I've been given the instructions "plant young plants only after the ground temperature reaches 70°F (21°C), about 2 to 3 weeks before the last frost."
I looked up ground temperatures in my area and it never goes above 60°F (15.5°C), measured 1m (3ft) down. So how are you supposed to measure the 70°F (21°C)? On top of the soil, 10cm (4in) below, or above or how?
The way I understand this instruction, I can never plant these. Or at least, it makes no sense to expect these temperatures 2-3 weeks before the last frost.