I always loved my university courses in physiology and biochemistry. I didn’t find them that easy, but even all those years ago they confirmed that life wasn’t just amazing at the whole organism or species level, but even more so down at the level of microbiology. By getting to understand, just a little, how living forms really tick (and for me at the time, this involved mainly humans and assorted domesticated animals), new horizons of wonder opened. The amazement intensified at the phenomenal complexities of design and function, down to molecular interactions which silently, and for the most part successfully, keep the individual going.

Unfortunately, as future veterinary surgeons, there just wasn’t any spare time for us to look at plant physiology or biochemistry. But as the season of autumnal leaf colour change is well and truly here now in West Wales, I thought I really should try to understand just what is going on within our leaves as they colour the landscape. Perhaps I would learn something new and maybe, a bit late in the lifetime of this garden, it too might benefit from having a more informed custodian. My starting point was a couple of internet review articles which I’ll provide links to (click here) by Philipe Matile “Biochemistry of Indian summer: physiology of autumnal leaf coloration” and (here) for “Why leaves turn red in autumn” by Taylor, Feild, Lee and Holbrook. Unfortunately they are fairly technical and packed with scientific jargon relating to biochemistry and microbiology, so I’ll attempt a simplifying summary which hopefully won’t offend any with degrees in plant physiology who might happen upon this.

We all know that most plants are green, and that this colour is the result of chlorophyll molecules, a complex pigment which has the fantastic property of being able to help capture the energy from the sun’s radiation and utilise it through a series of complex chemical reactions to create useful and larger carbohydrate molecules from very simple raw materials. This photosynthesis process is indeed at the very ‘root’ of most life on earth. The chlorophyll molecules don’t just float around in a chemical soup within the individual cells of a leaf structure. They’re organised on membranes within separate structures or organelles (think mini organs), called chloroplasts. Typically there are more than 100 of these chloroplasts in a single leaf cell. For all those perennial plants like trees and shrubs which live in temperate climates there is a big issue of what to do as the growing season ceases and winter approaches. Some evergreens have developed structures and techniques to retain leaves through this harsher season. Many ‘opt’ to lose their leaves completely and begin again the following year. But this is potentially very wasteful of resources, in particular of the complex molecules like chlorophyll, which the plant has had to create during the growing period. So, as the days shorten and temperatures fall, the plant starts to mobilise resources from the leaves and withdraw them to the woody more permanent parts to be available next season.

As an initial part of this process the chloroplast is structurally changed into a different form called a gerontoplast, and the chlorophyll molecules are then broken down, to save the useful components, and get rid of more toxic parts. As this happens the chlorophyll loses its green colour, and hence its greening effect on the leaf, becoming colourless, simpler compounds. As a result, other pigments which have been present within the leaf’s structure all along but been masked by the pervasive green, can now show themselves. In particular the yellows and oranges of many autumn leaves are thus created by the presence of various pre-existing carotenoid pigments. As with the shade grown Viburnum furcatum below. These normally hidden pigments have a role in protecting the chloroplasts and hence the leaf cells from internal oxidative damage, which can result from overactive sunlight-affected chlorophyll molecules. Viewed simplistically, the colour pigments help to dissipate excessive and damaging energy within the cell – bear in mind we’re thinking of an amazing little factory unit here, working overtime when the sun shines, with lots of energy and chemical production involved. How ‘clever’ to have your own in-built system of checks and balances to prevent overloads and what a fantastic economy of design and effort on the plant’s part. The spin off in autumn of the carotenoids’ moment, centre stage, is that we can marvel at the myriad of colour changes in our gardens and landscape at this time of the year.

But hang on a minute, what about the reds seen in some species of plants? Where does this colour come from in plants like the Viburnum plicatum mariesii below? Well, this is where the situation becomes even more complex. These red and purple colours are produced by different pigments in the anthocyanin group of molecules. And they don’t normally exist within these leaves’ structures. (Though they can exist in other plant materials – think ripe red apple skins, for example). It’s been suggested that in other circumstances where they do normally exist, these anthocyanins also have a role as chemical sunscreens preventing cellular damage – a role they’re well suited to since as red or purple pigments, they tend to absorb most of the blue-green light wavelengths.

