Either option "the 50 plants" vs "50 plants" would be grammatically correct. They have slightly different implications however, so one sentence could be better than the other depending on what the author is trying to convey.

"the" is used to refer to an exact or specific item (or set of items).
"Bring me a knife" is about an unspecified item. Any knife would do
"Bring me the knife" is about a specific item, so it is best used when there is no room for confusion over which knife is discussed; either there is only one knife available, or the sentence is about a knife that was previously identified.

In the sentence you posted, "watering the 50 plants in her yard" implies that these are all the plants in the yard (similar to the case above in which there is only one knife). On the other hand, "watering 50 plants in her yard" is less specific; it could mean that there are many more plants, but she waters an unspecified group of 50 plants.

Hope that was helpful,
-Patrick