Overwintering your plant is a great way to save rosemary topiary.
Question: How should I overwinter my rosemary topiary? I live in USDA Zone 5b.
Answer: Rosemary is not hardy in your area, so you'll have to overwinter it indoors. Rosemary is fussy about its winter quarters. If yours had been a regular rosemary plant, I'd have suggested just taking cuttings and rooting them indoors, but a topiary plant is a more valuable specimen. Since they take so long to grow, it's worth the effort of overwintering the whole plant.
Rosemary prefers an alkaline soil. If you used regular soilless potting mix, or your topiary is growing in the ground, repot the plant, this time incorporating a little crushed limestone into the mix. Position the plant with the top of the roots slightly exposed; cover the last half-inch with coarse sand instead of soil. This ensures excellent drainage near the crown.
Place the topiary in a cool sunroom or enclosed sunny porch. Temperatures should drop into the low 50s at night, and reach near 70 degrees during the day. Water just enough to keep the soil damp at all times, and use pebble trays to keep humidity high.