Ljeti biljke trebaju češće zalijevanje.

Ljeti means in summer or during the summer.

It is an adverb, not a noun phrase. Croatian often uses special seasonal adverbs like:

• ljeti = in summer
• zimi = in winter

So in this sentence, ljeti works as a time expression: In summer...

Biljke is the nominative plural of biljka = plant.

It is in the nominative because it is the subject of the sentence:

• biljka = plant
• biljke = plants

So biljke trebaju... means plants need...

Because it agrees with biljke, which is plural.

Croatian verbs usually agree with the subject in person and number:

• biljka treba = the plant needs
• biljke trebaju = the plants need

So trebaju is simply the 3rd person plural form of trebati.

Here trebati means to need.

So the structure is:

• biljke trebaju nešto = plants need something

In this sentence, the thing they need is češće zalijevanje = more frequent watering.

Croatian also uses trebati in another pattern:

• treba + infinitive = it is necessary to...

For example:

• Biljke treba češće zalijevati.
  = Plants need to be watered more often / It is necessary to water plants more often

So trebati can work in more than one way.

It is in the accusative singular, because it is the direct object of trebaju.

However, zalijevanje is a neuter noun, and for many neuter nouns the nominative singular and accusative singular look the same.

So:

• nominative: zalijevanje
• accusative: zalijevanje

That is why it does not change form.

Because here it means watering as a general activity, not separate individual watering events.

English does the same thing:

• Plants need more frequent watering.

Croatian uses the singular for that general idea:

• češće zalijevanje

If you wanted to talk about separate occasions, the plural would be possible in the right context, for example:

• dva zalijevanja tjedno = two waterings per week

Zalijevanje is a verbal noun, meaning watering.

It comes from the verb zalijevati = to water.

Croatian often forms nouns of action from verbs:

• čitati → čitanje = reading
• učiti → učenje = learning
• zalijevati → zalijevanje = watering

So zalijevanje is literally the act/process of watering.

In this sentence, češće is best understood as a comparative adjective agreeing with zalijevanje.

• čest = frequent
• češći / češća / češće = more frequent

Since zalijevanje is neuter singular, the matching form is češće:

• češće zalijevanje = more frequent watering

This can be confusing because češće is also the form used as an adverb:

• češće zalijevati = to water more often

So the same form can correspond to:

• more frequent before a neuter singular noun
• more often with a verb

Yes. A very common alternative is:

• Ljeti biljke treba češće zalijevati.

This uses the infinitive zalijevati instead of the verbal noun zalijevanje.

The difference is mostly stylistic:

• češće zalijevanje = more frequent watering
• češće zalijevati = to water more often

Both are natural. The version with zalijevanje sounds a bit more noun-based, while the version with zalijevati sounds a bit more verbal and direct.

Yes. Croatian word order is fairly flexible.

These are all possible:

• Ljeti biljke trebaju češće zalijevanje.
• Biljke ljeti trebaju češće zalijevanje.

The meaning stays basically the same, but the emphasis changes a little.

Starting with ljeti puts the time frame first:

• In summer, plants need more frequent watering.

That is a very natural choice here.

Because Croatian does not have articles.

So biljke can mean:

• plants
• the plants

and zalijevanje can mean:

• watering
• the watering

The exact meaning depends on context. English speakers often look for the or a, but Croatian simply does not use them.

It is one of the most common and natural ways.

For general seasonal meaning, ljeti is very idiomatic:

• Ljeti je vruće. = It is hot in summer.

You may also hear other expressions in different contexts, but for a general statement like this, ljeti is an excellent and very normal choice.