The first article I read couldn’t come up with a rational explanation of quite why an ageing leaf in its final dying stages, and when its biochemistry processes start to focus on economically salvaging useful materials from the soon-to-be-shed leaf, should suddenly begin to synthesise a completely new chemical form. Just as it was simultaneously trying to withdraw these resources into the woody centre of the plant. Matile speculated that perhaps it was just a curious non-functional anomaly, and as a result we benefited from an even wider palette of autumn leaf colour changes in the relatively few plants where anthocyanins were formed.

More interestingly to me with my gardener’s hat on… (which incidentally is worn most of the time to protect my ears from the sun – too many cases of amputating squamous cell carcinoma from the tips of cat’s ears is to blame! Perhaps we mammals could do with some anthcyanins in our ear tips. Wouldn’t that be fun? Never mind Spock ears, how about a race with red or purple tints developing in their ear lobes as summer approached) .. was the hypothesis in the second article.

This focused on leaf colour changes in Cornus sericea, and postulates that even at this terminal stage of leaf degradation and decay, the new pigment has a vital sparing effect on the leaf cells’ contents from becoming damaged by the effects of UV radiation. So in essence, even as the plant leaf is dying, it really is worthwhile making a completely new chemical to allow the process to complete more efficiently. The group of researchers were able to show that red Cornus leaves, where the anthocyanins are produced just before the chlorophyll molecules begin to be broken down, are indeed less susceptible to oxidative damage than equivalent leaves with no red pigments. Again how impressive is that as a design feature? At this point, since we have closely related Cornus alba ‘Sibirica’ growing in various areas of the garden, I thought I should get out with my camera. And finally, this is where all the above science led me. If you look at the Cornus leaves in various parts of the garden, and then at other species with red autumnal tints in the leaves, you do indeed find that the colours in many are most pronounced in those leaves exposed to full sunlight. Leaves more hidden and shaded at the plant’s centre often show no red colouration at all. Cornus alba ‘Sibirica’ planted in the most sunny location in the garden, and below the inner leaves on this clump. And below a clump in a slightly shadier part of the garden. So WHAT? I can almost hear you sigh.

Of what relevance is autumn leaf colour physiology to gardeners?

Well, just that perhaps the gardener armed with a basic understanding of autumn leaf colour physiology (which precluded me before now) would not only carefully select plants, or even specific cultivars of plants to give an autumnal leaf display bonfire, but also consider:

• Planting them, if possible, to maximise sunlight exposure, particularly if you’re after reddish tints.

This is the newish Sorbus ‘Olympic Flame’, performing well, but giving a relatively brief and early display. Many Sorbus cultivars don’t seem to retain coloured leaves for a very long period.

• Being aware that no matter how well exposed to sunlight they are, if they’re exposed to strong prevailing winds, the leaves may all be blown off before they’ve had a chance to colour up.

We planted several home grown Amalanchier canadensis in a block. They look fabulous in spring, but they catch the prevailing winds in an exposed position in the autumn, so this is all we ever see of their potential autumnal display.

• Planning for a seasonal progression of colour, just as one would do with spring bulbs. This is perhaps quite tricky to do, unless you visit a garden with a good progression of autumn leaf colour on several return visits, because even a week can see dramatic changes in colour. And it’s very difficult to glean from any text that I’ve seen, just how one species will relate its timing of leaf colour change to another species. Even worse, how it might then vary in your garden. And even worse still, how it will vary from year to year. Whoever thought garden design was easy? The other benefit of revisiting an autumnally interesting garden frequently, will be to give some idea of just how long a plant will hang onto its leaves, in various colour stages, and whether it tends to shed them all together, or sequentially over a longer period of time.

This is a view of Hydrangea serrata ‘Diadem’. Its terminal leaves consistently start to colour reddish purple in late June, and are retained until well into November. Do you consider this as autumnal colouration? I do. It’s certainly the longest display of changing leaf colour in our garden. Other trees like Parrotia persica, below, develop lovely autumnal tints but the leaves are then shed quite quickly in sequence along the branches, so the overall effect never, at least with us, seems to reach its full potential. Though our typically poor levels of late summer and autumn sunshine may well be involved with it not performing reliably in this garden.

