More pathetic spring onions

[ameno]

Perspex contains UV light protection to stop it going all brittle.
It reduces light that the plants need but not visible light.

UV stabilisation in plastics doesn't actually reduce light transmission. It's a chemical change which alters the bonds between the molecules, not some sort of coating over the top.
However, plastic itself (all kinds) does have notably poorer light transmission than glass, and even glass is certainly not 100%.

Plus of course since what he has is a potting shed that means it doesn't have 360 degree windows, or anything close, so plants will only get decent light all the while the sun is actually shining through the window.

[Marb67]

Well other plants in the same shed are doing ok after weeks if being in there. I tell you spring onions are very very difficult to grow in any conditions.