"The roots need a lot of water."
Translation:Rødderne behøver meget vand.
"Mange" is used for countable or plural collective nouns:
"Hvor mange penge?" = How much money (lit. How many monies?)
"Der er mange køer" = There are many cows
"Meget" is for uncountable nouns and to describe the degree of an adjective
"Meget vand" = A lot of water
"Hun er meget smuk" = She is very beautiful
Here, "vand" is an uncountable noun, and so takes "meget"