Green roofs make solar panels 3.6% more efficient

There’s probably 3 issues to look at based on studying solar passive house building principles.

Reflected/absorbed light, thermal mass holding onto absorbed heat over time and water evaporation.

All 3 will come into play here. The roof is likely already light coloured but with high thermal mass that absorbs and holds onto heat it does absorb over the day and radiating that nearby. Covering the roof with a lightweight and insulated material with a high solar reflectivity similar to wood over insulation for a wall would likely get part of the benefit. The other part will be the water evaporating from the plants carries energy back away.

So just painting a concrete roof white probably wouldn’t be equivalent on it’s own though I am really curious whether putting some lightweight siding over an insulation barrier would perform most of the same work.

Consider also ultimately someone needs to maintain all those plants not to overgrow the panels :)

This isn't terribly clear from the title, but I assume that they only tested plants, in which case evaporative cooling is also relevant.

They should have just compared it to a roof literally painted green (or white) though, as a control.