It can definitely be a bit tricky to get your Rosemary to survive the winter in climates where it can't grow outside. There are, however, some tips and steps you can follow to give your plant the best possible conditions:
If you live in a spot where the temperature doesn't get below freezing during winter you can keep your Rosemary outdoors during the colder months as well. If so just make sure that it has excellent drainage, that it gets the amount of light it needs and that the soil is kept on the dry side throughout.
If you live in a spot where the temperature gets below freezing you need to winterize your Rosemary according to these steps:
Check the soil Before moving your plant from where it has been during spring / summer, make sure the soil is dry to avoid mold growth.
Check for pests When your plant is moved to a new location and proceeds into the winter months, it might get a bit weak. Its normal resilience against pests is therefore lower. Make sure to check that your plant is healthy and free of pests before moving it to its winter location.
Lower the temperature Place your plant in a cool room. For example, a garage, shed, basement, attic, summerhouse or a cool stairway if you are living in an apartment. Recommended temperature for this plant:
Add extra grow light Pick a full-spectrum light bulb and place it 20-30 cm / 8-12 inches above the plant to prevent leaf burn. Keep the light on for 12-16 hours / day.
Reduce watering Cutting back on watering during winter is key to helping your Rosemary to survive. The amount of water it will need will depend on the amount of light it will get and what temperature it's placed in. But in general: it's better to water it a bit too little during winter than a bit too much (since Rosemary can recover from too little water pretty fast compared to too much water).
Keep an eye on it Keep checking your Rosemary from time to time to be able to stop any infestations at an early stage. Rosemary is especially prone to getting White flies and Spider mites if the temperature is a bit too high. It's also important to know that it isn't normal for Rosemary to drop leaves during winter, so if yours start shredding: something is going on! The most common causes are too much or too little water, too little light and too high temperatures. So, check the soil and the roots and make sure that everything is okay regarding the amount of light and temperature conditions.
“Is it starting to get leggy? Provide more light and if needed place it under a grow light to keep it happy during the colder months." — PLANTA TIPS