What are you planning on doing with them come spring? Roses do not do well as houseplants in the normal home enviornment. Even with grow lights year round they will not do well at all. In winter you actually want them to loose their leaves and go dormant. I'm not in your zone so can't speak to your winter conditions, but if you are trying to grow minis as house plants, it's usually a loosing proposition.

I would like to plant them in the ground next spring.
What is the best way to get them to go dormant this late in the year?

Hilary,

Do you have an unheated garage? If so, you could put them in there for the winter and they will go dormant.
I start many roses from cuttings and I've put them in my garage for the winter, watering lighly only a couple of times, or I've put them in a spare refrigerator for the winter too, with great luck. Come spring, set them outside and gradually acclimate to the sun, and plant them in their permanent homes.

Mindy

Thanks so much for the info! I have an unheated garage, and I will take your advice.

Thanks Again!

I found one in the trash in the fall (in southern Michigan) and brought it back to life. Now in August, it's dying, most likely because I didn't let it go dormant. Is there any hope?

If you are keeping it indoors, it isn't getting enough sunlight or humidity. It isn't because of not going dormant (roses grow in tropical areas). Roses are outdoor plants, it is difficult to provide the light they need by keeping them indoors. Also, it is difficult to keep the moisture level correct.