6" rockwool roots ebb & flow

Can I line the bottom of my 2x4 flood tray with hydroton? I didnt use a plastic film to cover the tray to block the light and the roots are dying coming out of the blocks.

Yes

Your plants will be fine w/o it but a cocomat would be better imo

Next time you can also cover all the blocks with a large piece of pandafilm or something like that.
No light, and the enviroment will stay damp. Ideal for your roots to absorb oxygen.

It is a bit more hassle to get your plugs in, but after that it is easy.

Should work alright.