THERE are some splendid autumn colours around and not just from the late chrysanthemums, Michaelmas daisies, Guernsey lilies and ornamental crab apples.
Let’s hear it for the deciduous trees and shrubs as they put on their final display of the year. Exactly how they achieve this chameleon-like transformation to their leaves is one of nature’s wonders.
We’ve moved the large pots of chrysanthemums into the greenhouse where they can bloom freely, as they are. A late Guernsey lily (Nerine bowdenii) display is illuminating part of the border as it does. And in several gardens assorted crab apples are strutting their stuff, not least the gorgeous Butterball. Then just when we think it can’t get much better, the leaves join in.
As friend Jean put her delightful floral arrangement together at last week’s Garden Club meeting, there was a comment on the attractive colour change in a sprig of maple she used. “Yes, I think it’s the breaking down of sugars in the leaf that cause it” she responded. At this point it was tempting to say “Sit up at the back, listen carefully, here comes the science lesson.
To understand this natural ‘fireworks”’display that is developing in gardens, parks and countryside at present, we need to see each leaf as a food manufacturing factory with a limited lifetime of one growing season.
The plant owns each factory and in order to survive hard times in winter, has to close them all down and retrieve any valuable supplies in the process. The signals or catalysts for change relate to light and temperature – shorter, cooler days.
A combination of these prompts chemical reactions within the leaves, as the plant begins to reabsorb energy from them, a salvage operation no less.
First to go are the chloroplasts which contain the dominant chlorophyll, so essential to the food-making process, whose green presence masks the colours of other pigments. They are also relieved of their protein and enzyme content which is consequently stored in the plant’s roots over winter.
Then we begin to see leaves in their true colours as various substances are broken down. Carbohydrates are converted back into sugars but during cold nights the process almost grinds to a halt.
When a sunny day follows, sugars left stranded in the leaf combine with anthocyanin pigments to produce intense red and purple colours. Eye-catching oranges and reds appear when bright, warm days are followed by night-time temperatures bordering on freezing point.
So what accounts for the range in leaf colour between different types of deciduous trees and shrubs? It is simply the different emphasis on chemical structure between species. This, along with light and temperature, is a factor in determining when the display starts and how long it lasts.
Have you ever noticed how some trees, benefitting from the proximity of a street light, tend to hold on to their leaves longer!
Once the plant has decided that salvage operations must cease, a chemical message is sent to the very point at which the leaf stalk joins it. The layer of abscission cells found there, are surrounded by enzymes and this causes them to collapse, so the leaf falls to earth.
This is the point where ever-resourceful gardeners step in and round up the windfall harvest with composting in mind. Either feed them into your Dalek-type bin, add to a compost area or, cram them into a black bin liner, tie it tightly and pierce a few times with a garden fork.
But surely there’s little food value left in the leaves when they fall, I hear you say. Ah well! A fitting analogy for this would be; just when we think the tube of toothpaste is empty, it is always possible to squeeze a little more out. Besides which, composted leaves are valuable for their soil sructure improving qualities alone.
The best autumn leaf displays can be found along our country lanes, in local parks and gardens open to the public. Howick Hall, Cragside, Wallington, Belsay and Alnwick are but some of the must visit places.
There is also the spectre of individual plants which are the highlight of private garden displays. Look out for wall displays from the Virginia creeper (Parthenocissus henryana), not one for faint hearts as it is vigorous, clinging to walls, guttering and anything it touches. The payback is an autumn leaf change from purple to orange-red. The crimson glory vine (Vitis coignetiae) and purple-leaf grape (Vitis vinifera Purpurea) are similarly popular and attractive right now.
Mention autumn leaf colour and acers (Japanese maples) spring to mind. They can be temperamental about draughts, frost, direct sunlight and dry soil conditions. These can be avoided by a strategic planting near wind-filtering shrubs and semi-shade, and adding organic mulch each year. We all have our favourites and this fellow’s is Acer palmatum Dissectum whose leaves are akin to a glowing fire.
Certain small ornamental trees and shrubs can represent a pleasant touchstone, as they reappear each autumn. In this respect Cornus kousa (Chinese dogwood), Cotinus coggygria (smoke bush) and Fothergilla monticola are like long-standing friends paying an overdue visit. So too the lance-like leaves of a red stemmed willow, Salix sachhalinensis `Sekka,` and Viburnum bodnantense `Dawn,` both of which are just colouring up nicely in this garden.