Learn Hardcore Japanese: syokubutu ni mizu wo yaruno wo wasurenaide kudasai.

The first を marks 水 as the object of やる (to give).
Then you nominalize 水をやる with の, making 水をやるの (“the act of watering”).
The second を marks that whole noun phrase as the object of 忘れる (to forget).
So the structure is literally “Don’t forget (忘れないでください) the act of giving water (水をやるのを).”

What is the function of の after 水をやる? Could we use こと instead?

の here is a nominalizer—it turns 水をやる into a noun-like phrase meaning “the act of watering.”
You can often swap の for こと to sound more formal or written:
植物に水をやることを忘れないでください。
Generally, の feels more conversational, while こと is a bit more formal or stiff.

Why is 植物 marked with に rather than を? Isn’t the plant what you’re watering?

You’re actually giving 水 to the plant, so 水 is the direct object (with を). 植物 is the recipient or target of that giving action, so it takes the indirect-object particle に. In English it’s like “give water to the plant.”

What’s the difference between やる and あげる when talking about giving water to plants?

Both can mean “to give,” but:

やる is more casual and often used for plants, animals, or children/pets.
あげる is more polite or neutral and used for people of equal or higher status.
You might hear 植物に水をあげてください in a polite context, but やる is still very common when referring to plants.

How is 忘れないでください formed, and how polite is it?

Take the verb 忘れる (to forget), form its negative 忘れない, add で, then attach ください. It’s a polite request meaning “please don’t forget.”

More polite than the plain negative imperative 忘れるな
A bit more formal than the casual 忘れないでね

Could we rephrase this sentence using 忘れずに? What would that look like?

Yes. 忘れずに means “without forgetting.” You can then put the main action after it. For example:

忘れずに植物に水をやってください。
忘れずに水やりをしてください。
Both mean “Please don’t forget to water the plants,” and they feel a bit more fluid by avoiding the double を nominalization.

Why is there no subject in the sentence? Who is being asked to do this?

Japanese commonly omits the subject when it’s understood from context. Here, the implied subject is “you” (the listener). So even without あなた, the sentence means “(You) please don’t forget to water the plants.”

Why nominalize 水をやる instead of saying 忘れないで植物に水をやってください?

Although 忘れないで植物に水をやってください is grammatically possible, it can feel slightly clunky because the negative request and the action are directly chained. Nominalizing (水をやるのを忘れないでください) separates the ideas cleanly: “Don’t forget the act of watering.” It emphasizes the watering as a thing to remember.

Can we use the noun 水やり (watering) here instead?

Absolutely. 水やり is a noun meaning “watering.” A natural alternative is:
植物への水やりを忘れないでください。
Here への marks the direction toward the plants, and 水やり makes the sentence concise and idiomatic.