Salvia guaranitica Large Form (foreground) with Euryops chyranthemoides, Miscanthus Zebrina and Salvia madrensis

Thank goodness for the cooler weather we have had this week, after the heatwave of last weekend. It's hard to find much solace in a wilting garden but some flowers provide us with the illusion of coolness even on the hottest days. These are the summer flowers in hues of blue, that bloom on unfazed by the heat. Perhaps it is the evocation of refreshing blue water that these flowers give, but they instantly make me feel a little better when I see them. The effect is most pronounced when the flowers are in a shady border. Blue is a very calm, agreeable colour that associates well with every other hue in the garden. Pure blue in flowers is not often found, however; with many so-called blue blooms being closer to mauve or purple. Along with white flowers, blue flowers remain visible at dusk for longer than those of any other hue: a bonus for us when the evening is the only time that it is bearable out of doors after a scorching day.

Salvia sinaloensis

Salvia sinaloensis

Salvia offer some of the bluest flowers in the plant world and at the moment I am enjoying brilliant blue Salvia guaranitica Large Form (ht 1.3 m, pictured at the start of the blog). This is not an invasive plant like the original species - which developed enormous tubers and took over whole garden beds - and its vibrant colour goes well with any other hue in the garden. At groundcover level, Salvia sinaloensis from Mexico with one of the bluest of all Salvia flowers. It spreads by underground runners to form a thick clump (though I don't regard it as invasive). The dainty flowers appear on upright stems (ht 25 cm) in summer. The leaves can take on purplish tints if grown in full sun, but it does also do well in light shade. It makes a good underplanting for yellow or lime-green blooms, such as those of Kniphofia 'Lime Butter' or Justicia brandegeeana 'Lutea', or for lime-green foliage. The blue sky Mexican sage, Salvia caudata, has small, intensely violet-blue flowers with white markings on the lower lip. It grows to a shrub about 1.5 m tall. The blooms appear over a long period from summer and into autumn. It is not a showy plant but the colour of the flowers attracts attention. It will grow in sun or part-shade and doesn't need any special conditions. Other Salvia out now include the inky-blue 'Indigo Spires' and the purple-blue Salvia forskahlei, which is useful for shade.

Browallia americana

Browallia americana

Another bright splash of blue at the moment comes from the self-seeding annual Browallia americana (ht 30-60cm), which has a mass of starry bloom with a white centre in summer and autumn. It is rarely seen for sale: if you are able to obtain even a single seedling from the garden of someone who grows it, you will have it forever. I started with one such lone plant and now have literally hundreds of them coming up each spring. They obligingly fill in any spare spaces in the garden, without swamping surrounding plants. Another self-seeding plant in my garden that is officially a perennial but behaves like an annual is Lobelia siphilitica (ht 60 - 90 cm). It is sometimes known as the blue cardinal flower and it blooms in late summer and early autumn with stout spires clustered in the recognisable hooded form of other Lobelia plants, in various shades of blue. It likes a sunny, moist position. I like to pair it with the beetroot-hued leaves of iresine herbstii 'Brilliantissima'.

Plectranthus zuluensis with Justicia carnea

Plectranthus zuluensis with Justicia carnea

Pastel blue flowers bedeck Plectranthus zuluensis, known colloquially as the Zulu spurflower, which grows into a loose shrub about 1.5-2 m in height. It has bold, striking plumes of soft blue flowers that appear in flushes through most of the year, with a brief respite in winter and early spring. It will flourish in sun or shade of any sort, and copes well with dryness. Plectranthus parviflorus is also a long bloomer, with slim spires of purple-blue flowers almost all year round.

Dichroa febrifuga

Dichroa febrifuga

Blue-flowered shrubs provide other cooling blue hues in my garden. As the mophead and lacecap Hydrangea macrophylla flowers fade, the so-called evergreen hydrangea (Dichroa febrifuga, ht 2-3 m) powers on: it an amazingly long flowering period and its large frizzy flower heads of denim blue have been a welcome sight on these hot days. It seems a very tough shrub and will grow well in sun or shade. If the soil is alkaline, the flowers will be pink!

Ceratostigma species in the garden at Government House, Sydney

Ceratostigma species in the garden at Government House, Sydney

Small shrubby Ceratostigma species (ht 50 cm - 1 m) - sometimes known as Chinese plumbago - are also starting to flower, with their dainty flowers of amazing blue. They have to be kept an eye on, though, for their tendency to self-seed. Semi-tropical shrubby Brillantaisia subulurgica (ht 3 m), is sometimes thought of as being some type of giant Salvia - however, it belongs to the Acanthaceae family of plants, which contains so many robust shrubs suitable for our Sydney climate. This one blooms from late spring until autumn, with a procession of tall spikes of hooded blue-purple flowers.

Let's hope the cooler temperatures continue for a while, but if the mercury soars, cool blue flowers will provide some solace ...