Si la raíz sigue creciendo así, tendremos que cambiar la maceta por otra más grande.

Because this is a real, possible condition: If the root keeps growing like this, we'll have to...

In Spanish, real conditions normally use:

• si + present indicative
• then a main clause in future, present, or another appropriate tense

So this pattern is very normal:

• Si llueve, nos quedaremos en casa.
• Si tienes tiempo, te llamo.
• Si la raíz sigue creciendo así, tendremos que cambiar la maceta...

You would not use the subjunctive after si in this kind of sentence.

Tendremos que means we will have to.

It is the future of tener que + infinitive, which expresses obligation or necessity:

• tengo que = I have to
• tenemos que = we have to
• tendremos que = we will have to

Here it fits because the speaker is talking about a future consequence of the condition:

• If the root keeps growing like this, we’ll have to change the pot...

So the timing is:

1. the root continues growing
2. as a result, in the future, they will need to do something

Seguir + gerund means to keep doing or to continue doing.

So:

• crece = it grows / it is growing
• sigue creciendo = it keeps growing / it continues growing

The sentence wants to emphasize that the growth is ongoing, not just happening once. That makes sigue creciendo the more natural choice.

Similar examples:

• Sigue lloviendo. = It keeps raining.
• Sigo estudiando. = I keep studying.

Here así means something like:

• like this
• in this way
• at this rate

So Si la raíz sigue creciendo así can be understood as:

• If the root keeps growing like this
• If the root continues growing at this rate

It refers to the way or speed of the growth that the speaker is observing.

Spanish often uses the singular when referring to something as a general part of the plant, even if in reality there may be more than one root.

Here la raíz can mean:

• the main root
• the root system, spoken of in a simple everyday way

This is very natural. In practical speech about plants, someone may say la raíz even when the idea is not scientifically precise.

The accent mark shows the stress and also helps indicate that a and í are pronounced in separate syllables.

• ra-íz

Without the accent, Spanish spelling rules would suggest a different pronunciation pattern. The written accent makes it clear that í is stressed.

So raíz is pronounced with two syllables, roughly ra-EETH in Spain.

This is a very common structure:

• cambiar X por Y = to replace X with Y / to swap X for Y

So:

• cambiar la maceta por otra más grande = change the pot for a bigger one / replace the pot with a bigger one

More examples:

• Cambié mi coche por uno nuevo.
• Tenemos que cambiar esta silla por otra.

Here por introduces the thing that will replace the original one.

In Spanish, otro / otra usually does not take the indefinite article un / una before it.

So Spanish says:

• otro libro
• otra casa
• otra más grande

Not:

• un otro libro
• una otra casa

In this sentence, otra means another one, and más grande describes that new pot:

• another, bigger one

Because otra is functioning like another one, and más grande adds extra description to that implied noun.

The full idea is:

• otra maceta más grande

But Spanish often avoids repeating the noun when it is obvious, so maceta is omitted after otra:

• cambiar la maceta por otra más grande

That literally means:

• replace the pot with another bigger one

This is a very natural Spanish pattern.

Más grande is the most natural choice here for a physical object like a pot.

• una maceta más grande = a bigger pot

You might sometimes hear mayor, but for everyday speech about size, más grande is much more common and straightforward.

So for a learner, más grande is definitely the safest and most natural option here.

Because the sentence begins with the si-clause:

• Si la raíz sigue creciendo así, ...

In Spanish, when this kind of introductory conditional clause comes first, it is normally separated from the main clause with a comma.

So this is standard punctuation:

• Si vienes, te ayudo.
• Si hace calor, abrimos la ventana.
• Si la raíz sigue creciendo así, tendremos que...

If the order is reversed, the comma is often omitted:

• Tendremos que cambiar la maceta por otra más grande si la raíz sigue creciendo así.

Yes, possibly, but the meaning shifts slightly.

• cambiar la maceta por otra más grande focuses on replacing the pot
• trasplantar means to repot / transplant, focusing more on moving the plant into new soil or a new container

So a natural alternative could be:

• Si la raíz sigue creciendo así, tendremos que trasplantarla a una maceta más grande.

That means:

• If the root keeps growing like this, we’ll have to repot it into a bigger pot.

Both are possible, but the original sentence is specifically built around the idea of changing one pot for another.

Because maceta is a feminine noun in Spanish, so it takes feminine words with it:

• la maceta
• otra
• más grande (same form for masculine and feminine here)

That is why the sentence says:

• la maceta
• por otra más grande

If the noun were masculine, you would see otro instead of otra.

Yes, it sounds natural and standard in Spain.

The vocabulary is very normal:

• raíz = root
• maceta = flowerpot / plant pot
• cambiar X por Y = replace X with Y

A speaker in Spain would understand it immediately, and it sounds like ordinary everyday language about plants.