We all know frost by sight. It’s so obvious, right? It’s that collection of pretty (if you don’t have tender annuals or vegetables that can be killed by it) ice crystals that have the potential to be deadly to “frost tender” plants. It tends to be most obvious from a distance on grassy sites. 

Not surprisingly, when night air temperatures are close to 32°F, cold air sinks and settles, which is the perfect recipe for frost to occur. If you want to be a successful gardener, it’s worth knowing that frost is most likely to transpire on a cloud-free night when the air is perfectly calm (no breeze or wind). Therefore, if there’s a lot of cloud cover or wind, frost is less likely to occur. Also, frost will occur first out in open areas where there is no tree cover. Plants near heated structures, under overhangs or trees, may receive just enough additional protection that frost may not damage them. 

Okay, so near 32°F is the crucial temperature for a killing frost on plants that are frost-susceptible. Temperatures at 28°F are what plant people call a “hard freeze”, and that seemingly small difference of 4°F will damage a much wider range of plants than just “frost”.

In the Chicagoland area, our average last frost date is May 10^th. What does that mean for spring planting? It means we need to be careful about rushing the season. Just because it’s been 65°F during the day, doesn’t mean it won’t frost at night. To be clear, “average last frost date” is just a guideline, not an absolute, but it’s a nice tool to keep in mind to possibly save yourself early spring planting heartbreak! 

Trust me, you’re not gaining anything by planting frost tender plants like tomatoes, peppers, begonias, salvia, basil (and many more listed below) far in advance of the frost-free date. Even with warm air temperatures, soil temperatures can still be cold enough that those heat-loving plants won’t be growing, progressing or very happy- even if they aren’t frosted!!! For those that insist on challenging that wisdom, I would say:

• “Have plenty of things available to cover and protect on cold nights” 

After those stern warnings, I wanted to share some helpful info so you know which plants are frost-tolerant and which ones are frost-tender.                      

Soil temperatures can be useful indicators of when it’s safe to plant certain “categories”. If you don’t have a soil thermometer (who does, even though they’re available and inexpensive at good garden centers?), starting April 1^st you can check Chalet’s home page (www.chaletnursery.com) for the current soil temp that we update twice weekly through May 15^th.     

By knowing and observing our favorite plants’ temperature tolerances we can forget fearing frost forever!