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To shade or not to shade

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Jim McColl Jim McColl | 07:00 UK time, Sunday, 17 July 2011

The Serac Greenhouse at Beechgrove

"The Serac Greenhouse at Beechgrove - As I recall, the house is of French design but is no longer available."

When there isn't enough light, plants become lanky and drawn, anaemic and disease-prone. On the other side of the coin, too much strong light will scorch the leaves of plants and in some cases it may even 'bleach out' the colour.

The sun is providing more than light itself of course, it is also responsible for heating the planet and it is this aspect I'd like to deal with as it affects plants in our glasshouses.

One of the problems we experienced in the Beechgrove Garden this spring was the incredibly fine weather! Lengthening days came with clear skies from morning to night. So what is the problem? Too much of a good thing, perhaps? The seedlings and young plants in the glasshouses romped away, quickly becoming overcrowded, so much so that we had to start the hardening off process earlier than normal to give them more space.

Truth to tell, some plants were getting rather large by the time it was safe to plant them out. "Should we apply some shading?" asked one of my young colleagues and that brings me to my point.

Firstly, I said if we reduce the light levels by shading, the plants will become even more stretched. We must try to reduce the temperatures in the glasshouses by ensuring that all ventilators are working efficiently. When they are fully extended we need to open doors, whilst splashing water about the floors, under benches etc to help cool the environment and provide a bit of humidity.

Young plants don't mind strong light so long as they have plenty food and water and the temperature is moderate but to quote an old song it was too damned hot! What's reasonable? When the glasshouse temperature soars into the high 20's Centigrade and beyond, many plants will suffer and as a result their performance will be significantly disrupted.

In human terms, they can't perspire fast enough! Transpiration in plants is like the cooling system in a car, it keeps the working parts from distorting by overheating. The root of the problem is that many small glasshouses do not have sufficient ventilation fitted.

Will shading help? There is a paint-on product that fades when the weather is dull and intensifies as the light levels increase. At the end of the season, it is easily removed by wiping with a cloth when dry. It is the solution that most of us would opt for because it is certainly better than doing nothing, but there is one thing it can't do and that is prevent the sun's rays from striking the glass surface even though it has that thin film of covering. As a consequence the glass heats up and that allows the temperature inside to rise because it is acting like a radiator. Paint-on shading may prevent some scorching but it will not help to reduce the temperature.

The new house complete with roll-down shading, spotted at Gardening Scotland.

"The new house complete with roll-down shading, spotted at Gardening Scotland."

What is the answer? Stop the sun's rays from reaching the glass of course and indeed, create a ventilated gap to part-insulate the glass surface! Will new technology be needed for this job? Not a bit of it, we old geysers can remember seeing glasshouse roofs protected with lathe blinds, rolled down over runners to keep them 4 - 6 inches off the roofs surface.

The point is, they were only used when absolutely necessary. At Beechgrove, we have a double-skinned polythene house called a Serac, which we use as a propagating house. It is fitted with a mesh fabric rollup blind as described (as I recall, the house is of French design but is no longer available).

At Gardening Scotland, the annual 3-day garden and outdoor living show held at the Royal Highland Showground in the first week of June every year, I spotted the answer - a new glasshouse with outside blinds fitted which can be rolled up and down as requires and held away from the glass itself to give that insulating gap. That is undeniably the best type of shading to provide.

Indoor blinds? They may reduce the light intensity and reduce scorching but they won't help to reduce the temperature, the heat has already come through the glass, into the confined space! This is not an easy problem to solve for someone who is away from home all day but external net or lathe blinds can be lowered on days when long periods of sunshine are forecast.

Jim McColl MBE presents BBC Scotland's the Beechgrove Garden.

Comments

  • Comment number 1.

    I had scorching on some of my tatties in pots in a coldframe this spring. As I'd never grown tatties in pots before I wasn't certain what would happen with them. But I kept them and watered them, and now I can't tell which were the scorched ones.
    I have since built a polytunnel, and am keeping the ends out for the summer for ventilation and to keep the temperatures down, though I still wash down the floor a couple of times on the hottest days. I'm considering covering it with a coloured tarp if it gets really hot (for more than a day), and now that I know, I'll make sure that I leave a gap between the cover and the tarp.

  • Comment number 2.

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