Rhubarb: A light frost will not harm rhubarb. However, if temperatures were cold enough to cause the rhubarb leaves to wilt or become limp then damage has been done and such leaves should be removed and discarded. Any new leaves that appear and are normal can be eaten.
Remember that the leaf blade of rhubarb is poisonous regardless of whether it suffered cold damage as it naturally contains oxalic acid. The leaf stalk is the edible portion of this plant. However, when leaves become frozen, the oxalic content of the stalks increases, making them dangerous to consume. You can find more information on rhubarb at: http://www.bookstore.ksre.ksu.edu/pubs/ep99.pdf
Asparagus: Asparagus does not contain poisonous substances but frost will cause the spear tips to wilt and give them an off flavor. Remove and discard any spears that show such damage. We also have a guide sheet on asparagus at: http://www.bookstore.ksre.ksu.edu/pubs/mf319.pdf (Ward Upham)