Happy St Andrews Day! As we celebrate our patron saint, I’m afraid I do wonder if he’d happily leave a warm, sunny Lake Galilee for a cold, ever-darkening Scotland to join in our festivities.

If he’d been a gardener, I’m sure he would blame winter’s icy temperatures or relentless wind and rain for killing off his plants, but shorter days and a miserably weak sun can also be the culprits. And warmer weather caused by climate change makes us fantasise about the exotic plants we’ll soon be growing in Scotland.

But the number of daylight hours and the sun’s strength are determined by latitude and this can never change. If you live in Glasgow, you’ll have a mere 7 hours and 29 minutes of daylight today. In Stornoway this is reduced by half an hour, with Lerwick a full hour less than Glasgow. I doubt if an extra hour in Penzance would lure me away from my native land.

These low light levels are damaging to most plants and, unless adapted to these conditions, they can’t photosynthesise efficiently. As a result, they’ll produce fewer of the sugars needed for respiration. They then draw on reserves stored in their roots and could gradually weaken, be susceptible to pathogen attack and might die.

How plants absorb light is measured in discrete units of energy called quanta. These quantum units are very low just now, approximately one quarter of what you’d expect in summer. And they vary hugely in one place, depending on direction and shading. Before writing, I took measurements next to a rosemary bush. I found the south facing level was 145 units, east and west 110, and north 85. Who can blame plants for leaning southwards?

So make sure plants aren’t shaded out by rotting vegetation or old pots, and keep greenhouse windows clean. They let 86% of available light in, but dirty glass can reduce this by up to 20%.

I go a step further by supplementing the low winter sun with grow lamps, targeting them at the likes of parsley and winter greens. A few minutes ago, when daylight was virtually over in the greenhouse, a grow lamp was still producing 210 quantum units till bedtime.

The sun’s colour spectrum also plays a vital role and plants can access different colours according to the season and time of day. They don’t use green, but reflect it, so look green to us. Short wave blue, violet and ultra violet light is most important for photosynthesis but refracts off particles in the air when the sun is low at dawn and sunset and in winter. This leaves mostly the long wave red, yellow and oranges.

Evergreens, like ivy and periwinkle, that are adapted for shade are not badly affected by this colour shift and continue growing normally. As do most houseplants which do well with poor indoor light.

Plants shut up shop when they can’t cope with reduced light. Lovage and globe artichokes die back and deciduous trees discard their leaves.

Others soldier on half-heartedly. Rock roses and feverfew maintain a clump of green, photosynthesise slowly and don’t do any energetic tasks, like flowering, setting seed or putting up fresh growth.

The same happens in the veg garden. Brussels sprouts swell gradually, but I find late varieties programmed for January and February picking simply don’t and can’t get as big as the catalogues promise. But at least good old Scottish kale keeps growing, if only a little.

You can’t seriously expect plants developed to deal with hot, dry and sunny environments to thrive here. They often have greyish green leaves with hairy or waxy cuticles which sunlight can’t penetrate for efficient photosynthesis.

Plant of the week

Scots Pine looks magnificent when low winter sun lights up the rust red undersides of its branches and the fine tufts of blue green needles trace intricate patterns against a pale winter sky.