People often ask where the warmest and coldest spots in Teton Valley are located. Especially now at this time of year, they wonder whether they should cover their plants against frost, which has already bitten a few Teton Valley gardens this year. The simple answer is yes, you should. Unless you have grown things on your plot of land for years and know it well, consider any forecast of low temperatures in the 30s F a potential threat to your green babies. Here’s why:
Weather measurements and forecasts are made for conditions at about a person’s head height above the ground. But down at your plant’s head height, the temperatures (and humidities and wind speeds) are probably quite different. Dark, bare soil or nearby asphalt will tend to create higher temperatures near the surface, especially on a sunny day. Lots of grass (or in my case weeds) will keep temperatures lower. On chilly mornings, cold air is denser and falls to where your plants live, so you might see 40 degrees F on your thermometer and frost on your cabbage. Unless you know exactly where those low, cold spots are and plan accordingly, these chilly nights might end your growing season any time now. After all, it is mid-September. Our average first freeze is almost a month away, but when have we had an average year around here lately?
Another factor that can pose a threat to your pumpkins is the proximity of tall mountains. Does anybody in the valley not have that? In the early mornings near sunrise, colder, heavier air comes rolling down the slopes into the valley below. It’s usually just a brief, very localized freeze, and maybe only thumb-height above the ground. Still, the mobile frost pockets rolling down from Fred’s Mountain were enough to make my tomato leaves recoil in horror a couple of times already this year.
Ironically, there are other places in the valley where down-slope winds keep things warmer than elsewhere. But even in those banana belts, it’s wise to have a plan like row cover ready to go at a moment’s notice. Wind is fickle, and even a small, brief change from usual can make a big difference in the temperature this time of year.
If you’re an avid camper, the same holds true for your favorite campsite. Especially if you bed down on the ground, have a plan, including a good closed foam mat below and warm DRY covers above, for a good night’s sleep. Goodnight!