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 June 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 26, so we don't dare plant that early. We wait a few days and by May 10 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 14 | Jan 30 | Jan 19 | Jan 7 | Dec 25 | Dec 2 | -- | -- | -- |
| Last 20° | Feb 21 | Feb 10 | Feb 2 | Jan 25 | Jan 18 | Jan 10 | Dec 30 | -- | -- |
| Last 24° | Mar 3 | Feb 23 | Feb 17 | Feb 11 | Feb 6 | Feb 1 | Jan 27 | Jan 20 | Jan 10 |
| Last 28° | Apr 7 | Mar 29 | Mar 22 | Mar 17 | Mar 11 | Mar 6 | Feb 28 | Feb 21 | Feb 12 |
| Last 32° | May 16 | May 8 | May 2 | Apr 27 | Apr 23 | Apr 18 | Apr 13 | Apr 7 | Mar 30 |
| Last 36° | Jun 1 | May 24 | May 19 | May 14 | May 10 | May 6 | May 1 | Apr 26 | Apr 19 |
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 6 | Dec 21 | Jan 3 | Jan 15 | Jan 30 | -- | -- | -- | -- |
| First 20° | Nov 25 | Dec 8 | Dec 17 | Dec 25 | Jan 3 | Jan 12 | Jan 24 | -- | -- |
| First 24° | Nov 6 | Nov 17 | Nov 25 | Dec 2 | Dec 9 | Dec 15 | Dec 23 | Dec 31 | Jan 14 |
| First 28° | Oct 23 | Nov 1 | Nov 7 | Nov 13 | Nov 18 | Nov 23 | Nov 28 | Dec 4 | Dec 13 |
| First 32° | Oct 7 | Oct 13 | Oct 17 | Oct 20 | Oct 24 | Oct 27 | Oct 31 | Nov 4 | Nov 10 |
| First 36° | Sep 21 | Sep 25 | Sep 29 | Oct 2 | Oct 5 | Oct 8 | Oct 11 | Oct 14 | Oct 19 |
Now that you know your frost dates, use our Garden Planting Calendar for Forest Grove, 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.
