I am guessing you mean perennials growing in outdoor pots as container plants.   The thing that would kill them is winter wind hitting the sides of the pots and freezing the roots, which are more exposed than perennial roots in the ground. I am assuming these are winter hardy plants, which is the definition of perennaials, and not tropical plants. If the plants are winter hardy, what you need to do is get them out of the wind. You could do this by putting them in a sheltered spot near the house or garage or in the garage, clustered together to protect each other. Cut off the top growth first, however, since it won't be photosynthesizing.

If they are not cold hardy, such a annual geraniums, bring them indoors and cut them back, but not all the way, and put them in a basement with some windows or another cool room with some light. Don't water them because you want them to go dormant.

They will not be okay with the average depth, unfortuately. They would just be buried alive if you did not move them. So dig up the hostas and replant after adding the topsoil. This is a lot of work, and so I suggest you divide them while you have them out of the ground so you will have more hostas. You do this by pulling apart the roots while they are out of the ground just before you replant them. Then replant each section of root about three feet apart from the others. If you do not want more hostas, just keep them whole and plant them back where they were.

I notice you used the word topsoil. Are you doing the work yourself? If you need more than a few bags worth, consider getting a yard delivered by Mass Naturals of Winchester. Their topsoil is very good and they deliver. A lot of what other companies sell as topsoil is really just fill, lifeless or full of weeeds. It's worth it to get good topsoil and not something that is under par. Your hostas will grow much better in good top soil, too. But then hostas will grow in almost anything.

If your lawn drains into a wetlands system, and many do, you should have an organic lawn. That means no Scotts turfbuilder. Paul Tukey has a new book out that is quite good on how to have an organic lawn.

 The biggest threat to your pond would be applications of nitrogen to "green it up" in the spring. Most of this runs off without being absorbed. When it hits a wetland it  spurs algae blooms which suck  the oxygen out of water so the fish sufficate. Very bad.

I do not fertilize my lawn at all, and it looks fine. The roots are deeper and more drought resistant and the grass forbs are tougher because they have not been coddled. I don't fertilize my flowers either. Ads for fertilizer companies have sold us on the idea that we need fertilizer.

The best way to fertilize your lawn is to use a mulching lawn mower which lets mitrogen rich grass clippings remain where they fall when cut. You can also apply a finely pulverized weed free organic fertilizer such as porcessed manure or sifted compost spread evenly over the lawn just before rain is expected. The best times to do this is in April and September, just before the two annual lawn growth spurts.