This was the first salvia I was ever aware of: the local council used it to spell out the name of the municipality in front of the council chambers. It is regarded as an annual plant, but it will often survive several seasons. There are many dwarf versions (ht 40cm) but the taller ones have more substance in the garden. A well-grown specimen can look like a small shrub, growing up to 80cm tall, and apparently this is what they were originally like before the plant breeders got to work on them and made them dwarf. I started out with ones from a punnet that were fairly dwarf and just let them self-seed, which they do quite freely, and they have gradually got taller over time - presumably reverting to the wild form.
In recent times, variegated forms of this plant have been available. The leaves have a scattering of yellow spots. The flowers seem to vary from specimen to specimen - I have seen bright red ones (as pictured) as well as ones with burgundy flowers rather like those of Salvia 'Van Houttei'. It can grow and flower quite well in shade and can be effective grown nearby yellow Abutilon or yellow-flowered bromeliads.
The original red 'bonfire sage' is probably the best known form but there are pink, purple, salmon and even white varieties. It flowers almost all year round and grow best in part-shade or even quite dense shade rather than full sun. The flower spikes are quite large so it makes a good impact amongst other semi-tropical plantings. It is effective with large tropical foliage plants, such as cannas and elephant's ears (Alocasia species). I trim off spent flower spikes regularly to encourage new ones to form. They can be propagated by cuttings or seeds.
Recently, the cultivars known as 'Go Go Scarlet' and 'Go Go Purple' have been released. They are tall forms of Salvia splendens, growing to 1.2 m and like the species, will bloom in sun or part-shade. Their flowers are densely clustered and they are said not to self-seed. Because of their shallow root system, they are easy to dig up (even when quite large) and move to another spot, enabling an instant effect to be created!
Salvia splendens is regarded as a good plant for bees!