Ποτίζω το υπόστρωμα με το ποτιστήρι και φυτρώνουν νέες ρίζες.

Because Greek verbs usually already show the subject.

• ποτίζω = I water
• The ending -ω tells you it is 1st person singular.

So Greek often leaves out the pronoun εγώ (I) unless it is needed for emphasis or contrast.

• Ποτίζω το υπόστρωμα = I water the substrate
• Εγώ ποτίζω το υπόστρωμα = I’m the one watering the substrate

This is very common in Greek.

φυτρώνουν is the 3rd person plural present of φυτρώνω.

So it means:

• they sprout
• they grow

In this sentence, the they is νέες ρίζες (new roots).

So:

• φυτρώνουν νέες ρίζες = new roots grow / sprout

Greek word order is more flexible than English word order.

In English, you usually need:

• New roots grow

But Greek can also very naturally say:

• Φυτρώνουν νέες ρίζες

This is still understood as new roots grow, because Greek relies more on verb endings, articles, and context than English does.

Both orders are possible:

• Φυτρώνουν νέες ρίζες
• Νέες ρίζες φυτρώνουν

The first one can sound a bit more like roots are starting to appear/grow.

Mainly from the verb and the meaning.

• φυτρώνω is normally intransitive here, so it does not take a direct object.
• That means νέες ρίζες must be the subject: new roots are the ones doing the growing.

There is also a grammar point here:
for feminine plural nouns like ρίζες, the nominative and accusative forms can look the same in Modern Greek. So sometimes you identify the role from context rather than from the ending alone.

Because the adjective must agree with the noun in gender, number, and case.

• ρίζες = feminine plural
• so new must also be feminine plural: νέες

That gives:

• νέες ρίζες = new roots

Greek adjectives always match the nouns they describe.

Because both nouns are neuter singular.

• το υπόστρωμα
• το ποτιστήρι

The article το is the neuter singular form of the.

In this sentence:

• το υπόστρωμα is the direct object of ποτίζω
• το ποτιστήρι comes after με

In Modern Greek, the article is used very often, usually more often than in English.

It means with the watering can or using the watering can.

Here με means with and shows the instrument used to do the action.

So:

• Ποτίζω το υπόστρωμα με το ποτιστήρι
  = I water the substrate with the watering can

Modern Greek does not have a separate instrumental case, so it usually uses με + accusative instead.

Both verbs are in the present tense:

• ποτίζω = I water / I am watering
• φυτρώνουν = they grow / they are growing

In Greek, the present tense can be used for:

• something happening now
• a habitual action
• a general process
• instructions or descriptions of a procedure

So this sentence can sound like:

• a general statement
• a step in plant care
• a description of what happens when you do this

Its basic meaning is and, but in context it can also feel like and then or and as a result.

So here:

• Ποτίζω το υπόστρωμα με το ποτιστήρι και φυτρώνουν νέες ρίζες

can be understood as:

• I water the substrate with the watering can, and new roots grow
• or more naturally in context:
• I water the substrate with the watering can, and then/as a result new roots grow

Greek often uses simple και where English might choose and, so, or then depending on context.

In everyday Greek, υπόστρωμα literally suggests something like an under-layer or base layer.
In gardening or plant contexts, it means the substrate, growing medium, or the material the plant is rooted in.

So in this sentence it is not just any ground or soil word; it is a more specific plant-care term.

Yes. Greek allows some flexibility.

For example, these are also possible:

• Ποτίζω με το ποτιστήρι το υπόστρωμα και φυτρώνουν νέες ρίζες.
• Νέες ρίζες φυτρώνουν όταν ποτίζω το υπόστρωμα με το ποτιστήρι.

But the original sentence is very natural and straightforward.

A useful thing to notice is that Greek often keeps the most important new information later in the clause, which is one reason νέες ρίζες can come after φυτρώνουν.