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