Spring Frost

Mornings in April start with negative temperature, mainly due to high elevation (1180m above see level) and my tunnel does not really offer much protection.  I am stuffing  one gallon containers (8 so far) filled with water in  ope that this will provide some relief form low temps in early morning.  Was thinking about creating some solar heat pump/collector and accumulator using the soil in the bed but my bed is not deep enough to placing pipes to heat the ground, maybe will have to come up with different process for storing heat during the day and increasing heat mass.  I guess, will have to do some research and experimentation in the near future.