Not sure if that's a good idea. They don't realy like wet feet. Perhaps "flowing" a few times per night may work better.
(Don't know for sure. Never tried that)

i have simalar circumstances and now that time has past scince your posting, what did you do and how did it work out?

How about an aquarium heater for the reservoir?

I did put in an aquarium heater and set the timer to flow a little more often. I put in two small electric heaters for night time. It worked okay for a few nights and then as the outside temps. plummeted to -15 degrees I thought I would put in an extra heater. Sometime in the night the third heater tripped the breaker and I lost one other heater and the aquarium heater and the water pump. Lost all the brocolli! I am looking for a solar pool cover for next year. I decided to just wait a couple months and then start with my regular garden veggies for the summer. I hope for better success next winter.