Soil temp is where pots have the advantage over in-ground. The soil in containers warms really quickly compared to the ground. This is why I can plant my toms much earlier than my friend at the farm. Even when the surface seems warm enough, I have found it plenty cold still at the bottom of that planting hole!
Another trick for cold soil is to warm it up with hot water before planting. The heat from the water will dissipate into the surrounding soil and raise the soil temperature overall.
I want to try warming the ground at my mom's with black plastic this year, at least before planting (she's a bit fussy about the look of things). That is an impressive temperature difference!
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