casting a shadow

[Ian]

Hi All,

I am trying to understand light level requirements for Paphs. My books suggest that a good and easy test is whether your hand casts a shadow on the leaves - the suggestion is that there should be no distinct shadow. (I think that the assumption here is that we are talking about sunlight rather than artificial lighting.)

Reading this forum I understand that different groups of Paphs have different light requirements. However I only have one fairly small greenhouse, so the light levels are going to be fairly similar everywhere.

Here are two pictures taken today, showing the shadow cast by my camera outside the greenhouse and inside the greenhouse on a sunny afternoon in England (yep, really!). The shadow inside the greenhouse is faintly visible under the 50% shade cloth I currently have in place.

What is your advice - is this shadow test a useful one? (It is certainly easy to do.) If you were to do this in your growing conditions would there be more or less of a shadow cast?

I will be grateful for any help.

Regards,

Ian

 

[ehanes7612]

The hand shadow test is not reliable as you can still have too much light without casting a shadow...as in a greenhouse where the light is scattered and diffused. you can only gain information about relative light levels with your hand (or anything else) as you move it through your grow area..but this is only relative and not absolute measurements. One way to approximate absolute levels is using a white piece of paper (maybe neutral gray) and your camera metering system (if you have an slr) but I cant remember the conversion factors..or you can just buy a light measuring meter for 40 dollars

I would say 50% may not be enough shade , especially in summertime..unless you have a means to shade them more inside

 

[Ian]

Thank you for this. I guessed that the cast shadow method was very approximate, but I had not considered that it could be misleading (i.e. that in a greenhouse there could be no cast shadow but too much light).

Ah well, what might have been a simple 'rule of thumb' bites the dust! (A pity because I generally have my hands with me when I am in the greenhouse, but do not always carry a light meter.)

Ian