Answer: Unfortunately, it is very difficult to keep a rhododendron in a container under home conditions so I would recommend replanting it as soon as you can. It is better to plant it now than to wait any longer into the season.
The good news is that rhododendrons bloom from buds that are set the year before, so your plant is apparently capable of setting buds in your garden. In other words, it would have bloomed even if you had left it in the ground.
Sometimes rhododendrons take several years to become reestablished after transplanting and will fail to bloom while they are becoming rooted. Sometimes flower buds form but are killed back during excessively cold and/or windy winter weather. Some varieties are more winter hardy than others, so since the past winter was fairly mild this may be the cause you had no blooms until now.
When you replant, set it in a location that is extra sheltered during the winter with good protection from the wind. Also be sure the soil is evenly moist yet well drained, meaning not sopping wet or saturated. When you replant it, make sure to plant it no deeper than it grew before.
Mulch lightly with two to three inches of organic mulch such as pine bark. This will help keep the soil evenly moist and also help feed the soil slowly over time as it decomposes. You can also try to assure good flowering by fertilizing each spring with a slow release granular fertilizer for acid loving plants such as Hollytone; read and follow the label directions.
Good luck with your rhododendron!
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