My son would like to use part of my greenhouse to grow myrtle or ivy topiaries throgh the winter. Problem is my greenhouse has poor heat and low light, often going below 50 F, but never freezing. It can also gets pretty damp and buggy as I use it for overwintering tender perrenials/tropicals. Will either myrtle or ivy grow enough to train over the winter in these conditions? Thanks.

To induce a growing period, you will need to successfully imitate light and temps of the usual growing season and this will only work with tropicals or tender perennials. These type of plants are used to growing all year round and don't neccesarily need the rest period.

I believe once the temps drop and the amount and instensity of light decreases, hardy plants will 'rest' up in preparation for the upcoming growing season.

if you use tropicals and commit to growing under lights, and warming your greenhouse, then I think it is entirely possible.