Kad začinsko bilje nikne, može brzo uvenuti bez vode.

Both kad and kada mean when.

• kad is the shorter, very common form in everyday Croatian.
• kada is slightly fuller and can sound a bit more formal or careful.

So:

• Kad začinsko bilje nikne...
• Kada začinsko bilje nikne...

Both are correct here.

Začinsko bilje means herbs or more literally seasoning herb vegetation / herb plants, but grammatically it behaves as a singular neuter noun phrase.

• bilje = herbage, plants, herbs
• začinsko = seasoning, spice-related

Even though the meaning in English may feel plural, Croatian treats bilje as a collective/mass-type noun, so the verb is singular:

• začinsko bilje nikne
• začinsko bilje može uvenuti

not mogu, because the subject is grammatically singular.

Because it has to agree with bilje.

The noun bilje is:

• neuter
• singular
• here in the nominative case

So the adjective must match:

• masculine singular nominative: začinski
• feminine singular nominative: začinska
• neuter singular nominative: začinsko

That is why you get začinsko bilje.

Nikne is from the verb niknuti, which means to sprout, to come up, to emerge from the ground, or to start growing.

So Kad začinsko bilje nikne... means something like:

• When the herbs sprout...
• Once the herbs come up...

It refers to the moment the plant has emerged, not the whole ongoing growing process.

This is a very common Croatian pattern.

Croatian often uses a perfective present after kad to talk about:

• a future event
• a repeated general situation
• one event happening once another event is completed

So Kad začinsko bilje nikne, može brzo uvenuti bez vode. does not mean simple present in the English sense. It means something like:

• When the herbs sprout, they can quickly wilt without water.
• Once the herbs have sprouted, they may quickly wilt without water.

This is normal Croatian usage.

Because the grammatical subject is začinsko bilje, which is singular.

Even though English often says herbs as a plural idea, Croatian bilje behaves grammatically as a singular noun, so the verb must also be singular:

• začinsko bilje može = the herbs/herb plants can

If you used a clearly plural noun, then you would use mogu, for example:

• Kad začinske biljke niknu, mogu brzo uvenuti bez vode.

That version uses biljke = plants, which is plural.

Croatian usually does not need subject pronouns when the subject is already clear.

Here the subject is already stated:

• začinsko bilje

So Croatian simply continues with:

• može brzo uvenuti

A pronoun such as ono would usually be unnecessary and sound heavy unless you wanted special emphasis.

Uvenuti means to wilt or to wither, usually with the sense of reaching that result.

It is the perfective partner of venuti.

A simple way to think of them:

• venuti = to be wilting, to wilt as an ongoing process
• uvenuti = to wilt completely / to end up wilted

In this sentence, uvenuti fits well because it refers to the result that can happen quickly after sprouting and lack of water.

Brzo means quickly. It is an adverb, so it modifies the verb phrase:

• može brzo uvenuti = can quickly wilt

Its position is natural and common. Croatian word order is fairly flexible, but this order sounds neutral:

• može brzo uvenuti

You could also hear:

• može uvenuti brzo

but that is less neutral in everyday style.

The preposition bez always takes the genitive case.

So:

• voda = nominative
• vode = genitive singular

That is why the sentence has:

• bez vode = without water

Other examples:

• bez šećera = without sugar
• bez pomoći = without help

It is in the nominative case because it is the subject of the sentence.

You can see this from the structure:

• Kad začinsko bilje nikne = When the herbs sprout
• začinsko bilje može brzo uvenuti = the herbs can quickly wilt

The same noun phrase is understood as the subject throughout the sentence.

Because Kad začinsko bilje nikne is a subordinate clause introduced by kad.

Croatian normally separates that kind of clause from the main clause with a comma:

• Kad začinsko bilje nikne, može brzo uvenuti bez vode.

This is similar to English writing:

• When the herbs sprout, they can quickly wilt without water.

Yes, possibly, depending on nuance.

For example:

• Kad začinsko bilje iznikne...

Izniknuti can also mean to sprout up / emerge. In many contexts it is very close to niknuti.

Still, niknuti is perfectly natural here. A learner should mainly understand that Croatian has several related plant-growth verbs, and the exact choice can depend on style, region, or the speaker’s preferred nuance.

Most naturally, it expresses a general fact or general possibility:

• once herbs have sprouted, they can quickly wilt without water

The modal verb može helps give that general meaning:

• može = can / may / is able to

So the sentence is not just about one particular herb at one particular moment. It sounds like a general observation about plants.