"Light is at least as important for plants as water."
Translation:Licht ist mindestens genauso wichtig für Pflanzen wie Wasser.
Maybe with time they will introduce more pop-up messages whenever these new phrases are first encountered. Duolingo's still a relatively young learning site.
I happen to like "...every as bit as..." better than "..at least as..."
"als" is used when there is a difference between the things you compare. For example: "Hans ist größer ALS Paul"="Hans is taller THAN Paul"
If the there is no differnce between the things you compare, you use "genauso [adjective] wie" Example: "Licht ist GENAUSO wichtig WIE Wasser"="Light is as important as water"
So "Licht ist für Pflanzen genauso notwendig wie Wasser" would be a correct sentence, but for this translation you would also need the "mindestens"