We are reaching that time of year when we all watch that morning low temperature very closely. Some of us like to plant our precious vegetables a little early to get a jump on the season but if you are a cautious gardener you may want to hold off a little longer than some.
If you search the web for the average date of the last frost for Boise you may come up with everything from May 8th to May 26th. Tonight I talked to a long time gardener who uses the date of April 22nd but he also says that he does need to cover his vegetable from time to time.
Our temperatures vary from year to year so we need to look at averages or what the National Weather Service call "normals" for an area in order to quantify what is most likely to happen.
The NWS chart below shows we have a 50/50 shot of hitting 36 degrees on by May 26th. 36 degrees is used for the critical temperature in which frost can form on the ground. Official temperatures are recorded at ~5 feet above the ground so 32 degree readings can be present at ground level. Many gardeners use this date as the "safe" date meaning by this point you should be frost free. This is not techincally the case but you can decide how much risk you want to encounter by planting earlier.
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% Probability of Indicated Temperature or Lower on or After Date | |||||
| 90% | 75% | 50% | 25% | 10% | |
| 36 | May 10 | May 13 | May26 | June 8 | June 14 |
| 32 | April 17 | April 26 | May 10 | May 18 | May 27 |
| 28 | April 3 | April 13 | April 24 | May 5 | May 12 |
| 24 | Feb. 21 | March 4 | March30 | April 15 | April 20 |
| 20 | Jan. 26 | Feb. 5 | Feb. 18 | March 4 | March 25 |
Here are the charts for some other areas around the valley:
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