Hi Nancy,

Thank you for contacting the UNH Cooperative Extension Infoline with question about how to overwinter your rosemary plant.

A fact sheet about herbs prepared by UNH Extension offers this advice about rosemary plants.  "Other perennial herbs, such as rosemary and bay, are not hardy in New Hampshire. Grow these and other tender herbs in containers that can be set out in the garden during the frost-free growing season and overwintered in a sunny indoor location."

https://extension.unh.edu/blog/2018/05/growing-herbs-containers  This fact sheet provides information about how best to grow herbs indoors including specific information about rosemary.

Here is additional information about rosemary plants.  "Tender perennials such as lemon verbena, bay, rosemary and scented geraniums must be taken into the house or a warmer location in cold weather. To avoid transplanting shock, grow them in pots year round, so they can be brought indoors easily in the fall.

Potted perennial herb plants can be saved and moved indoors a few days before frost. First move them to a shaded location (porch) so they start to adjust gradually to the change. Then, before bringing them indoors, examine the plants for any insect infestation. Once indoors, treat them as houseplants. Rosemary is an evergreen and will stay green all year."

Below it a segment from an UNH Extension blog that discusses the question you asked.

Lisa asks: A friend gave me a rosemary plant from his herb garden. How do I keep it happy through the winter -- it seems to dry out quickly.

Emma: Keeping rosemary alive through the winter is a real challenge. One of the main keys to success is starting with a plant that was grown in a container all season long. Plants that are dug out of the garden often don't survive due to the stress of repotting. Rosemary plants also need to be placed in the proper location inside the home. They need bright sunlight and should ideally be set in front of a south facing window. We have also found that growing rosemary in a cooler room, somewhere between 45 to 55 degrees F, can improve survival. Watering rosemary is where things get the most difficult. It is important to wait to water until the potting soil is very dry, water well enough that water drains from the bottom of the pot into the saucer. You can expect to have to water less when plants are kept at cooler temperatures.

Rob asks: Leaves curling on rosemary - beginnings of mites, but treated with neem oil. Moisture problem maybe?

Emma: Curling leaves is often a sign of moisture stress. Watering is the single most difficult aspect of keeping rosemary alive through the winter. Plants often dry out quickly and can suffer from drought, but they will also rot if they are kept too moist. We recommend keeping the plant in a room that stays between 45-55 degrees F, to reduce watering needs.

Another option for dealing with spider mites is to spray the plant foliage with a heavy stream of water every day or so. This will knock the small mites off of the leaves, and they are often unable to successfully make it back to the plant.