This is the type of simple advice I need! Does it harm the plant or just make its growth unwieldy?
What other herbs is this true for? I've got sage, Italian parsley, oregano, thyme, rosemary, and the aforementioned basil growing in herb boxes. I've just randomly snipped from the sides or tops, which seemed to have made sense, though sage now seems to be treated like basil. True? Anyone willing to offer a quick primer guide on herb care (snipping vs. pinching, sun exposure, and water requirements would be of help). Thanks as always!
Wow. This is putting me to the test.
In general, when you strip individual leaves from a plant, you're reducing the plant's ability to photosynthesize. The bare stem left behind can serve no biological purpose and will not re-grow those leaves.
"Pinching" refers to the practice of removing a stem just above a node (nodes are where leaves or other stems develop). This stimulates the plant into re-growing new stems, resulting in a bushier plant - meaning more leaves to harvest.
I don't have time to research this, so here are a few guidelines off the top of my head. As I'd hate to disseminate mis-information, I welcome any corrections.
Basil is a warm weather annual. Don't set out plants until nighttimes lows are above 55F. Pinch young plants just above the third set of true leaves, and pinch each new stem above the third set of new leaves. By the fourth iteration of this process, you'll be getting significant amounts of leaves, and the plant will become distinctly bushy. Keep after it, because pinching will also delay flowering. Once flowers develop, the entire plant will start to taste harsher. If you see flowers, pinch back hard ("hard" in this case is jargon meaning to go further back on the stem - at least three nodes). Since basil is an annual, you can only delay this flowering impulse for so long; eventually the plant will flower anyway or will just fade away (which is fine, because it dies not longer after setting seed anyway). If you want to keep your plants going after the cool weather returns, you can do so by taking soft cuttings and rooting them. Keep the new plants in a warm, sunny spot. But, as with all clones, the new plant is not really a baby - it will have a significantly shorter life span than the parent plant. Some varieties of basil respond better than others to cuttings. In my experience, the ornamentals last a lot longer than the culinaries. African blue basil, for instance, can be kept going for many years through vigorous pruning and stem cuttings, but it doesn't taste very good.
The other popular annual herbs, like cilantro, chervil, and dill, prefer cool weather and have very short life spans (2 months at most), and there's not much to do to prevent them from bolting. Bolting is when the plant kinda wholesale gives itself over to reproduction and sends up massive flower stems. Game over. The plants get funky tasting when they reach this stage. You can delay it once or twice by cutting out the entire bolting portion (if you study the plants every few days the change in foliage will become quite evident), but bolting is a response to hot weather. The best way to keep a supply of these all season long is to continually (like, every two weeks), sow new seed. These plants do not need to be pinched as basil does. They grow multiple stems from the crowns, so just remove stems as you need them.
Parsely is actually a biennial (meaning it has a 2 year life span). It will mostly die back in the winter and re-grow the next year. But, that second year's growth is entirely devoted to reproduction, and once again, it affects the flavor of the plant. So for culinary purposes treat parsely as an annual. It will last well into winter if lightly mulched (say, with autumn leaves).
The perennial Mediterranean herbs, like rosemary, oregano, marjoram, savory, and thyme, are woody when mature. Again, don't strip leaves, but rather take stems as you need them. Allow the plants the chance to develop and branch out somewhat before whacking 'em. You'll notice as they grow that the older stems will lignify (turn woody). Cutting woody stems doesn't hurt, except for rosemary, which will become stunted. Remove the soft, tender stems of rosemary. But if you start too soon, you'll really stunt the plant.
These herbs, by the way, need very well drained soils not too rich in organic matter. If you're growing them in pots, make a mix of a lot of sand with some native soil and compost (use a product like Leafgro if you don't have compost). They all get very large, though, so either put them in the garden next spring or start again if you have to keep them in pots. Also, don't keep them moist. Once established, they should be allowed to dry between waterings. And don't water them from overhead - this region is already too humid for them, so keeping the leaves wet just encourages fungal diseases. Water the soil.
As youngs plants these perennials do well in a rich potting mix, but as they age the roots will start to rot from all that moisture.
Thyme has an interesting growth habit, often browning out and dying at the original crown within two years, but staying green at the tips. If you bury some of the stems (either woody or soft) while they're still attached to the plant, they'll root at those sites - this is another form of cloning and will keep the plant alive for many years. Rooting them from cuttings is trickier.
I can't remember much about sage. I haven't grown it in years. It's a desert plant, so provide poor, well-drained soil and allow it to dry between waterings. I think you want to harvest soft stems (kind of like pinching basil) only, but I just don't remember.
All of these plants, whether cool-weather or hot-weather, will take as much sun as you can give them. A little morning or evening shade won't hurt, but otherwise they need to be in full sun.
That was probably waaaaaay more info than you wanted.