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 7, so we don't dare plant that early. We wait a few days and by April 21 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° | Jan 28 | Jan 13 | Dec 30 | Dec 8 | -- | -- | -- | -- | -- |
Last 20° | Feb 3 | Jan 21 | Jan 10 | Dec 29 | -- | -- | -- | -- | -- |
Last 24° | Feb 21 | Feb 12 | Feb 5 | Jan 30 | Jan 25 | Jan 19 | Jan 12 | Jan 3 | -- |
Last 28° | Mar 15 | Mar 4 | Feb 24 | Feb 17 | Feb 11 | Feb 5 | Jan 29 | Jan 21 | Jan 10 |
Last 32° | Apr 22 | Apr 12 | Apr 4 | Mar 29 | Mar 23 | Mar 17 | Mar 11 | Mar 3 | Feb 21 |
Last 36° | May 14 | May 6 | Apr 30 | Apr 26 | Apr 21 | Apr 17 | Apr 12 | Apr 7 | Mar 30 |
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 17 | Dec 31 | Jan 14 | Feb 3 | -- | -- | -- | -- | -- |
First 20° | Dec 15 | Dec 28 | Jan 8 | Jan 20 | -- | -- | -- | -- | -- |
First 24° | Nov 27 | Dec 10 | Dec 20 | Dec 29 | Jan 7 | Jan 16 | Jan 27 | Feb 12 | -- |
First 28° | Nov 12 | Nov 21 | Nov 27 | Dec 2 | Dec 7 | Dec 12 | Dec 18 | Dec 24 | Jan 1 |
First 32° | Oct 21 | Oct 30 | Nov 5 | Nov 10 | Nov 15 | Nov 20 | Nov 25 | Dec 1 | Dec 10 |
First 36° | Oct 5 | Oct 11 | Oct 15 | Oct 18 | Oct 21 | Oct 25 | Oct 28 | Nov 1 | Nov 7 |
Now that you know your frost dates, use our Garden Planting Calendar for N Willamette, OR 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.