Her Dark Materials
Colour in the garden is so much a matter of personal preference and taste. I don’t have a designer’s eye for colour, but I do have some strong favourites where flower colour is concerned. There are certain shades of yellow which simply scream ‘weed’ at me. My eyes may see Inula hookeri and much as I like the flower form, my brain thinks ‘dandelion’. For me you can’t beat reds, oranges and purples. But more recently I’ve been noticing colour in stems and leaves, particularly black. Of course ‘black’ in horticultural terms isn’t true black, but is usually more of a very dark purple or mahogany brown.
I first began to appreciate plants with black stems and leaves a few years ago when I was in Cally Gardens, a nursery in Dumfries and Galloway. A clump of a tall herbaceous perennial carrying umbrella-like heads of rosy-purple flowers held aloft by black stems caught my eye. I had to have it, not because I was attracted to the impressive flower heads, but because of the dark stems. It turned out to be Eupatorium maculatum Atropurpureum Group 'Gateway'.
My eyes were then tuned into looking for plants with black stems and foliage. So along with the eupatorium I walked out of Cally Garden with Phlox paniculata 'Logan Black' and Actaea simplex 'James Compton', both of which have very dark foliage and stems. And so it began…
Amongst others, Ligularia przewalskii, Lysimachia ciliata 'Firecracker', Hemerocallis 'Corky' and Artemisia lactiflora 'Guizhou' have all found their way into the garden along with a couple of sedums with dark purple ‘flesh’. In fact the dark stems of H. 'Corky' look good alongside Sedum telephium 'Purple Emperor'.
In planting my new acquisitions out, I started wondering whether a black, purple or brown leaf or stem colour could interfere with photosynthesis. On doing a little research I found that such plants function quite normally regardless of their apparent colour.
The colour of anything comes down to what colour light the surface reflects. So the normal green leaf reflects green light and absorbs red light. Black parts in plants contain anthocyanins, these are the pigments which give fruit and veg such as blueberries, red cabbage and aubergines their dark purple colour because they reflect red and purple light. The pigments are present alongside chlorophyll (green) and so don’t stop the plant from photosynthesising, but depending on the relative concentrations, the green colour can be masked completely.
Plants grown in the shade may produce more green chlorophyll to compensate for lower light levels, in which case the green starts to come through again and the purple/black colour is less vivid. This is really apparent in some of my tender succulents such as Aeonium arboreum 'Atropurpureum' and Crassula perforata (black edges to fleshy leaves) which spend the worst of the winter months on a bright windowsill. After a couple of months indoors their dark colour is fading and more green is coming through. But the dark colour returns to their leaves after a few weeks outdoors in full sun.
However my A. simplex 'James Compton' is planted in semi-shade and the very dark brown/black leaves have lost none of their depth of colour.
So why did plant anthocyanins evolve? What evolutionary advantage do they provide when present in leaves and stems? There are a number of theories; in some species they are thought to protect leaves from UV light which can damage plant tissue just as it can human tissue, in other species it is thought they can add a disagreeable flavour which puts predatory animals off. Hmm, I’m not convinced by this; Ligularia dentata 'Desdemona' which has deep red brown stems and leaves is a martyr to slugs, so much so this plant acts as a good decoy for hostas!
Whatever the practical reason behind the colour, I think that well placed next to contrasting colours, white and pale green work well, horticultural black brings an extra visual dimension to a garden.
First published in the South Pennine Group Newsletter Spring 2013
and subsequently in Cornucopia Issue 34.
© Copyright for this article: Toni Frascina
This article was taken from a copy of Cornucopia that was published in 2014. You could be reading these articles as they are published to a national audience, by subscribing to Cornucopia.