I asked Marcus about this and he has kindly sent a reply. He may at some point join our forum but as you can imagine since his trip to Europe he is now very hectic with work and family.
At Lambley Nursery, we grow a dozen or more selections of Salvia nemorosa and the related hybrids S. x sylvestris, and S. x superba. These are collectively known as Meadow Sages, and the varieties we cultivate include, 'Lubecca', 'Ostfriesland', 'Amethyst', 'Caradonna', 'Wesuwe', 'Mainacht', 'Blauhugel', 'Scheehugel', 'Viola Klose', 'Marcus', 'Tanzarin', and several of our own selections. Perhaps the most beautiful is the naturally occurring Salvia nemorosa ssp tesquicola, with its rich claret coloured bracts and violet flowers.
Our Nursery and gardens are situated in the windswept plains of central Victoria, Australia, with temperatures from -7C to 45C. This group of Salvias are frost and drought tolerant, thriving in our conditions. Our gardens are watered only 3-4 times per year, during the extreme dry of our summer months.
These herbaceous Salvias emerge in early spring, and flower strongly for 8 weeks until early summer. By late December we cut the entire clumps to the ground, leaving neither stem nor foliage. For mass plantings we use a powered hedge trimmer to save time and labour. Though this seems severe, within a week new foliage emerges, and by February (six weeks after pruning) the salvia's are in full flower again. The flowers hold well until late autumn. This second flowering is usually a shorter height than the spring display, as the rainfall is much less for this time. During late autumn the plants are cut to the ground again, to make way for the emerging under-planting of spring bulbs.
This mid-season pruning will also encourage other garden plants to flower more strongly in the Autumn: Achillea hybrids, and the globe flowers Echinops bannaticus and Echinops ritro will also benefit from this treatment, flowering strongly again in the autumn.
I urge you to try this technique: the well timed hard prune in summer saves one from otherwise diligently dead heading all season, and rewards with a strong second flowering, giving strength to the Autumn garden!
David Glenn, the proprietor of Lambley has penned some more detailed notes regarding these Salvia's on our website,
http://www.lambley.com.au/garden_notes/in_my_garden_salvia_nemorosas