This SUPERthrive mix/solution has repeatedly show to will help the plants through the cold Hill Country Texas winters. Do not add liquid fertilizer as the nitrogen ingredients that make up the fertilizer are more likely to freeze and cause capillaries in the plants to freeze and possibly burst - thus either killing or significantly damaging your plants. Keeping plant beds moist will also be beneficial as it typically does not freeze deep into the soils as it my in northern areas.
Plastic covers on most plants will cause more damage than benefits. Light sheets or cloth tarps are much better in protecting the plants from the chill and allow the plants to breath vs. sweat.
Wrap extra sensitive plants with incandescent mini-holiday (not LED) lights under tarps or without tarps as they provide some warmth and not burn the plant or the tarps.
A good hint is to stop organic fertilizing of plants around the mid-September time frame (synthetic fertilizer should stop mid-August or earlier) and and do not begin using fertilizers again until after the potential of the last freeze. Low nitrogen organic fertilizers can usually being treatment use after mid-March.