I don't know for sure what the correct pruning time for lavender might be, so I hope someone else jumps in on this who has more knowledge than I do, but here is my own experience.
I live in zone 5b in Canada, so my timing is probably going to be very different from yours. I have pruned my lavenders twice now, both times in the late spring - that would be around late May for me. Why I wait that long is to see which branches start showing signs of life, since in our cooler climate lavender is not evergreen but drops its leaves over the winter. Once I can see where the new growth will be I go about trimming out all of the dead and/or weaker sections.
If the plant has grown a little wild or scraggly looking I will also give it a general trim to give it a more pleasing shape. This I tend to do either during spring pruning or right after it finishes blooming for te season - or both if necessary.
I don't know if any of this is helpful to you or not. I believe the plant hardiness zones in Greece are mostly 9 and 10 aren't they?
I don't really know much more than Ginny but I'll add my thoughts. I have lavender, but none of them have been around long enough to need to be pruned. But if I were going to trim them, I would probably do it in the early spring (just after the last frost date for your area). If you do it earlier than that, then it may stimulate tender new growth which could get hit by frost, but if you wait and do it too late then you may delay when you get flowers. I'm assuming you're in about zone 9 like the other people from Greece who I've seen post here, so unless you had a really bad winter you won't have to worry about it dropping its leaves like Ginny does, so you can prune it a bit earlier in the year than she can.
I prune my lavender after it's bloomed (or else when I collect the blooms). Is it in a sunny spot? It likes a lot of sun but not a lot of humidity. I have some lavender plants that are over 10 years old and still going.
My lavender plants die back in the winter, so I prune them in the spring. They do fine in the several locations all around our gardens. The humidity here does not seem to be a factor, and it is REALLY humid all the way from spring into fall most years.
I have 3 dwarf lavender bushes, (2.5 yrs old) and have yet to do a good pruning.
I have rabbits that nest under them in the spring and the babies just love the leaves... so I leave them alone.
Last year I did cut back some of the dead wood - but they certainly need a "trim".
So, if I don't get nests this spring, I will trim them back a bit.
I've read that they're pruned back in springtime when you see the first new buds forming along the stems. Cutting back to about 8" high (depending on variety) should give you blooms. Do not keep it in a wet spot.
actually -- i purposefully leave them long so the bunnies do nest (though my plants are quite gangly right now)... their only other choice is out in the field where the coyote roams.
Last season, most the nests got "munched" which i hate seeing -- so i'd rather she digs lil holes in my lavender and roses -- 'cuz i just love watching the babies romp and play.
I am totally into lavenders; here's my advice.
You can prune them after all danger of frost is gone. Old leaves can take frost, new growth can't.
Then, prune off 1/3 of the green part. Don't cut until the old wood. By doing that, the plant has no way of feeding the roots anymore, it will go into shock and possibly die.
You can prune in stages; prune an inch, two weeks later prune another inch, but still not into the old wood.
Christie - I'd like to surround a new group of five roses with lavenders so I'd want short in front, taller at the sides and tallest in back. Can you give me the names of the most fragrant for each category - small, medium, tall?
I always felt Provence was the most fragrant. Lately I've read the fields of Provence, France are planted with Munstead! What is your opinion?
Christie - Thank you for this information about not pruning into the old wood. It's only through sheer luck that I haven't killed one of my poor lavenders - just because I haven't felt the need to trim back that far yet! Now I know better, and I'm sure my plants are grateful too :-)
I have all kinds of lavenders planted together, pink, white, blue-ish, every kind you can imagine!
I don't know which ones are most fragrant by name, but in my opinion, the more colour, the more fragrant. My pink and white ones don't smell strongly to me.
For size, if you are good with pruning, you can keep them all at a desired height. Just don't let them get any higher than you would like them to be. As long as you start with small, young plants, it is possible, because they don't have mature stems.
In that case, you can plant them all together and not block any views.
I like the fact that all my lavenders are different. It looks cheerful, more interesting ( but informal too) and the fragrance is all the same; you don't know who is the strongest.
I bought a great little book, only on laveders, 19.95 at Borders; "Lavender. How to grow and use the fragrant herb" By Ellen Spector Platt.
It is very informative and has great pictures to enjoy too.
Actually there are very fragrant white varieties of lavender. Different species of lavandula and different cultivasrs have more fragrance. For fragrance, look for cultivars grown for the perfume industry. The Gardenwatch is a good place to find reliable suppliers devoted to lavenders.