They love acid soil so I would use fertilizer for rhododendrons, azaleas and camelias. Spring time.
Have they flowered in previous years? my favourites!

Unless you know your soil is actually alkaline or neutral, any general purpose fertiliser such as Growmore will do - both these trees are fine in neutral to acid soil. Apply chemical fertiliser when growth begins, usually mid to late March or April.

Thank you very much to both of you for your advices. Bamboo, I have neutral to slight acidic soil in my large garden, it is also rich in humus, however, I did not put any fertiliser to these young trees since they were planted 2 years ago. I just noticed last year, that they had very small leaves, specially Cornus florida did, as if missing something. They bloomed, but not so wonderfuly, as it is in magazines.
Bamboo, as I do not live in England, for me trademarks of fertilisers you have in England are unknown. Can you specify what is Growmore rich in? I use annualy "horn shavings" (do not know, if this is a correct English term) in my garden and plants usually like it, too :)

You need to check what's known as the NPK - it's usually shown as a ratio of 3 numbers, and in the case of Growmore, it'll be 7:7:7 or sometimes 6:6:6. Its a granular product, takes 6 weeks to break down, so only needs one application at the start of the growing season, though you could do a second 6 weeks later if you wanted. Rake or turn it into the soil at the base of the plant.

Thank you. 6:6:6, that sounds like Devil´s number :)
Well, now it is clear. I know NPK fertilisers.

Do you know, I nearly made a joke about the devil's number myself, lol. And I knew you'd know about NPK... universally used I think.