https://earthsky.org/earth/why-dont-evergreen-trees-change-drop-leaves/

The above is the easily Googled answer to your original question before you edited your question into your inability to overwinter Ipomopsis.

Biennial plants that form a rosette - and some perennials that overwinter as a rosette of leaves usually do not like mulch of any kind placed over the leaves during the winter. Putting plastic over the plants during the winter probably isn't a good idea, it would heat up excessively on mild sunny days and provide no protection during freezing temperatures. The idea is to moderate the extremes of temperatures during the winter when overwintering plants in a cold climate, not to keep them warm!! Once it gets cold, outdoor plants should stay cold, and not experience wide fluctuations of temperatures.

What is probably the most important factor in overwintering borderline perennials or things that tend to die during the cold months is DRAINAGE. Ipomopsis rubra is native to dry, sandy or rocky soils. Plant it in an area where the drainage is good and you won't have to fuss with it.

To answer your original question, evergreen plants DO photosynthesize during the winter months in a cold climate, but the metabolism of evergreen plants is slowed down considerably during colder weather - at freezing temperatures or below the process probably stops.

I would lean toward covering with a fairly generous amount of leaves held in place with flexible sticks stuck into the ground to form a low mounded dome.

The above is the easily Googled answer to your original question before you edited your question into your inability to overwinter Ipomopsis.

(i added to the post to give context to the subject line.)

What is probably the most important factor in overwintering borderline perennials or things that tend to die during the cold months is DRAINAGE.

For sure (as I acknowledged in my post). I know very well that it isn't the cold that is the deciding factor for the survival of these plants in my location.

evergreen plants DO photosynthesize during the winter months in a cold climate

Yup. I googled that.

Hey 'peren'. What I think I will do is mound lots of leaves (as you suggested) over a smallish area which has many rosettes and then on top of this I will lay the thin plastic. (Leaves then plastic is exactly how this individual overwinters his in the ground Salvia Guaranitica i.e. https://www.youtube.com/watch?v=TESG_ilxT9g)

(I have so many of these first years throughout the garden that I think I can safely do this 'experiment' for if it fails and the rosettes in question die I still have lots to fall back on...fingers crossed.)

(I should have counted how many 2nd year plants I had this past summer....probably between 6 and 9. I so like having them in our garden as they are unusual to see in my neck of the woods and hummingbirds enjoy them.)

(From mid July 2023)