Last average frost date was April 29th here in my zone, but May is an untrustworthy, fickle she-beyotch. Friday, I just started putting out my herbs in pots, and the basil had to come back in with me last night.
If you’re in the northeastern U.S. and Ontario, We have a frost warning scheduled (again) for tonight. If you’ve put them out in pots, bring them in, if you can.
If you can’t, the wisdom of the internet collectively agree on two important steps…
1) Water them thoroughly. The water helps insulate them from rapid changes and can help preserve a plant even if the water sprayed on the plant turns to ice.
2) Cover them. Bucket, upside down flowerpot, tarp, blanket, whatever. The freezing air is your enemy. Anything you use to cover a plant should come all the way down to the soil. This allows the thermal energy captured during the day in the soil to radiate heat back up into the cover and air around your plant, rather than back into the atmosphere.
In my very heavy planter, it appears that the watering was a sufficient step. I will note that my planter is up against the brick wall of the house, which would radiate some heat, as well as the planter itself has thermal mass built up in the large quantity of water stored in the bottom half. For day 2, since it’s been on the cold side both days, I think I may err on the side of caution and cover them… just in case.