Salvias do offer a range of blue flowers not to be found elsewhere, however there is not a ‘true blue’ which is neat and tidy and a cure all for a lot of situations. I think all of these gorgeous blue salvias come from places in Mexico and further south where they don’t contend with frost.
Many people see the gorgeous blue patens used overseas a lot but they don’t realise that it is probably just grown for the one summer and discarded and started anew for the following summer. In Australia patens is generally at its best for maybe two years, that is if you manage to get it through to the second year. It is winter dormant and so often slugs and snails eat the new growth about July when you are not in the garden a lot and don’t notice it is happening. The plant puts out more new growth but if it is continually eaten out it is either wiped out completely or a very weak plant. It does set large seed which grow easily. If you are prepared to grow it carefully, the large blue flowers are a great reward.
About twenty years ago punnets of salvia patens were sold in Victoria at least, but these days very few nurseries grow even individual plants.
Those lovely plantings of patens you see in Europe would also be well fertilised and cosseted. I don’t think many gardeners in Australia carefully fertilise. We have a much longer growing season and get results easier than they do in Europe so gardeners here are not usually so particular.
There are some lovely blues in salvia macrophylla and sagittata and variations. However they are not real tidy plants generally. Salvia macrophylla can get rather large and bulky, even more than a couple of metres high if it has something to lean on. The ‘worst’ thing about macrophylla is that it is inclined to set a lot of seed so there could be a forest fairly easily. It is winter dormant in cold areas at least.
Sagittata has gorgeous blue flowers but the plant is inclined to be floppy and would probably benefit by pruning. I think straight sagittata would grow to about a metre left to its own devices. It is likely best grown with good exposure and not crowded as things tend to be in my garden. There are crosses of sagittata and macrophylla which have the breathtaking blue flowers but the plants can be large – maybe 2m x 2m and more and perhaps favour a bit of shade in hotter areas. They have large arrow shaped leaves which are not the easiest for tidy pruning. They repeat flower well and give me a lot of pleasure.
There is salvia cacaliifolia with sprays of true blue flowers. It is a kind of crawly evergreen plant but can bulk up to about a metre high by about 1.5 metres wide. I believe it prefers part shade, especially in temperatures over 30C. The foliage is arrow shaped again and a solid attractive green.
Scutellariodes is a ground cover with true blue flowers. However it doesn’t flower a huge amount and likes shade. The flowers are about a 10 cent piece size and lovely.
Salvia guaranitica and all its variations are very close to ‘true blue’ and are easy to grow, nearly all are invasive in fact. ‘Black and Blue’ has lovely blue flowers out of blackish calyces on shorter plants than the species guaranitica which is usually about 2m. ‘Black and Blue’ is definitely invasive which is not such a bad thing if you know before you plant it and can plan it in with that in mind.
Salvias chamaedryoides and ‘Marine Blue’ are easy to grow in full sun and dry conditions but they do not have quite the ‘true blue’ flower colour. Chamaedryoides are not always the easiest cuttings to grow in summer as they rot off when you have to water them to keep them alive. Masses grown in large nurseries get the conditions they prefer. Young plants of ‘Marine Blue’ look the same as chamaedryoides but can grow towards a metre high when flowering whereas chamaedryoides should stay around 25cm or twice that in flower.
When people want an exact colour it makes me nervous that we are both on the same page and that I will supply just what the client wants as opinions of colour vary.
There seems to be unlimited species of lavender blue flowered salvias in all sorts of shapes and sizes and also a lot of indigo blue and pale blue that are easy to grow and long flowering but if you want ‘true blue’ you would have to be prepared to seek them out and give them attention in the garden.