[brinylon]

I guess part of the question is obvious about the amount of light roots usually get in the ground, but I wondered if I would actually be harming my plants (celeriac in this case), if I used some old pasta sauce pots I found. Do some plants benefit more / less than others?

[Trillium]

Are the pots long term or just short term until potting outside? Either way, there's usually enough soil in the pot to keep the roots shaded. I don't think they really like much direct sun.

[iwantanallotment]

I've got cabbages, lettuces & other bits growing nicely in clear plastic lids from DVD/CD tubs, but they will be planted before the roots reach the sides/bottom & become 'exposed'.
Got things in chicken trays, Pot Noodle tubs, punnets & allsorts - anything with sides and a bottom I can make holes in will do  

[diggerjoe]

I collect all the plastic water cups from the office water cooler and ours are a thirsty lot and put a hle in the bottom and use them for potting on. I did get them to try biodegradable paper cups but they didnt like them pity as they make good pots for runners. Got all my courgettes sweetcorn and beans in them and everything is growing well. I'm now wonder what else I can recycle from the office  

[cawdor2001]

DJ, i use old plastic cups too they are great, as deep and just wash and reuse after planting out, perfick  

Cawdor

[gobs]

And then there is soilless growing systems too, you should be fine.

[brinylon]

so what happens when the roots get the sun?