Whatever I have handy to be honest. Usually blankets then tarps/plastic/emergency blankets on top. If you put the tarps first, all the moisture from the plants transpiring condenses and falls, soaking the plants, if you do blankets or sheets then tarps, the fabric holds most of that moisture and they stay mostly dry. I still give them a blow out after I uncover them... and you don't want direct contact with the buds if it can be avoided. Look at this hillbilly shit... clothesline and pins, tiki torches, tent poles... fuckin MacGyver deluxe bhuuud.
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The black cherry I had to do plastic and just dry her in the morning, she was just too big. I think I added some sheets the next time, but she was still wetter than I like... but not frozen. That's my blanket fort before adding the plastic.