[Kleftiwallah]

In the weak sunshine of the spring, would there be any benifit to early crops if I placed white plastic sheet 'down sun' to reflect any weak sun back onto the plants, or are the usefull frequencies not affected?

Over to you.  Cheers,   Tony.

[DD.]

I would imagine that it's similar to the aluminium foil trick employed with tomatoes etc. on window sills.

That works.

[mumofstig]

when the late Geoff Hamilton showed us how to make a cold frame, he always said it was worth painting the inside white to reflect light 

[Lawrence]

I haven't got round to it yet, but I bought an "emergency" tent from the 99p shop.
This is exactly what I bought it for, it is made from silver mylar type plastic and is very reflective.
It was the silver foil trick that inspired me!

[Kleftiwallah]

Thanks for those replies, looks like I've got a few boxes of wine to get through this winter!

Cheers,   Tony.

[Auntiemogs]

Let us know if you need any encouragement Tony, I'm sure a fair few of us could be persuaded to keep you company!   
Maybe that should be our next competition?  Guess how many wine boxes Kleftiwallah will have to drink to make his reflective barrier.  Come next spring he'll say "what barrier"!