kanyousyokubutu no mizuyari wo suru.

Here の links two nouns. It is not only for possession; it can also show that one noun modifies another.

So 観葉植物の水やり means something like:

• the watering of houseplants
• houseplant watering

A helpful way to think about it is that 観葉植物 tells you what kind of 水やり it is.

水やり is a noun meaning watering or giving water, especially to plants.

It comes from:

• 水 = water
• やり = the noun-like form of やる

So 水やり is the activity of watering something.

In Japanese, this is a very common set expression for watering plants.

Because 水やり is functioning as a noun here.

Japanese often uses the pattern:

• noun + をする = do noun

So:

• 水やりをする = to do watering

This is similar to patterns like:

• 勉強をする = do studying / study
• 掃除をする = do cleaning / clean

The particle を marks 水やり as the thing being done.

Yes. That is a very natural alternative.

Compare the two:

• 観葉植物の水やりをする
• 観葉植物に水をやる

They are close in meaning, but the focus is a little different:

• 水やりをする focuses on the activity or chore of watering.
• 水をやる focuses more directly on giving water to the plants.

So if you are talking about a task on your to-do list, 観葉植物の水やりをする sounds very natural.

Traditionally, Japanese often uses やる when giving something to plants or animals, so 水やり became the standard fixed expression.

That said, in modern everyday speech, many people also say:

• 植物に水をあげる

So:

• 水やり is a very common established noun
• 水をやる is traditional and common
• 水をあげる is also heard in modern speech

For learners, the safest thing is to recognize 水やり as a standard word meaning watering.

Because in this sentence, 観葉植物 is not being marked as the indirect object of a verb. Instead, it is modifying the noun 水やり through の.

Compare:

• 観葉植物の水やりをする
  = do the watering of houseplants

• 観葉植物に水をやる
  = give water to houseplants

In the second sentence, に marks the target that receives the water.
In the first sentence, 観葉植物 is simply part of the noun phrase 観葉植物の水やり.

Yes, but that is normal in Japanese.

Japanese often leaves out the subject when it is clear from context. So this sentence could mean:

• I water the houseplants
• I’ll water the houseplants
• (Someone) waters the houseplants

The exact subject depends on the situation.

This kind of omission is extremely common and natural in Japanese.

する is the plain non-past form.

In Japanese, the non-past can refer to:

• a habitual action: I water the houseplants
• a future action: I’ll water the houseplants

So context decides the time.

For example:

• in a routine context, it may mean I water the houseplants
• in a plan or schedule context, it may mean I’m going to water the houseplants

Yes, it can be.

観葉植物の水やりをする。 is a grammatically complete plain-style sentence.

However, in real life it may also sound like:

• a statement in a diary
• a note on a to-do list
• a simple description of one’s action

Because it is plain form, it can feel a little clipped or matter-of-fact if said by itself. In conversation, speakers often add context or use a polite form such as:

• 観葉植物の水やりをします。

Normally, Japanese would write it without spaces:

• 観葉植物の水やりをする。

Pronunciation:

• 観葉植物 = かんようしょくぶつ
• 水やり = みずやり
• をする = pronounced roughly o suru

So the whole sentence is read:

• かんようしょくぶつ の みずやり を する

The spaces in your example are just there to help learners see the parts more clearly.