ookiku naru ni turete, uekae to sentei ga hituyou ni naru.

Because 大きい is an い-adjective, and when an い-adjective is used before なる to mean become ..., it changes to its -く form.

• 大きい = big
• 大きくなる = to become big / to grow bigger

This is a very common pattern:

• 暑い → 暑くなる = become hot
• 早い → 早くなる = become early / become fast
• 大きい → 大きくなる = become big

So 大きいなる is not correct Japanese.

〜につれて means as ..., in proportion as ..., or along with ....

In this sentence:

• 大きくなるにつれて = as it gets bigger
• as it grows, ...

This pattern is used when one change happens gradually, and another change happens along with it.

A simple way to think of it:

• X につれて Y = as X changes, Y also changes

Examples:

• 年を取るにつれて、記憶力が落ちる。
  As one gets older, memory declines.
• 気温が上がるにつれて、虫が増える。
  As the temperature rises, insects increase.

So here, as the plant or tree gets bigger, repotting and pruning become necessary.

That に is part of the grammar pattern 〜につれて.

The clause before it is:

• 大きくなる = become big / grow bigger

Then you attach:

• につれて = as..., along with...

So it is grouped like this:

• 大きくなる + につれて

It is not that なる itself is taking に here for some separate reason. The に belongs to につれて.

Because the sentence contains two different ideas of becoming:

1. 大きくなる = become big / grow bigger
2. 必要になる = become necessary

So the full structure is basically:

• As it becomes bigger, repotting and pruning become necessary.

This repetition is completely natural in Japanese. It may sound a little repetitive in English if translated very literally, but in Japanese it is normal.

必要 behaves like a な-adjective (often called a nominal adjective).

With なる, な-adjectives use に:

• 静かになる = become quiet
• 元気になる = become well / lively
• 必要になる = become necessary

So the pattern is:

• い-adjective + くなる
• な-adjective + になる

That is why:

• 大きい → 大きくなる
• 必要 → 必要になる

Because 植え替えと剪定 is the thing that is becoming necessary.

• 植え替え = repotting / transplanting
• 剪定 = pruning
• 植え替えと剪定が必要になる = repotting and pruning become necessary

Here, 必要 describes what is needed, and the thing that is needed is marked with が.

A literal breakdown is:

• 植え替えと剪定 = repotting and pruning
• が = are the things that...
• 必要になる = become necessary

So が marks the item(s) that are necessary.

植え替え means replanting, transplanting, or very often repotting, depending on context.

It comes from:

• 植える = to plant
• 替える = to change / replace

So literally it is something like changing where something is planted.

If the sentence is about a potted plant, repotting is probably the most natural translation.
If it is about a tree or garden plant, transplanting or replanting may fit better.

剪定 means pruning.

It is a standard word, especially in gardening, bonsai, landscaping, and plant care.
In everyday conversation, it is common enough if the topic is plants, but outside that context it is a bit specialized.

So if this sentence is about caring for a plant or tree, 剪定 is exactly the right word.

Yes. Japanese often leaves out the subject when it is clear from context.

In this sentence, something is growing bigger, but it is not explicitly named. From context, it is probably a plant, tree, bonsai, or something similar.

So the sentence naturally implies something like:

• As it gets bigger, repotting and pruning become necessary.

Japanese does this all the time. If the context already tells you what it is, there is no need to state it again.

Sometimes, but につれて is especially good when talking about gradual change and one change accompanying another.

• 〜につれて strongly suggests that as one thing changes step by step, another thing changes too.
• 〜とともに can also mean along with, but it is often a bit more formal and broader in use.

For this sentence, につれて feels very natural because the idea is:

• the plant gets bigger little by little
• the need for repotting and pruning also increases accordingly

So につれて is a very fitting choice.

The comma separates the dependent clause from the main clause.

• 大きくなるにつれて、 = as it grows bigger,
• 植え替えと剪定が必要になる。 = repotting and pruning become necessary.

This comma is very natural in Japanese writing because the first part sets up the condition or background, and the second part gives the main point.

It is not absolutely required in every sentence of this type, but it is common and helps readability.

It can mean either, depending on context.

Literally, it is become big.
But in natural English, when talking about living things such as plants, children, or animals, it is often better translated as grow.

So:

• 大きくなるにつれて
  can be understood literally as as it becomes bigger
• but more natural English is often as it grows

That is a translation choice, not a difference in the Japanese grammar itself.

It means repotting and pruning are necessary.

The particle と simply links the two nouns:

• A と B = A and B

So the sentence presents both actions together as things that become necessary.

If the speaker meant either one or the other, Japanese would usually use a different wording.

Yes, but grammatically it is not a verb here. It is a な-adjective / adjectival noun meaning necessary.

So:

• 必要だ = is necessary
• 必要になる = becomes necessary

In English, we often translate this more naturally with need:

• 植え替えと剪定が必要になる
  = it will need repotting and pruning

That English wording is natural, but in Japanese the structure is still become necessary, not a verb exactly like English need.