They are not dead. Their leaves have been frozen. Roots SHOULD be OK, however if they were left on the benches unprotected in 18 F temps, you may have some root damage. The leaves froze because the trees were/are still active. The tree isn't fully dormant, which means the roots are not fully prepared for deep freeze temperatures. This cold trough of air is dangerous because it has arrived early, before trees have become fully hardened off for it.
You Chinese elm was most likely damaged just as your maples have been. It's just not showing it yet. Chinese elms are less cold hardy than maples...
BTW, a plastic cover is the WORST thing you can use to protect trees. It traps condensation, which freezes on the under surface of the plastic against the plant, as well as on branches and twigs. both can cause dieback on those areas. Plastic also doesn't provide much, or any, insulation from cold.
Unexpected frosts can leave gardeners scrambling for anything to cover their...
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