Staghorn Fern Platycerium

Mitch is right. Generally Staghorns are grown in shady areas, under the eves or under a tree, perhaps a shade house. If we don't expect the temperature to be below 29 - 30 degrees, they will probably be OK. If there is a 'heavy frost warning' where the temperatures may dip to say 25 degrees, I would try to provide some additional shelter.
A simple way is to drape an old bed sheet over them, (plastic sheeting is not as good) it does not take much to help ward off frost damage. If it is predicted to be 'very cold' you might think about rigging a 100 watt light bulb under the sheet. Be sure the sheet does not touch the bulb.
Some depends on where you live. If you are in a low area, it tends to get colder, higher areas not quite so much. At our nursery in Poway (kind of up on the hill) we have never had any damage from frost on our Staghorns. But lower areas can be cloder. About 5 or 6 years ago, it did get quite cold and damage was very evident in the lower areas, I notieced some Phoenix robelini palms on Pomerado Road that were very brown (the lowest part of Pomerado) Light frost should not be an issue, just watch the weather forcasts and plan accordingly.

My staghorn fern is starting to look lousy, I live in Poway, in the mobile home park, I covered up my staghorn with a trashbag, now I'm looking at big brown area, like a rust color on my leaves. Will it be okay. Is this from the cold. I did purchase Gro Power Tablets a few months ago and put a few on top of the moss. I hope that didn't harm anything. Any suggestions would be greatly appreciated.

Next time you cover your staghorn, don't use a trashbag; it doesn't do much in the way of insulating and it could have caused that brown spot if the bag was touching that part when the cold hit. When you cover plants in order to protect them from frost damage, it's important to elevate the cover so that it doesn't touch the plant leaves--so temporarily insert stakes or something into the fern in order to elevate the sheet. Also, it is very important to remove the sheet first thing the next day, as things could get too hot in there for the plant if the sun hits it. This is another thing that could have happened with the brown spot--if the sun hit the staghorn in the morning and the plastic was still on. As far as fertilizer tablets, did you put them in the area that is now brown?

I put them at the base of the fern where the moss is, just threw it on top of the moss. I put 3 tabs in it.