Propogating hydrangea salvia

When placing a bag over cutting to prevent moisture loss , should it be air tight eg a twist of wire around top ridge of pot ?

 

It depends on how much attention and energy you want to spend on the cuttings. If you keep it air tight then you have humidity but if there is to much moisture, the cuttings can rot. I personally leave it a little loose, with a some air flow, and then just control how much water there is in there by looking at the condensation on the bag.
Hydrangea are very easy and don't even need a cover if you mist them enough. I have now moved on to placing cuttings in a normal propagator with clear plastic lid because they get more air flow and have less problems with rotting if I get the watering wrong.

 

I, too, use a small propagator with a clear lid that has air holes which can be open or closed. You can up the moisture level by lining the base with a piece of capillary matting and keeping this damp.

 

I put individual pots of cuttings in plastic bags and fasten the tops loosely with twist ties or elastic bands so that there's a little bit of ventilation. If it gets too damp in there it's easy to open up the bag for a while and then fasten it up again. If I have a number of small pots, I stand them in a tray with a rigid propagator top with ventilation thingummies that you can adjust.