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Growing Points: ‘Rain, rain, go away…’

May 1, 2013  By Dr. Mohyuddin Mirza

Here we are in May and a lot of hard work was done in planning,
scheduling, advertising, the labour and the time management to reach
this stage of the crop.

Here we are in May and a lot of hard work was done in planning, scheduling, advertising, the labour and the time management to reach this stage of the crop.

I’m in and out of many greenhouses each spring to see the fruit of your labour and love. This feature includes pictures of what you might see in a greenhouse in April and May.


Seasonal bedding plants greenhouses often open their doors in the middle of April and customers are eager to come in. They may buy a few plants early, but the major sales take place around the Victoria Day weekend. If the weather is good, all bright and sunny, everything will be buzzing! Customers will be loading up their carts with flowers, vegetable transplants and supplies.

5370-Wall-of-colour  5370-Hanging-baskets

This grower always impresses me with the team’s dedication to create a wall of colour (at left) and plenty of hanging baskets (at right). It is so enjoyable to walk along these walls of flowers and marvel at the colours that have come along during the last decade, along with, of course, the fancy names as well!

5370-Containers 5370-Wooden-benches

This grower used to have old wooden benches and recently installed flood benches in the sales area. They were just being filled in when I took this picture. The new benches have changed the entire look of the greenhouse and customers are going to enjoy this experience.

■ Once in a while the weather does not co-operate. I have seen cloudy and rainy weather affecting crops, customers, and, of course, growers in many ways.

We’re not just talking about cloudy and rainy weather; it could also be a hailstorm or a severe thunderstorm. On one occasion, while touring a greenhouse with the grower, the sky quickly became dark and a severe hailstorm hit the area.

The greenhouse plastic was leaking at several places. However, we were more worried about the plants in a nearby shade house. While they were quite damaged, the surprising thing was that they did recover, though it took until August.

When customers don’t come, growers will, of course, worry and get depressed that after all their hard work, there will be no one to take home such beautiful plants.

■ Two main things happen when there is cloudy and rainy weather:

  • Under low light conditions, plants cannot carry out  photosynthesis.
  • This means they are not making enough food to maintain and sustain themselves.
  • The relative humidity goes up to 100 per cent and when the temperature drops at night time, dew can form on leaves and stems.

After six to eight hours under such conditions, Botrytis grey mould can become an issue. It does not take long for this fungus to spread and cause damage.

The following pictures are there just to alert you as to what can happen and show the sequence of events.

5370-Water-droplets 5370-Geraniums-und

The picture at left shows water droplets that resulted when plants were watered late in the afternoon. A similar situation can arise when high humidity levels combine with cool night temperatures, resulting in the formation of dew. This is ideal for Botrytis grey mould to germinate and infect leaves. You can see a leaf on the plastic tray (photo at left) already showing signs of Botrytis infection.

■ On previous page, the lower right photo is of geraniums that experienced four days of cloudy weather. They did not get watered and hence no fertilizer was applied. The temperature was not reduced and the plants stretched. Lower leaves started turning yellow and have to be removed before they are ready for sale. And by “sale,” I mean sold at a reduced price.


This photo shows what happens when Botrytis grey gets out of control. Plants should be removed from the sales area well before these symptoms appear.

■ Here are a few strategies to try if it gets cloudy and rainy during May:

  • Reduce the temperature by at least 2 C (the 24-hour average).
  • Use circulation fans to keep the air moving.
  • Vent as much as possible.
  • Watch for symptoms of nutrient deficiencies. The most common of these is yellowing of lower leaves due to nitrogen deficiency.
  • Prepare plants for bright sunny days after the cloudy weather passes. This can be done by lowering the feed EC.
  • Check the roots when it is cloudy for over a week. Root rot can set in and that has to be managed. n

Dr. Mohyuddin Mirza is a greenhouse consultant. He would welcome comments at

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