GRASS
Just to educate some of the great unknowing, grass on an airport is in most cases, a managed commodity. There are contractors who will mow the product and use it as hay or silage, thus generating income for the airport.
There is also the question of bird control. Most birds do not like walking in grass that is too deep for them to poke their heads above it to look for danger, so grass is normally deliberately allowed to grow tall enough to hide the birds completely. That helps to prevent birds from gathering on airfields and is an aid to the prevention of bird strikes, thus aiding air safety.