I have a little hedge of boxwood which I let alone for a year. This year I would like form it a little bit.
When is the right time to gently cut and trim boxwood? Can I do it several times during the year?
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If you did not do it after planting originally, nor in spring last year, you will need to remove the top third of growth, maybe a bit more depending on how much it has grown, in order to create new growth lower down the plant to ensure you finish with a dense hedge rather than its being leggy and bare for the lower half. As for trimming at other times, see your other question and its answer.
Boxwood are "pinched" or thinned in the summer, and sheared in the winter. If you shear them in the spring, all the beautiful new growth will be removed, leaving last year's old growth. Once you see the buds in Spring it's too late to shear, unless you really must.
Thinning is very important. Rather than repeat it all here, I'll just reference a really good paper on Boxwood Pruning, by Peter Deahl.
Shearing is not as important. Some boxwood, esp English, look wonderfully rustic when left unsheared. If you want a formal "box" hedge or topiary, shearing is the only way. But if you do this in the winter, when the new growth happens in spring, the leaves cut and killed by the shearing will disappear under the fresh new growth spurt. Even if you shear regularly, pinching is still just as important.