How do you protect outdoor potted plants on a screened in porch?

Are you allowed to semi shelter your plants loosely with plastic sheeting? I have used a tomato cage that you can purchase at a nursery or Walmart in the spring. Slide the cage over your plant(s) in their pot. Drape and semi secure plant plastic wrap around the cage. Place in sunlight not shade. Water as necessary. Use twist ties or clothes pins to secure your temporary green houses with the plastic wrap. I couldn’t tell where about you lived. This should do the trick. Don’t forget to read a page everyday out of your Farmers Almanac.

Don't let the plastic touch the leaves of the plant, if it gets really cold, it could damage the leaves, flowers, etc. Where I live, you have to take in what you want to keep in pots, but when it is cold only for a few hours overnight, I cover them with sheets with good luck. Potted plants are susceptible to the roots freezing in pots, so try to keep them out of the wind if possible and if it will be a freeze for a more prolonged period, bring them in until it is warmer out. My hibiscus are living in the garage right now, until we can rearrange our back room/library to fit them all in for the winter along with other p9ots that we had outside that we want to keep. I hope this helps you overwinter your plants, Marie!

I'm assuming your "outdoor" plants are not winter hardy. Some tropical plants will die down over winter and come back out in spring. Others can stand cool but not cold temperatures, so an enclosed porch could be adequate. Still others cannot stand for the temperature to drop below 60ºF and are better off being brought indoors. Do a search on each of your plants for this info.

Which way your porch faces determines whether it's worth winterizing. A south-facing porch is ideal, and east and west facing are OK. North-facing porches never get any sun and stay cold, even on a mild, sunny winter's day. If your porch faces north you should bring your potted plants in and set up some florescent lights.

To winterize cover the screened area with plastic. Most building supply stores sell a utility weight plastic that is good for one season. But some types are better than others: https://www.doityourself.com/stry/4-types-of-greenhouse-plastic-to-use

Before you install plastic or any other weatherizing material you must get your landlord's permission, because you will need to put some holes in the porch. The plastic is usually stapled to the porch studs, though I suppose you could try duct tape. A better way to attach the plastic is to sandwich it between the porch studs and a 1x2" furring strip to protect the plastic from wind stress. No matter how smooth and stretched you install the plastic, storm winds will cause it to flap, and that pulls the plastic against the staples which causes holes. The furring strip spreads out the wind stress. Ideally you would also place plastic over the outside of the screens to get a dead-air space between. You may also need weatherstripping around the patio door.

Plastic won't keep the porch warm by itself, so you will probably need a heat source. However, running an electric space heater is expensive. And you should never use an extension cord, so it's best to have an outlet on the porch. A small propane heater is much cheaper to run. Do a search for types and sizes.