Answer: Astilbes are very cold hardy, down to about -34 C, so it should be hardy for you if planted in the ground. If you could move the barrel to a sheltered location, where the roots would not be exposed to the maximum cold, that would be better than trying to overwinter the plant in the house. An unheated garage, for instance, would be a place to consider. Leave the plant outside until hard freezes have killed it back, then move it to shelter. Keep the soil barely moist while the plant is resting and move it back outside as soon as the harshest cold has passed. Your goal is to maximize the time it is outside but protect it from the most extreme temperatures so the roots don't freeze and to avoid repeated freeze thaw cycles. While it is outdoors you could also mulch the soil surface, use several inches of organic mulch around but not over top of the plants.
Astilbes require an evenly moist soil to do well, especially when planted in sunnier locations. Morning sun or dappled light all day is enough to keep these plants happy. Full sun all day or afternoon sun is usually just too hot a location for them, although the wetter the soil the more sun they can handle.
Since it is difficult to keep a small container evenly moist during hot or dry weather, the larger container is a good idea. It will take some time for the plant to root out into the new soil, however, so check with your finger to see if and when you need to water because the original potting mix may dry out faster than the surrounding soil. Also, keep in mind that wind can dry them out quickly. You could consider using one of the water holding polymers mixed into the soil, these can help reduce watering needs.
The dried leaves will not repair themselves, so go ahead and trim the browned and dried parts off. New growth may even be able to conceal the foliage damage in a few weeks.
These plants also require a rich soil; container-grown plants require frequent feeding in addition to the carefuly watering. Use a water soluble fertilizer or a long lasting slow release fertilizer per the label instructions. Taper off feeding in the fall to allow the plant time to harden off before winter.
The flowers will change color as they age and begin to fade. Drying out would accelerate the fading process just as moister soil will help them last as long as possible. When they look ugly to you, trim them off.
Enjoy your astilbe!
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