I cant tell by the pics you posted but if they are close enough maybe you should consider covering them with a pvc frame and 6 mil plastic, make a little hoop green house. That will give you maybe 5 or 10° warmer. You can get a small heater that run off propane bottles and run it on the cold nights. Here is a link to one:
http://www.righttoolusa.com/p/Coleman-BlackCat-PerfecTemp-Catalytic-Heater-1020328.html
Good part about it is at the same time its heating its giving off co2.
Just thought I would add you would have to leave it on its lowest setting cause those things really heat up a small area.
I use one in a 12x18 room and it warms it up about 5° in a 1/2 hour or so. On low the canister lasts about 8 hours.