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 1. 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 6, so we don't dare plant that early. We wait a few days and by April 16 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 31 | Jan 15 | Dec 30 | Dec 7 | -- | -- | -- | -- | -- |
| Last 20° | Feb 11 | Jan 28 | Jan 17 | Jan 7 | Dec 25 | Dec 4 | -- | -- | -- |
| Last 24° | Feb 22 | Feb 13 | Feb 5 | Jan 30 | Jan 24 | Jan 17 | Jan 9 | Dec 28 | -- |
| Last 28° | Mar 16 | Mar 6 | Feb 27 | Feb 21 | Feb 15 | Feb 10 | Feb 4 | Jan 28 | Jan 18 |
| Last 32° | Apr 15 | Apr 7 | Apr 1 | Mar 27 | Mar 23 | Mar 18 | Mar 13 | Mar 7 | Feb 27 |
| Last 36° | May 1 | Apr 26 | Apr 22 | Apr 19 | Apr 16 | Apr 13 | Apr 9 | Apr 6 | Mar 31 |
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 19 | Jan 5 | Jan 20 | Feb 13 | -- | -- | -- | -- | -- |
| First 20° | Dec 5 | Dec 21 | Jan 2 | Jan 15 | Jan 30 | -- | -- | -- | -- |
| First 24° | Nov 22 | Dec 4 | Dec 13 | Dec 21 | Dec 29 | Jan 6 | Jan 15 | Jan 30 | -- |
| First 28° | Nov 6 | Nov 15 | Nov 21 | Nov 26 | Dec 1 | Dec 6 | Dec 11 | Dec 17 | Dec 26 |
| First 32° | Oct 24 | Oct 31 | Nov 6 | Nov 10 | Nov 15 | Nov 19 | Nov 23 | Nov 29 | Dec 6 |
| First 36° | Oct 12 | Oct 18 | Oct 22 | Oct 25 | Oct 28 | Oct 31 | Nov 3 | Nov 7 | Nov 13 |
Now that you know your frost dates, use our Garden Planting Calendar for Portland, 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.
The Victory Seed Company is family owned and operated, working to help fellow gardeners succeed by selling popular vegetable, herb and flower seeds. Since the 1990s they have been heavily involved with the preservation of rare, open-pollinated, heirloom seeds. Visit them at www.VictorySeeds.com.
