You may not want to bother removing the flower-bearing stems on many types of salvia that are grown for hummingbirds.
These stems simply dry up by themselves and since our annual salvias generally do not produce seeds in great number; this need not be a concern for you. Salvia coccinea does produce seeds on these stems. Some gardeners recommend that flower stems should be removed from plants once the flowers stop looking attractive in order to prevent the plants from producing seeds, which can reduce future blooming. This does not happen with salvia coccinea, which produces both lots of seed and lots of flowers all year long. Once any part of a plant turns brown and brittle it may be removed without harming the plant, since brown and brittle indicates a dead part of the plant.
If you want to promote branching in salvias, you can cut off the tip of each stem, which will usually result in two new stem tips being produced from the axils of the last pair of leaves left on the plant. When growing fall blooming salvias you can cut or pinch back the stem tips regularly until about the end of June, then stop cutting to allow the plants to produce flowers. It is necessary to remove only the growing tip of the stem to encourage branching, try to cut back after a few inches of growth has occurred to encourage the maximum number of branches. Try not to remove more than an inch or so of the stem, but if your salvias have gotten too tall you might remove much more than an inch to prevent them from becoming weak-stemmed and falling. This type of cutting back is good for salvia elegans, s. mexicama, s. iodantha, s. madrensis, s. involucrata, s. puberula, s. slendens 'van houtte', s. leucantha, s. purpurea, s. gesneriflorae, and perhaps other varieties. Cutting back isn't usually necessary for salvias that bloom all summer, because cutting back would delay flowering. Once salvias flower, they generally branch out below the old flower stem automatically, so a species like s. guaranitica doesn’t need to be cut back. Some like to tidy their salvias and clip off the spent flowers, this is your choice but not really necessary.
Each salvia variety can have its own set of pruning instructions. Some need hand pruning to each stem, while others can just be clipped-off, and still others can be sheared. Learning from experience what is best for your salvia varieties is sometimes the best way to achieve the look you want. Gardening is an art of trial and error or trial and success.
General rules can apply to pruning salvias:
Woody stems are pruned down to the lower set of new leaves -- ex. greggii. Softer, brittle stems are pruned to the ground (or new growth if it's started already) -- ex. leucantha
Where the leaves are basal (low to the ground in a circle) you only clean away dead leaves and remove any old stems.
Shrubby, but not woody salvias are pruned rather short -- down to lower new leaves -- ex. microphillia, involucrata, elegans, "Black & Blue.”
Note from a PCMG member: I have Black & Blue salvia guaranitica and where I have it, it must be very happy because I have to prune it hard a couple of times during the summer just so we can walk by it. I usually cut the spent blooms away to keep it flowering all season. It grows so vigorously that I have to dig some of the roots up every year just to keep it confined.
My Victoria blue salvia is beautiful what about pruning or taking cuttings from it?
Wait until you see new growth. Use the “asparagus” rule to trim salvias. Bending the stem until it snaps (towards the outer end) will reveal the approximate area where the transition of the woody growth to new green growth is. This is also the point where cuttings are taken.
Treat shrubby woody-stemmed sages like greggii and microphyllas that form twiggy, woody growth like roses. Definitely do not cut these to the ground.
Cutting to the ground is only good for those salvias that form short stolons underground and send up new shoots, like leucantha, some microphyllas, guaranitica, and others. If the plant was rooted and set into the ground with no nodes beneath the soil line, it will not send up shoots in any case. You can often tell this has happened when there is a thick trunk coming from the soil, and the first node has a multitude of stems coming from it. Cut that off, and you have a dead plant.
Questions about cutting back salvia greggii:
Re-blooming salvias, such as salvia greggii should be pruned back periodically during the summer. To make the job easier, use hedging shears, and remove only the spent flowers and a few inches of stem below.
Note from Parker Master Gardener: In Parker County, salvia greggii needs to be cut back to look better, mine gets rangy looking and the wood gets brittle and does not bloom as well. Plus pruning in February will make it thicker and healthier looking in the spring. I suppose you could just leave the salvia greggii alone but mine does better cut back by one third every February.