Thank you for your question. Your plants have probably suffered more from being trimmed at the wrong time than having been trimmed harshly. If you trim a plant at the wrong time in its annual cycle, you may damage the flower buds or remove the part of the plant on which those buds will be formed. Let's talk about the correct trimming and care for each and with a bit of TLC they should become blooming garden participants again.

The best time to prune rhododendrons is immediately after bloom (in the NYC area that is late May until mid-July but may be slightly earlier in your milder growing region). If you wait too long, buds will already have formed for the following year and will not re-form if lopped off. In general, azaleas and rhododendrons need minimal pruning. If you need to do more drastic or remedial pruning, please refer to our guide on Azalea and Rhododendron Maintenance.  

To bloom properly, your rhododendrons should be in dappled shade or limited sun (not full sun or full shade) and in slightly acidic soil (pH 4.5 to 6).  You need to feed them with an acid loving shrub food during the growing season.

Camellias also grow in slightly acidic soil and prefer some protection from the harshest midday sun. They do need strong sun for most of the day, however, to develop flower buds and also require soil that drains well.  If camellias are to be pruned, they should be pruned in late spring immediately after blooming. If refer you to information from  the American Camellia Society on proper pruning for camellias.

Your andromeda (Pieris japonica) is a third, acid-loving plant. I imagine that your location near the coast is relatively cool and moist, in which case pieris can tolerate full sun. Like your other plants, this one should be pruned only just after it has finished blooming or the buds for the following year will be destroyed. 

None of these plants needs to be pruned hard as a rule but will tolerate some shaping.  It is possible that your plants will take more than one season to recover, but if they are planted in the correct amount of sunlight, acidic soil and receive the water and nutritional care they require, you should be able to reestablish an annual blooming cycle once you adjust the pruning schedule.

Please let us know if we can be of further help.

Courtesy of NYBG Plant Information