How are things going? If what you planted is reliably hardy, no protection should be needed, especially if put in ground in spring. Where the ground does not freeze, heaving is not a problem. Attempting to keep plants that should go dormant in vigorous growth mode at the wrong time of year could be extra stress, not helpful. Unlikely in Z8 but worth mentioning. Couldn't tell if you are talking about plants in pots or in the ground? What plants? If you are zone-pushing Z9+ plants, I don't think I would use any kind of plastic. A pile of leaves is my usual method. Some paving bricks adjacent to the roots (on S side,) or dark-colored jugs of water, dark rocks, can also help keep the ground slightly warmer. Regardless of method, excellent drainage is usually a key factor in successful zone pushing. Wet + cold = rot. Dry + cold = exponentially higher chances of survival.

"...will the seedlings be destroyed by the freezing temperatures (even down to -10 F.)?" Yes they will. The first thing I would do is move them to an area where the pots will be sitting on the ground. Ground does not conduct cold as does concrete and provides a medium to insulate roots. Next, you need to 'insulate' them as though they were in the ground. One easy way is to throw leaves (or anything of the like) on the lot and hide them for the winter. If you can tip them so that water does not accumulate in the top of the pot and freeze do so. You don't want that to happen. I will use my veggie garden for this in late fall and bundle the pots with soil from the garden. Then I take shredded leaves and just throw them on top hiding everything. Another good place to keep them is up against the house. I have pots lined up under the veranda up against the basement wall. These are not 'tipped' since they get just a bit of snow and rain every now and then. The outer exposed edges of the pots are snuggled by bags of leaves and the bottoms rest on fine gravel. The issue with seedlings in pots is that they have small roots and should those be in the ground they would have a higher chance of survival since some roots would have reached below the frost line. In a pot they grow to the sides where things get cold in the winter. :O)

Melissa is right that this winter's continuous snow coverage is not a good test of mulch protection against the freeze thaw cycle. I also mulch plants in the spring to conserve moisture and discourage weed growth so most of my plants go into winter with some mulch around the base. Just not the 6-12 inches that is often recommended and none over the crowns. When some plants set seed I remove much of the mulch around them to promote self seeding. Then those have no mulch over winter. Like Helen I leave stems to catch snow and blown leaves. I should have asked if plants growing in early spring due to no mulch grow as well as those plants that have been mulched. Does the new growth on the uncovered plants get frozen off and therefore set them back compared to covered plants? It is difficult to know when the optimal time is to removed mulch. If not removed soon enough plants can get smothered or new growth under the mulch is so fragile it dies after being uncovered. I know that one is to remove mulch in layers but that rarely happens in my garden. I think Helen is right that one can lose plants if they are covered or not. So is it worth the work, both in the fall and spring? And the answer is probably different for each of us depending on how much risk we are willing to take. The reason for asking the question is that as I am getting older and my gardens are getting larger I want to cut back on the amount of work but not in areas where it is really needed.

If a plant is fully hardy for your climate zone, there is usually no need to provide any protection in winter. The exception is those plants that have already developed flower buds that can be damaged by prolonged cold temperatures, like some kinds of hydrangeas. Some perennials do not survive winter in colder zones (and many milder ones as well) because they will not tolerate wet soils or the combination of cold AND wet, as is often the case in winter. Providing exceptional drainage for these types of plants - salvias, agastache, echinaceas, etc. - will help. And some perennials are prone to winter damage from heaving or exposure of the roots during freeze/thaw cycles. Mulching well to insulate the soil will help. Making sure plants go into winter well-hydrated will help also - some are subject to drying out in cold winds and a moist soil (NOT wet) is a better insulator than soil that is dry. Other than that, tender perennials or borderline hardy plants should be protected. Google to find out the specifics for what you have. Otherwise, mulch and winter snow cover is all the winter protection you need.