Most perennials propagate through other means than just seeding, many send out side shoots, or grow rhizomes, or tubers, to sustain the parent plant year after year, and then when the parent plant puts out seeds, their purpose is to spread out to other areas. In many cases this is not needed in your garden, so when the flower buds are past bloom you should dead head (remove past blooms) as much as possible to encourage new flower buds, and keep it blooming as long as possible. The reason for this is when a plant puts out flowers, and those flowers are fertilized, the process for creating seeds starts, when those seeds reach maturity, the plant releases hormones to indicate that it has completed it's mission, reproduction, so by dead heading your past blooms before seeds are made, you incite the plant to try harder to make seeds, increasing overall yield.
Another suggestion on perennials is to tip off the main stalk, doing this encourages side branches, which will make your plants much fuller looking when in bloom. When it comes to herbs though, you will want to pick off any flowers that develop, flowering on herbs slows down the foliar growth, and picking them helps side growth, in most cases it is the foliar growth you desire in herbs, except for the medicinal herbs and flowers, where the most desirable part of the plant are the flowers, like Echinacea.
Now for the biggest mistake people make when perennial gardening.
It's late October, and most of your perennials are done, there has been a hard frost or freeze, and not much is going on, the stalks are getting all ragged looking, the leaves have either dropped, or turned brown and shriveled, and the whole area looks like a mess, at this point many people will cut everything back to the ground to give the area a cleaner look, and this is the first mistake, after the plants have all died back LEAVE it as it is till spring, all the debris laying on the ground is going to start breaking down over the winter, this provides both insulation for the roots, and some small amounts of heat, as the micro organisms break down the organic matter produced by the plants, this is the natural cycle, and in many cases, if not followed, these plants will not come back the next season, then in spring when you do clean up what's left, you should cut everything back only up to about 2-3 inches from the ground, because many plants have root crowns at the soil level, and if it is cut too short, the crown is removed and the plant will die.
A word about feeding your plants!
Most people are familiar with the NPK values on fertilizer packages, but what do they mean? There is a simple way to remember how these values relate to your plants, The first value is the Nitrogen content, the second is the Phosphorous content, and last but definitely not least, Potash, now you need not remember this to make these values useful, just remember over, under, all around, meaning the first value supports the growth of the foliage(over), the second supports the root system(under), the last one, general health and resistance to disease(all around). When you see something like 10-10-10, this is a general food for veggie gardens and tomatoes, when you see 15-30-15 this is also for general feeding, but for flowers, tropicals, and shrubs, when you see 10-6-4 this is for evergreens and acid loving plants, how does this help you? When a plant is establishing itself it needs lots of N for foliar growth, then as the nitrogen depletes the P become more prominent in producing flower buds, at this point the plant is working very hard, which is where the K comes in to sustain the plants resistance to stress(high temps, droughts, insect infestations) and disease. If you were to look at the many different plant foods in stores, you will see the ones for grasses, are primarily N, and the "bloom booster" foods for flowers, are highest in P, and the remedies for plant problems are highest in K.
DO NOT USE FERTILIZERS FOR AT LEAST TWO WEEKS BEFORE HARVESTING ANYTHING YOU PLAN TO INGEST, and you can help to flush them by heavy watering a day or so before harvesting.
When picking herbs, or medicinal flowers, early morning harvesting will provide the highest quality, as their oils are at their greatest concentration, as the day progresses, photosynthesis starts converting sunlight in to sugars, then as the plant respires over the night, the sugars are converted to starches and stored.
