20 Jan 2016

Winter wonderland.

Its winter... and although it arrived late it is definitely here, its cold, wet and often frosty.  Its the time of year when the trees reveal their bare structure, our herbaceous plants are in the most part no where to be seen except perhaps for a deceased skeletal reminder of their beauty from a warmer period, very few seedlings are emerging except perhaps some of tougher wild natives, flowers are also few and far between.

Living in the U.K this is the norm and I think it is very easy to take this for granted but have you ever asked why?, why do the trees lose their leaves?, why do herbaceous plants die back?, why aren't seedlings emerging?  Why the lack of flowers?.  I hope the much simplified explanations below help answer some of these questions.

In a temperate climate there is a definite distinction between the hottest summer days and the coldest temperatures that winter has to offer, this has a profound effect on how plants have evolved to deal with this.

Deciduous trees drop their leaves as a survival strategy, having a canopy full of leaves in summer enables the trees to harvest sunlight for food, basic biology 101 right?, leaves however are organs that allow gaseous exchange and are the final step in the movement of water up through a plant (a process called transpiration), in a nut shell water is lost via the leaves and the amount lost is greatly increased during windy periods.  Wind adds a second element of jeopardy in that the combined surface area of all those leaves adds up to a great deal, a few hundred thousand leaves on an average English Oak Quercus robur, while the semi-wind permeable nature of the canopy helps a little it still offers considerable resistance to the wind which translates into stress on the trunk and branch structure.  The result can be damage to the wood of the tree, root damage caused by the rocking motion of the tree or worse still total upheaval of the entire root plate in extreme examples.  Light levels are much reduced this time of year due to the tilt of the earth  in the relation to the sun so the trees are unable to make as much food as they can in the summer when light quality and quantity is much higher.  The conclusion is that it makes sense to drop the canopy, reduce the chance of wind damage during a time when relatively little food can be photosynthesised anyway.  The leaf fall decays at the base of the tree and is taken back into the soil by worms to be made available as nutrients, these nutrients can be taken up by the tree and used to grow leaves once again.

So what about flowers?

To answer this we need to remind ourselves that flowers aren't there for our amusement or to decorate our homes and give a florist a living, they are sexual plant organs which have the sole purpose of facilitating pollination.  In winter insect repellent sales drop through the floor and the all too common image of day trippers frantically trying to swat flying insects as they enjoy a picnic is a distant memory, there just aren't as many insects around.  If you want to spread your genes it makes much more sense to time your flowering period so that it coincides with the correct part of the insect life cycle.  As with trees another limiting factor is light which is required in a good amount to fuel the hugely energy demanding process of flowering and create the pigments that prove attractive to pollinators.

The majority of the plants we have in flower now in the walled garden originate in Asia such as the Chimonmanthus praecox, Cornus mas, Daphne bholua "Jaqueline Postil", Hamamelis spp. and the Viburnum x bodnantense, they have some other common features in that their flowers are all rather small, very mute and washed out in colour and all very aromatic.  This is explained by the need to keep flowers small and robust to avoid wind damage, the colour can be explained by the difficulty to synthesise the necessary plant pigments in the reduced light and heat, this leaves rather inconspicuous flowers that aren't easily detected from any great distance by their pollinators.....at least not visually anyway.  Often the plants listed above along with Edgeworthia chrysantha are often detected via scent before sight, it is this tool that the plant uses to help the pollinators locate the plant until its flowers are more obvious.


Seedlings and more accurately seeds are a marvel, they enable a mother plant to package away an embryonic plant complete with a packed lunch to give it a start in life (no crippling student loans here), when the seeds germinate they have just enough fuel to unfurl the cotyledons (baby leaves) in the hope they find light and are able to begin to make their own food.  Its an excellent strategy but it is not enough?, if seeds germinated as they dropped from the tree they would surely perish in the winter months while they wait for warmer and lighter seasons to arrive.  Nature has this covered, seeds can be placed in a state of dormancy, this is where the seedling will NOT germinate even if it is placed in favourable temperatures and moisture unless it has first undergone a treatment to remove the dormancy.  This treatment could be as simple as a period of cold, telling the seedling that winter has passed and it now must be spring.  Another form of dormancy employed by Sweet Peas Lathyrus odorata is a physical dormancy which is simply a thick seed coat that stops the seed taking in water and gases until it has sufficiently been degraded by passage through the gut of an animal, frost/thaw, abrasion or a number of other factors.  Plants are surprisingly good at timing the germination of the seedlings and even vary the thickness of the seed coat to stagger germination to avoid an "eggs all in one basket" scenario.  Other types of dormancy exist and can be very complex, this is a result of the need to coincide with the seasons of the locale in which they evolved.

The interesting fact remains that plants evolved nearer to the equator where this seasonal light variation is less pronounced tend to respond more in relation to wet/dry season cycles than the light levels which are more constant.

As gardeners we aren't allowed to go dormant and sit for warmer more favourable weather, it is now that we look at our hard landscaping, work on dormant plants, improve our soils and plan for the season ahead.  Yes its cold and frosty but without this inclement period we wouldn't have the seasonal variation that allows us to witness the growth cycles of the plants and the wildlife that accompanies it, Winter is a time to catch your breath and prepare to hit the ground running as time and nature wait for no man.

1 comment:

  1. As always, a fab post. We are prepared for clearing up blackened foliage at the moment. Jx


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