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