You don't "trim back" a Physocarpus (NineBark); you remove no more than 1/3 of the stems (the longest ones) at the base of the plant, at the crown of the root ball (ground level). This reduces the shrub's overal size, reduces density, allows light to reach the interior of the plant and air to circulate, preserves the natural shape of the plant, promotes health and vigor, and does not dimish flowering of the remaining stems. It should actually increase bloom production, assuming the remaing stems have not been previously "trimmed back" !
Cut off the old flower stems leave the leaves.
After they have flowered. Do not cut below green wood as they do not sprout from brown stems.,
If you cut back the flowering stems of lupins after the flowers have faded you will get a secondary flowering. Any other cutting back should be to the ground in Spring when growth restarts.
If the plant's basic structure has been established, then just remove dead stems and one or two older shoots at the base, then shorten side shoots to about 3 inches or 8cm. The main stems that have flowered at the end are also cut back to the nearest side shoot, but all other main stems are left as they are.
You can cut down the stems on a potatoes but this may slow their overall growth. The stems are a vital part of the plant which provide nutrients and grow considerably as it does.
Not necessarily, the stems died after fruiting and fresh shoots are produced from the base each year
There are a LOT of different types of salvia. Some are perennial and some are annual. If yours is a perennial you should cut it back for the winter. Recommendations differ for different climates, but usually you can cut it back to 6-8 inches. Leave the stems a little taller in Prairie climates.
If you mean individual stems throughout the season then yes, you can trim back daylilies after they flower. Once all the buds on the stem have flowered remove the whole stem, but not the leaves.
yes, you trim the spent flower stems off, but you don't trim into old wood
Grafting
Can I assume you mean Wisteria? If that is the case summer pruning is the proper way to control it and encourage flowering. Shorten back the shoots that grow from the main stems after flowering.