I understand that not all cultivars of rosemary are hardy enough to survive our zone,
and read that it’s possible to hard prune rosemary in the fall and force it into dormancy. My question is: How cold does it need to be kept to remain in dormancy? How warm does it need to be kept to stay alive?
Termed a “tender perennial” Rosemary (Rosmarinus officinalis) can be hardy to about 14 degrees F (-10 degrees C) perhaps more with sharp drainage.
Mark Cullen in The New Greener Thumb sums up the position succinctly. “It can withstand frosts and cold but is unlikely to survive winter in climates harsher than Zone 8”.
Rosmarinus officinalis ‘Arp’ is the hardiest cultivar surviving in zone 7.
Native to hot, dry climates Rosemary hates the cold and wet of northern winters and is more likely to die than remain dormant if left outside, cut back or not. If we have winters where the temperatures did not dip below freezing then it might stand a chance to survive but until that time your best strategy is to bring it inside for the winter as soon as temperatures dip to freezing and keep it inside until all danger of freezing is over.
In The Container Gardeners Bible you will find the recommendation “Rosemary does not tolerate freezing temperatures so it must come inside for the winter. But that shouldn’t stop you from letting it grow into a shrub; put its pot on a wheeled platform to move it around as the weather decrees. Rosemary plants are killed by excess watering more frequently than they are killed by neglect so make sure the container potting soil drains freely.”
Good luck with this tender perennial.