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