The Colour of Plants at Twilight

by Brian Denison

Some time ago Judi (Barton) asked me why plants look different at Twilight, so here is my definitive answer which I thought I would share with you. It’s all about the composition of light and light scattering.

Fig 1

Light is simply a collection of electromagnetic waves in the visible spectrum. Violet/blue light have the shortest wavelengths and red light the longest. Green, yellow, and orange are in the middle. These rainbow colours are shown in Fig 1.

When a ray of light travels through the atmosphere some of the colours are scattered out of the beam by air molecules and airborne particles, changing the final colour of the beam. At sunset when the path through the atmosphere is much longer, the violet, blue and green components are removed almost completely leaving the longer wavelength orange and red hues (fig 2).

Fig 2

In other words, light may be white or yellow at midday, gradually turning redder as it approaches sunset due to light scattering.

Evening sun is lovely, but twilight turns a garden into a place of enchantment. Colours that dazzled disappear and paler tones come to life. This is the time when white, pale yellow and light pink create really pleasing effects.

We also owe some more of our most spectacular phenomena to light scattering – not just the red of sunset, but also the blue of the sky, the white of clouds and not forgetting rainbows.

The sky is blue: we all know that, but why? Air molecules, like oxygen and nitrogen for example, are small and thus more effective at scattering shorter wavelengths of light (blue and violet). The selective scattering by air molecules is responsible for producing our blue skies on a clear sunny day (Fig 3).

Fig 3

But why is the sky not indigo or violet (shorter wavelengths than blue). This is because the sun emits a higher concentration of blue light waves in comparison to violet or indigo. Furthermore, our eyes are more sensitive to blue rather than violet hence to us the sky appears blue.

So why are clouds white? Cloud droplets have diameters of 20 microns or so (larger than gas molecules) and are able to scatter all visible wavelengths more or less equally. This means that almost all the light which enters clouds will be scattered. Because all wavelengths are scattered, clouds appear to be white. When clouds become very deep, less and less of the incoming light makes it through to the bottom of the cloud, giving these clouds a darker appearance.

Lastly rainbows (Fig 4), but refraction rather than scattering is the mechanism. Most raindrops are spherical, and it is this spherical shape and their size that provides the conditions for a rainbow to be seen.  Light passing from air to a raindrop at an angle slows and changes direction, in a process called refraction (fig 5). Refraction splits the light into the colours of a rainbow just like a prism. The sun’s position (angle) and that of the observer are particularly important, which is why rainbows are not seen so often.

I hope you have enjoyed the science!

Brian Denison

Plant of the Month: November

Saxifraga fortunei

by Sue Gray

Saxifraga fortunei

Had Covid19 not forced the cancellation of the AGS event, originally scheduled to take place at Harlow Carr on October 14th, I am sure that during the past couple of weeks I would have been planting some of these little jewels which I would have been unable to resist buying.  I say ‘little jewels’, and they are usually sold by nurseries specialising in alpine and woodland plants, but although some are grown as ‘specimen’ plants, largely for foliage effect, many are robust, very good garden plants which are extremely useful at this time of year.

Saxifraga fortunei was first introduced to Great Britain from China by the English Botanist William Jackson Hooker, and named for the Scottish Plant Hunter Robert Fortune, in the mid 19th century, and is native to China, Japan & Korea.  Whilst the species form has white flowers, there are now many named forms in colours from white, through various shades of pink, to darkest red.  I would describe them as ‘quiet plants’, almost the Autumn equivalent of hepaticas; you hardy notice the foliage emerging but when it becomes crowned with the delicate flowers, they are such a welcome sight at this time of year.

They are herbaceous perennials and, according to the official advice, prefer a shady, or part shady spot with the usual ‘moist but well drained’ soil. However, I find that by October, when the sun has lost much of its strength, few gardens can really be said to be in ‘full sun’ so S. fortunei can withstand a more open position, and this, now sadly unlabelled, plant (right)

performs beautifully in my garden in a relatively open spot.   From my experience, and this may just be me, I find that the white and pale pink varieties are more floriferous. 

S. fortunei ‘Reica’

The deep pink and red varieties are more dramatic with the flowers set against often dark foliage, but I find the flowers can be rather more sparse, as shown on this plant of S. fortunei ‘Reica’.   

When I started thinking about writing this, I got out my copy of ‘Plant Finder’ to check on some of the named varieties.  I expected to find them all recorded, in alphabetical order under the heading ‘fortunei’, but instead I discovered them all, in common with all other named varieties of other Saxifraga species, in alphabetical order, spread throughout the full list of Saxifraga entries, followed by (fortunei) and (5) so, for example the  entry for one of the best known varieties ‘Wada’ reads ‘Wada’ (fortunei) (5). Turning to the front of ‘Plant Finder’ I discovered that there are 15 classifications of Saxifraga; class 5 is entitled ‘Irregulares’ and describes plants in this class as ‘shade-tolerant, usually herbaceous perennials forming rosettes of broad, palmately lobed leaves, with leafless stems bearing lax panicles of small flowers with short upper, and longer lower petals, in summer or autumn’.

Shortly after discovering all this, I was in the company of a very knowledgeable friend and said to him, ‘explain to me, in words of one syllable that even I will understand, about Saxifraga classifications’.  He gazed into space, as if thinking ‘where do I begin?’, but then said, ‘I shouldn’t worry about it’, so I won’t, but just enjoy these lovely plants for what they are.  I am indebted to our Secretary Pat who has sent me some images taken at Holehird recently, and to a friend with whom I was visiting Harlow Carr and who photographed various examples on display in the Alpine House.  Their images appear below.

You may not be surprised to hear that, having spent some time thinking about these lovely plants, I think I can accommodate several more in the garden, and an order has been sent off to one of my favourite nurseries!