All the leaves have fallen from the Cornus ‘Sibirica’ here, revealing the deep red stems. Whilst below, the leaves of Sorbus sargentiana (rear centre) are only just starting to colour up by this date, the leaves of Liquidamber ‘Worplesdon’ (left front) are only about half way through a 2 month display, whilst to the right the leaves of a Cornus kousa chinensis have barely begun to colour up at all.The late golden leaves of Hamamelis ‘Aphrodite’ with, again, the red tinged leaves of Hydrangea ‘Diadem’.

• Consider the value of heightening the pattern of autumn leaf colour change by using evergreens or simple green leaves which don’t exhibit any leaf colour changes, as foils to all these autumnal foliage fireworks.

The pale green leaves of a cutting of Hydrangea arborescens ‘Annabelle’ centre stage, amongst more autumnal tints. The dark green of a clipped yew to the left of some Cornus ‘Sibirica’ leaves, and behind, some blue sky.

• A 3 hour drive yesterday into England made me more aware that there are actually very few trees native to the UK which exhibit any real red colouration to their leaves. Early in the season the Spindle tree, Euonymus europaeus, named because its wood was valuable in former times as a strong timber for making spindles for wheels does turn red.

The earliest tree/shrub in our garden with autumn colour, Euonymus europaeus ‘Red Cascade’ above. There is a native Dogwood, Cornus sanguinea, apparently so called because the long straight stems were used by butchers for making skewers, ‘dags’ or ‘dogs’, as they used to be called. The majority of trees chosen by gardeners for red or purple autumn colours in the UK will therefore have come from other regions of the world. And in all probability from areas where consistently sunny autumn weather has nudged plants over millions of years into developing these super late season colour rich sun screens. We don’t have any Cornus sanguinea in the garden, but do grow this cultivar of Cornus kousa, ‘Miss Satomi’, which is normally promoted (and indeed this is why we bought it) for its long lasting peachy pink coloured bracts in early summer. These are beautiful, but it’s also a great and reliable later season plant for autumn leaf colour with us, even in part shade.

• Finally on top of all the above, the actual or even relative timings of autumn leaf colour are hugely variable. It’s generally thought that clear sunny days followed by cool frost free nights will give the best displays. This seems a logical corollary from all the above about sunscreens and anthocyanin production – and why ‘fall’ in the Eastern states of the U.S.A. is so consistently spectacular. This year in our garden, after another pretty grey though mild month, we’ve had one of the best displays ever, albeit much earlier than in recent years and fleeting, as strong Easterly winds are now bringing lots of leaves down. Maybe the slightly sunnier than usual September was the key factor in setting this display off? Certainly for the first time in years, most leaves will be off the deciduous trees well before bonfire night on November 5th. Below is another later, but reliable red leaved display from Acer griseum, with again the uppermost and most sun exposed leaves showing the greatest reddening.

The big bulb plant for this year is now over bar 150 ‘Flaming Purissima’ Tulips, which will go in, come November. Fearing future creaky joints we opted to blitz the garden this year and in the end were relieved to clear them all, 6,000 plus, without too many ill effects. Who on earth thought 100 Kg of bulbs to plant in one autumn was a good idea?

It was definitely worthwhile making a note in the spring of areas needing more bulbs to improve the scene, and although I guess we’ll still need a bit of a top up, by choosing cultivars which we now know from observation set seed well, in future years we hope it’ll be more benign seed scattering or sowing rather than the use of a digging bar and mulch that adds to their numbers! One of the many areas tackled. Digging bar to create the hole, bulb in, back fill with leaf mould, and then cover the lot with cardboard sheet, topped off with more leaf mould or wood waste. The picture below is a few weeks after the above, and shows early yellowing of the underlying grass…. By spring, given our wet climate, the cardboard should have rotted down and so the bulbs can push through without issue. Next autumn, we’ll plant up with ground cover perennials. Our ‘No Glyphosate’ approach to managing a new area for cultivation.

Finally, in the garden, moving a pot disturbed this rather fine frog. One of my favourite moths, the Merveille du Jour, appeared beside the back door, and had to be photographed with the aid of our wind up torch. I still haven’t sorted out my flash!Armed with all the thoughts above about siting plants for best autumn colour, where shall I plant these home grown tree seedlings next autumn? Malus tschonowskii, assorted crab apples and Acers.