Answer: In general, it is better to plant things in the ground than in a container, and a plant set out promptly before it becomes stressed will usually perform better in the long run. The reason for this is that when planted in the ground the roots are protected from excessive heat build-up and from abrupt swings in soil moisture; in the ground, fertility is more constant as well since there is less leaching. Mid to late summer is the most difficult time of the growing season to try to maintain a plant in a container -- the heat is very stressful and can make it necessary to water frequently, sometimes even twice a day if the plant is outgrowing its container. If you can shade the pot (but keep the plant in the sun) or perhaps insulate it in some way for the summer, can maintain an even soil moisture, and fertilize periodically per label instructions, you might be able to keep the plant healthy, assuming the pot is large enough to accommodate its growth. The larger the container, the better the chance the plant will survive for a long time; a half barrel is a nice size to work with.
Then for winter you will need to protect the roots from freezing and thawing by either setting the pot down in the ground or insulating it in some other way such as by placing it in a cold frame. You will also still need to keep the soil moist when it is not frozen. Once again, it is much easier to keep the plant in the ground during winter, as well.
Having said all that, this is a fairly sturdy perennial so with good care, regular repotting and some luck you might be able to keep it in a container from year to year. Good luck with your salvia!
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