Is there something I can spray on my plants to provide protection during late spring freeze events such as potassium, sugar, or carbohydrates?
The simple answer is no, there is not something that can be sprayed on plants to give them protection from late spring freezes. However, there are all sorts of trials and testimonials using foliar potassium (Nutri-K, Agro-K), carbohydrates, and sugars sprayed onto plants a few days prior to cold temperatures setting in to help plants survive. Each product is basically acting like antifreeze (prevent nucleation) in the cell to prevent internal freezing and cell bursting. It is very difficult to get good data on farms on in research field trials to verify when/how/if these products actually work. Conditions vary so much from site to site and year to year that we don't have replicated data to stand by (remember that ISU is reserach based guidance). At this time, I cannot say there is solid evidence of any product being affective consistently and in replicated in trials. A lot of factors go into it including timing, product, and rate (and probably a bunch of other things). It appears there are other products (see recent ISU lab research with salicylic acid on lettuce) that can be applied to provide protection but nothing I am going to go out on a limb to recommend at this time (again because I don't have solid research to back me up here).
It has been done and tried before. Nutri-K and Agro-K are readily available locally. Carbohydrate and sugar solutions intended for foliar application can be found by many hydroponic suppliers. Salicylic acid (must be food grade) can be purchased in bulk quantities from food manufacturing company/suppliers. I do not know what rates these products would need to be applied at, timing of when they would need to be applied, and I do not know what side effects might occur if you did spray any of these products on your plants in hopes of providing freeze protection.