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 22. 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 29, so we don't dare plant that early. We wait a few days and by May 8 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 1 | Feb 22 | Feb 16 | Feb 12 | Feb 7 | Feb 3 | Jan 29 | Jan 23 | Jan 14 |
Last 20° | Mar 10 | Mar 3 | Feb 27 | Feb 22 | Feb 18 | Feb 14 | Feb 10 | Feb 5 | Jan 29 |
Last 24° | Mar 31 | Mar 23 | Mar 18 | Mar 13 | Mar 9 | Mar 4 | Feb 28 | Feb 22 | Feb 15 |
Last 28° | Apr 22 | Apr 15 | Apr 10 | Apr 5 | Apr 1 | Mar 28 | Mar 24 | Mar 19 | Mar 12 |
Last 32° | May 10 | May 4 | Apr 29 | Apr 26 | Apr 22 | Apr 19 | Apr 15 | Apr 10 | Apr 4 |
Last 36° | May 22 | May 17 | May 14 | May 11 | May 8 | May 5 | May 2 | Apr 29 | Apr 24 |
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 16 | Nov 25 | Dec 2 | Dec 8 | Dec 14 | Dec 20 | Dec 26 | Jan 3 | Jan 14 |
First 20° | Nov 3 | Nov 13 | Nov 20 | Nov 25 | Dec 1 | Dec 6 | Dec 12 | Dec 19 | Dec 28 |
First 24° | Oct 27 | Nov 4 | Nov 9 | Nov 14 | Nov 18 | Nov 23 | Nov 27 | Dec 3 | Dec 10 |
First 28° | Oct 20 | Oct 25 | Oct 29 | Nov 1 | Nov 4 | Nov 7 | Nov 10 | Nov 13 | Nov 19 |
First 32° | Oct 4 | Oct 8 | Oct 12 | Oct 15 | Oct 17 | Oct 20 | Oct 23 | Oct 26 | Oct 31 |
First 36° | Sep 20 | Sep 26 | Sep 29 | Oct 3 | Oct 6 | Oct 9 | Oct 12 | Oct 16 | Oct 21 |
Now that you know your frost dates, use our Garden Planting Calendar for Coulee Dam, WA 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.