We are happy to offer an over wintering service for lavender and rosemary plants, as we can provide these plants with the sunny yet cool conditions they prefer through winter. But what if you really want to keep yours on hand to harvest right through the winter?
1) Rosemary requires cool conditions through winter. Although rosemary comes from a warmer climate it does enjoy a contrast between the seasons. If you think about how it is in the Mediterranean through winter it is light jacket weather - especially at night. There may even be occasional frosts. One of the biggest challenges for rosemary indoors on the prairies is the forced air heating that can make plant spaces near a window too hot. The ideal location for a rosemary indoors is a cool sun room or large window (e.g. patio door).
2) Rosemary needs lots of light. This a full sun herb and the winter simply can be challenging on this front. Although rosemary handles the decreased hours of daylight just fine it is important to maximize sunlight by choosing a space where the sun reaches it with out obstruction. If there are no excellent windows available full spectrum grow lights are a great alternative.
3) Rosemary requires humidity... but not too much. Coming from the humid sea-side hills of the Mediterranean means that rosemary has evolved in a humid environment. During the dry winter months it is easy for the plant to become stress due to low relative humidity (made worse by the forced air heating). That said the plant can also run into trouble if it is misted; the water droplets can set-up the perfect conditions for powdery mildew to occur. So what to do? Keep your plant away from heat vents is a great start. Second you can place little bowls of water near your plants to create passive humidification. We also recommend giving your plant an occasional shower which freshens up the foliage and also creates some short term humidity. Last but not least water your rosemary deeply while avoiding soggy roots.
4) Rosemary thrives on a little fertilizer right through the winter months. The plants do much better when they get a light fertilizing approximately once per month. We recommend using Evolve All Purpose Organic.
Another easy solution is to apply Jobe's Organic fertilizer spikes every three months.
We also always top up our rosemary (and other houseplants) with Sea Soil compost in fall then again around March.
5) Prune your rosemary often. By cutting back the stem tips on a regular basis you can stimulate vigorous growth on your indoor rosemary. It is helpful to do this even if you are not needing any rosemary for harvest - just get in the habit of nipping off the very stem tips. By contrast you will want to avoid cutting deep into the old woody growth - doing so leads to very slow regrowth or even stunting of the plant.
The last thing we can say about indoor rosemary is that it is helpful to bring it indoors before weather has gotten too cold. They will look just fine even as frosts become regular events; but the plant stresses considerably when moved from cool conditions straight into warm indoors. Early September is ideal (note for next year's calendar!). If you are still planning on moving a rosemary in from outside you can refer to our recent blog post on this topic here.
Hope this helps with this popular indoor gardening goal.