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