I live in northern Colorado on the plains and the rosemary that thrives in the summer never makes it through the winter in a pot. It slowly dries up and dies even though I water it and keep it in a sunny place.
What should I do to keep it alive?
While I am not intimately familiar with rosemary per se indoors in Colorado, I am aware that many plants are remarkably sensitive to humidity and this comes up particularly with plants like ginger (which I have grown indoors in central Utah as a kid).
The first thing I would do is assume that the problems may be related to a drop in humidity and consequently extra water loss. After all, Colorado gets cold, and heating makes air very dry.
If this is the case, the first thing to do when you bring it in doors is to cut the plant back somewhat. By doing so, you reduce the number of leaves and the respiration of the plant.
The second thing I would think about trying to do is spray the leaves with water every day or so (and if necessary a couple times a day).
The third thing I would do is periodically check the soil, which should be damp but not wet.
A fourth thing (rosemary-specific here) that you could try is to put the plant in a cooler part of the house with a humidifier. Rosemary is hardy to zone 7 so your pipes will freeze before your indoor plants will be killed by frost. Cooler weather retards respiration and water loss.
Hope this helps!
Yeah I have this problem in Utah, it is just way too dry in the winter with the forced air heating... You can put the plants in a clear trash bag supported with a stick and make a micro greenhouse... This winter I have killed lots of things:
Small olive tree, A couple of small umbrella acacias, A windmill palm, All of my citrus dropped leaves, A couple of morangia ovalifolia... Which could come back...