Reader Inquiries

Answers by Rick Donnan

Should I use twin skins if building a polythene greenhouse?

I am considering building a single span poly house to grow speciality cucumbers. I have a friend who says twin skins are essential if I plan to heat the greenhouse, but others warn that the twin skin loses light. What do you recommend?

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How do I convert a formula in ppm to amounts of individual fertilisers?

I have received several successfully proved formulas for the crops I grow, however they are expressed as ppm (parts per million) of the various nutrient ions. How to do convert these to the practical amounts of fertiliser to use?

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Water quality needed for different hydroponic crops? – Part 2

Following on from last month’s question on water quality, here are some more guidelines. These list the Dutch guidelines for the maximum levels of sodium that are acceptable in recirculating solutions for a range of crops.

I have also included more recommendations as to different water sources, especially the collection of rainwater.

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Water quality needed for different hydroponic crops?

I am considering setting up a small commercial hydroponic farm using a recirculating system. I have read that water quality is very important for hydroponics, especially when recirculating. Is this correct and can you give me some guidelines as to the quality required to grow different crops in hydroponics?

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Should I use a PAR meter for greenhouse climate control?

My greenhouse climate and irrigation controllers are years old and use a lux sensor for light measurement. I was recently told that I should be using a PAR (Photosynthetically Active Radiation) meter. This is apparently much better than a lux meter. Is this correct?

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Can we use chlorine dioxide to prevent diseases?

Can we use stabilised chlorine dioxide continuously in the irrigation system to prevent diseases and biofilm (on a system that recirculates drain water)?

Can we spray chlorine dioxide directly on crop leaves, to prevent diseases in general, specially fungal diseases?

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What is IPM (Integrated Pest Management)?

I have been reading about IPM and am somewhat confused as to whether it is biological pest control. If so, it must be difficult to buy and manage the beneficial insects before the pests get overwhelming. Is there a different explanation?

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Do you need to whitewash a greenhouse with UV protective glass?

Your question implies that the main function of applying whitewash to a greenhouse cover is to prevent the transmission of UV (ultra violet) light into the greenhouse. This is not correct. The main function of whitewash is to reduce the heat load upon the crop in the greenhouse by reflecting a significant proportion of the incoming Infra Red (heat) portion of solar radiation. This is aimed at periods of high solar radiation, that is, around summer. See More

How do I grow hydroponic mint?

I have read the article on mint in your Issue 132. I am also trying to grow mint in hydroponics in India. Would you please guide me in a few things:

  • What is water pH and TDS levels? 
  • What is the temperature to be maintained in the greenhouse?
  • What is the pest control method used? 
  • What nutrients are used?


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Should I use copper fittings in hydroponics?

I plan to set up a small hydroponic system in my village allotment. I have some copper fittings left over from a kitchen renovation. I am concerned about using these because, although we have very little pilfering of produce from the allotments, the lure of stealing copper may be higher. Is there something cheaper and less attractive I can use instead?

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