[BennB]

The main issue is usually night time temps, which a bag won't help with and I would think you would be asking for mold/fungus problems with high humidity during the day and cool at night.

I agree that if if you try it, either open the tops or make sure there are some good sized holes in the side to let out excess heat and humidity.

I live in Seattle, where it is still cool too and I don't like to put plants in the ground until the end of May, but if I wait too long I won't get any fruit.

I second the wall of water route, an old standby and a good option. You can buy them in 3 packs on-line for about $10.

I was also wondering how well a couple layers of bubble rap would work, and it is good to see somebody else has had success with that!.

But the big issue with any of these approaches is that plants are still in the cold soil and if you get a cold snap that lasts for more than a night or two, you could be in trouble.