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Tomatoes are very sensitiveTomatoes are very sensitive to frost; it will kill them. Even cold weather is harmful to their health, although it is not as deadly as frost. When you say that your plants "look sad" do you mean that they are limp and floppy or do sections of the leaves look very dark green and mushy like lettuce that's been in the fridge for too long? if that's the case... severe damage has occurred. It's a good idea to cover them with plastic sheeting or individually with inverted clear plastic jugs at night and uncover them during the day until all threat of frost has passed. As for will know for sure within the next week or two. Also, keep the plants and soil wet as the moisture raises the temp needed for frost to form.