Rules for pruning can be as arcane as the secret rites of the Free Masons.
Or they can be simple.
Frank's Pruning Rule # 1
Prune when you feel like it. There may be an aesthetic price to pay--lost flowers for a season, some frost damage--but odds of killing a plant are very, very, very low. Better to just get the pruning done when you're motivated rather than letting it slide for another year.
And all plants evolved being grazed on by something: cattle, deer, giraffes. A plant's evolutionary programming is such that when it's been pruned, it thinks something grazed on it, and it must send up some new shoots that can eat sunlight and feed the roots. Like this sturdy little tree.
Frank's Pruning Rule # 2
Prune in months that begin with the letter "J".
That means January, June and July. If you are willing to wait for optimal times of year to prune, here's a mnemonic that can help you get there. And this rule even works for procrastinators because the month following the J months is also a good pruning month, i.e. February and August.
The winter pruning months--January and February--would be not-so-good for spring blooming plants as their flower buds have formed, even though they might not be visible. But the worst case is that you lose the flowers for a season. So if you really need to prune because the plant is blocking a path or something, go to it. But these months are best for plants that bloom in summer, fall or winter, such as gardenia, sasanqua camellia and daphne odora respectively.
The summer pruning months--June, July and August--are best for the spring blooming plants, such as forsythia, azalea, and spiraea. This is also a good pruning time because it gives plants enough time to grow out and harden that new growth before the cold weather can burn the leaves. Again, frost-burnt leaves aren't pretty, but the odds of killing the plant are very low.
Frank's Pruning Rule # 3
Try to keep the clippings on site, especially from perennials and perennial grasses.
Granted most of us don't have the power equipment to shred tree limbs and shrub branches to use as mulch, but if you have some wooded land where you can pile brush (and throw some good soil on it to speed up decomposition) that would be better than burning fossil fuel to have the city or a contractor haul it to a dump. With prunings from ornamental grasses or herbaceous perennials I can usually keep them onsite. If perennials are growing in a lush bed, I chop them back a few inches at a time and let the clips fall where they may to decompose out of sight on the ground. With ornamental grasses, since the best time to cut the back is late winter (end of February here in zone 7) I spread them on the ground and cover them with mulch, as that's also an ideal time to get that chore done. The bulbs and perennials haven't come out yet and so the mulch can be spread more easily. And if it's covering a few armfuls of ornamental grass clippings that will feed the earthworms, so much the better.
Keep the prunings on site and keep the pruning rules simple. As you get more experienced you can add some nuances of pruning to your skill level. I'll post about that in the future. Meanwhile get a bit of pruning done in the cool part of the day so you can enjoy a video of Texas Chainsaw Massacre inside on these hot afternoons.