Kad budemo imali više vremena, posadit ćemo još sadnica uz ogradu.

Because after kad when you are talking about a future situation, Croatian normally uses future II in the subordinate clause.

So Croatian says:

• Kad budemo imali više vremena... = When we have more time...

not:

• Kad ćemo imati više vremena... in this meaning

A good rule of thumb:

• main future statement: usually future I
• clause with kad, ako, čim, nakon što referring to the future: often future II

Here the main clause is:

• posadit ćemo još sadnica uz ogradu

and the kad clause uses future II:

• kad budemo imali više vremena

This is one of the biggest differences from English future clauses.

It is future II of the verb imati (to have).

It is formed with:

• a form of biti: budem, budeš, bude, budemo, budete, budu
• plus the l-participle of the main verb

So here:

• budemo = we will / we are to
• imali = participle of imati

Together:

• budemo imali = when we will have / when we have

A useful note: the participle agrees in number and sometimes gender.

• budemo imali = masculine/mixed plural
• budemo imale = all-female plural

So if a group of women is speaking, you may hear:

• Kad budemo imale više vremena...

This is the normal Croatian way of writing future I with many verbs ending in -ti.

From the infinitive posaditi, Croatian removes the final -i before adding the clitic form of htjeti:

• posaditi
  • ćemo
• becomes posadit ćemo

This happens with many verbs:

• raditi → radit ću
• pisati → pisat ćemo
• gledati → gledat ćeš

So posadit ćemo is the standard Croatian form.

This is about aspect, which is very important in Croatian.

• saditi = imperfective, focuses on the process or repeated action
• posaditi = perfective, focuses on a completed action

Here the meaning is a single completed future action:

• we will plant some more seedlings

So Croatian naturally uses the perfective verb:

• posadit ćemo

If you used saditi, it would sound more like:

• planting in general
• being engaged in planting
• repeated or ongoing planting

In this sentence, the idea is once we have more time, we’ll get the planting done, so posaditi is the better choice.

Because više (more) usually requires the following noun to be in the genitive.

The noun is:

• nominative singular: vrijeme = time
• genitive singular: vremena

So:

• više vremena = more time

This is very common in Croatian with quantity expressions:

• puno vode = a lot of water
• malo novca = little money
• više vremena = more time

Since time here is an uncountable idea, Croatian uses the genitive singular: vremena.

Because još here means more / additional, and in this quantity meaning Croatian often uses the genitive plural after it.

The noun is:

• nominative singular: sadnica = seedling / sapling
• nominative plural: sadnice
• genitive plural: sadnica

So:

• još sadnica = more seedlings

This works like other quantity expressions:

• puno sadnica = many seedlings
• nekoliko sadnica = several seedlings
• još sadnica = more seedlings

A small detail that can confuse learners: the genitive plural form sadnica looks the same as the nominative singular. But here it is definitely plural in meaning.

The preposition uz usually takes the accusative and means things like:

• along
• by
• next to

The noun is:

• nominative: ograda = fence
• accusative: ogradu

So:

• uz ogradu = along the fence / by the fence

That is why you see ogradu, not ograda.

Yes. Kad and kada both mean when.

In this sentence:

• Kad budemo imali više vremena...
• Kada budemo imali više vremena...

Both are correct.

The difference is mostly style:

• kad = a bit shorter, very common in everyday speech
• kada = a bit fuller, sometimes slightly more formal or emphatic

So a learner can safely use either one.

Because Croatian often drops subject pronouns when they are not needed.

The verb already tells you the person and number:

• budemo imali = we will have
• posadit ćemo = we will plant

So Croatian does not need to say mi unless the speaker wants emphasis or contrast.

Compare:

• Kad budemo imali više vremena, posadit ćemo još sadnica uz ogradu.
  Normal, neutral.

• Kad budemo imali više vremena, mi ćemo posaditi još sadnica uz ogradu.
  More emphatic: we will do it.

The word order can change somewhat, but this version is very natural and standard:

• Kad budemo imali više vremena, posadit ćemo još sadnica uz ogradu.

Croatian word order is more flexible than English, but not completely free. You can move things for emphasis, for example:

• Posadit ćemo još sadnica uz ogradu kad budemo imali više vremena.

This means the same thing, just with the main clause first.

What usually stays important is:

• kad budemo imali remains together as the future clause
• uz ogradu stays as a prepositional phrase
• the future form posadit ćemo stays in its normal shape

So yes, the order can vary, but the original sentence is a very good neutral model to learn.

Yes, it can.

In future II, the participle agrees with the subject in gender and number. So:

• mixed group / default plural: budemo imali
• all-female group: budemo imale

That means both of these are possible:

• Kad budemo imali više vremena...
• Kad budemo imale više vremena...

In many grammar explanations, the masculine plural form is shown as the default, but in real Croatian an all-female group can definitely use imale.