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 18. 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 20, 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. 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° | Feb 7 | Jan 26 | Jan 16 | Jan 7 | Dec 29 | Dec 16 | -- | -- | -- |
Last 20° | Feb 25 | Feb 14 | Feb 6 | Jan 30 | Jan 22 | Jan 14 | Jan 3 | -- | -- |
Last 24° | Mar 4 | Feb 21 | Feb 13 | Feb 7 | Jan 31 | Jan 25 | Jan 17 | Jan 8 | Dec 22 |
Last 28° | Mar 22 | Mar 12 | Mar 5 | Feb 27 | Feb 21 | Feb 15 | Feb 9 | Feb 2 | Jan 23 |
Last 32° | May 4 | Apr 25 | Apr 18 | Apr 13 | Apr 7 | Apr 2 | Mar 28 | Mar 21 | Mar 12 |
Last 36° | May 18 | May 12 | May 8 | May 4 | May 1 | Apr 28 | Apr 24 | Apr 20 | Apr 14 |
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° | Dec 3 | Dec 15 | Dec 25 | Jan 3 | Jan 14 | Jan 28 | -- | -- | -- |
First 20° | Nov 21 | Dec 5 | Dec 15 | Dec 25 | Jan 4 | Jan 15 | Feb 1 | -- | -- |
First 24° | Nov 15 | Nov 25 | Dec 3 | Dec 10 | Dec 16 | Dec 23 | Dec 30 | Jan 8 | Jan 26 |
First 28° | Nov 1 | Nov 12 | Nov 20 | Nov 27 | Dec 4 | Dec 10 | Dec 17 | Dec 25 | Jan 5 |
First 32° | Oct 20 | Oct 27 | Nov 1 | Nov 5 | Nov 9 | Nov 13 | Nov 17 | Nov 22 | Nov 29 |
First 36° | Oct 6 | Oct 11 | Oct 15 | Oct 19 | Oct 22 | Oct 25 | Oct 28 | Nov 1 | Nov 7 |
Now that you know your frost dates, use our Garden Planting Calendar for Newhalem, 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.