 3 | Apr 25 |
Last 36° | Jun 29 | Jun 21 | Jun 15 | Jun 10 | Jun 5 | Jun 1 | May 27 | May 21 | May 13 |
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° | Oct 22 | Nov 2 | Nov 10 | Nov 17 | Nov 24 | Nov 30 | Dec 7 | Dec 15 | Dec 26 |
First 20° | Oct 11 | Oct 20 | Oct 27 | Nov 2 | Nov 7 | Nov 13 | Nov 19 | Nov 26 | Dec 5 |
First 24° | Sep 28 | Oct 5 | Oct 11 | Oct 15 | Oct 19 | Oct 24 | Oct 28 | Nov 3 | Nov 10 |
First 28° | Sep 19 | Sep 24 | Sep 28 | Oct 1 | Oct 4 | Oct 6 | Oct 9 | Oct 13 | Oct 18 |
First 32° | Sep 5 | Sep 10 | Sep 14 | Sep 18 | Sep 21 | Sep 25 | Sep 28 | Oct 2 | Oct 8 |
First 36° | Aug 23 | Aug 29 | Sep 3 | Sep 7 | Sep 11 | Sep 14 | Sep 18 | Sep 23 | Sep 29 |
Now that you know your frost dates, use our Garden Planting Calendar for Bonners Ferry, ID to know when to sow and transplant your various vegetable plants!
Data is provided by the NOAA National Centers for Environmental Information. Photography by Garden.org member TBGDN.