First, 33F is not a hard frost, it's not even a frost at all (must below freezing or 32F for it to be a frost) A "hard frost" is considered to be a sharp drop in nitghtime temperatures where both the air AND soil temperatures drop below freezing....something in the mid to low 20's at this time of year could possibly qualify.

You never want to put plastic over a plant with foliage (especially tender, new foliage) and leave it on during the day as well. Just like a greenhouse, daytime temperatures and sunlight will cause the temperature under the plastic to escalate drastically and you will burn your plants. If you want to resort to any plastic for protection, put it on at night and remove it in early morning, otherwise use a woven material made for this purpose, often sold as remay or harvest cloth. But if you are looking at temperatures only in the 30's, you don't need to bother at all.