Darting in cold temps has several issues to consider. Freezing meds, to shock and pneumonia in the darted deer. Just because someone did something once doesn't make it proper or smart practice. It would take too long to try and type out in one post all the factors to consider. Long story short, if you can wait for temps closer or above 32 then wait. Otherwise learn how to best care for the animals during the process. It can be done safely, but you should know how.