I would leave well enough alone. I am guessing that when you pruned it you sheared the outside and didn't remove branches and that is why it has dense foliage. In the future, when pruning a rhododendron it is best to use a hand shears and prune back toward the center thick branches. Remove branches that don't grow outwards. Eliminate areas where branches cross. Try to open up the plant to get air flow. However, I would not do it now.

Here is a link that might be useful: How to grow rhododendrons and azaleas.

Thank you, Rhodyman!

I guess the second part of my question, that I forgot to ask :) is: how long will they keep growing? All summer long? Or will they eventually stop earlier?

Thanks again,
Cumpaniciu

Rhododendrons normally have a growth spurt. They will open the buds they formed last summer and they will grow to maturity. If you fertilize you can get them to continue with more flushes of growth during the summer.

However summer growth also depends on the weather. Here in Pennsylvania they will grow all summer and must harden off in the fall because of our severe winters. However along the west coast you get a Mediterranean climate which can be dry in the summer. If you irrigate or live in one of the wetter areas, they will grow all summer. However in the wild they grow in the spring and in the fall. For the same reason, your trees get two growth rings each year while they only get one growth ring here. I used to raise Douglas Fir Christmas trees and they grew half as fast here as in Oregon and we had a much longer growth season, but we only had one growth season per year while in Oregon we had two.

Thanks again!

They get a lot of water, the garden is irrigated. Ok, I can only imagine how they'll look next spring :)

I will follow your tips next year.

Cumpaniciu

Just a quick followup: the growth spurt stopped around mid July, and now the plants look just like they looked back then, which is not as bad as initally looked.