У этого ростка тонкий стебель, но крепкий корень.

In Russian, possession is very often expressed with the pattern:

У + possessor in the genitive + possessed thing

So:

У этого ростка тонкий стебель, но крепкий корень.
literally = At this sprout, [there is] a thin stem, but a strong root
natural English = This sprout has a thin stem but a strong root.

This is one of the most common Russian ways to say have/has.

Compare:

• У меня есть книга. = I have a book.
• У дерева толстый ствол. = The tree has a thick trunk.

So here у introduces the owner/possessor: this sprout.

Because the preposition у requires the genitive.

Dictionary forms are:

• этот росток = this sprout
• after у it becomes у этого ростка

That is why both words change:

• этот → этого
• росток → ростка

This is completely normal after у when it means possession.

Russian usually omits the present-tense verb to be in simple statements.

So instead of something like This sprout has..., Russian often just says:

У этого ростка тонкий стебель...

The verb is understood.

A more literal expanded idea would be something like:

У этого ростка есть тонкий стебель...

But есть is often omitted when the point is describing a characteristic rather than simply stating existence.

Here the sentence is describing the sprout’s features, so leaving out есть sounds natural.

Because they are the things being talked about as the sprout’s features:

• тонкий стебель
• крепкий корень

In this possession pattern, the possessor goes after у in the genitive, but the possessed nouns are usually in the nominative.

So:

• у этого ростка = of/at this sprout
• тонкий стебель = a thin stem
• крепкий корень = a strong root

The adjectives match the nouns:

• тонкий
  • стебель: masculine singular nominative
• крепкий
  • корень: masculine singular nominative

Both adjectives are masculine singular nominative, agreeing with masculine singular nouns:

• стебель is masculine
• корень is masculine

So the adjectives must also be masculine singular nominative:

• тонкий стебель
• крепкий корень

The ending -ий here is just the normal dictionary/nominative masculine form for these adjectives.

Compare:

• тонкая ветка = feminine
• тонкое растение = neuter
• тонкие листья = plural

Because но means but, and in Russian it normally introduces a contrasting clause or phrase, so it is preceded by a comma.

So:

У этого ростка тонкий стебель, но крепкий корень.

This is standard punctuation, just like English often uses a comma before but.

Yes, you can think of it that way.

The full idea is:

У этого ростка тонкий стебель, но у этого ростка крепкий корень.

Russian does not repeat у этого ростка because it is already clear from the first part. This is a very natural omission.

So the sentence means:

• this sprout has a thin stem
• but it has a strong root

Yes, Russian word order is flexible, but this order is the most neutral and natural for introducing the possessor first:

У этого ростка — topic: we are talking about this sprout
тонкий стебель, но крепкий корень — comment: what it is like

This word order feels like:

As for this sprout, it has a thin stem but a strong root.

Other orders are possible, but they change emphasis and may sound less neutral. For example:

• Тонкий стебель, но крепкий корень у этого ростка.
  This puts more emphasis on the features themselves.

For a learner, the original order is the best model to remember.

They describe two different qualities:

• тонкий стебель = a thin/slender stem
• крепкий корень = a strong/sturdy/robust root

Крепкий often means something like solid, strong, firm, sturdy, depending on context.

So here the contrast is:

• the stem is delicate or narrow
• the root is strong and well-developed

That contrast is exactly why но is used.

Russian has no articles.

So тонкий стебель can mean:

• a thin stem
• the thin stem

and the exact interpretation comes from context.

In this sentence, English naturally uses a thin stem and a strong root, but Russian does not need separate words for that.

Yes, it is grammatically possible, but it changes the feel slightly.

• У этого ростка тонкий стебель... sounds like a description of the sprout’s qualities.
• У этого ростка есть тонкий стебель... sounds more like emphasizing that such a stem exists or that the sprout possesses one.

In many descriptive sentences about body parts, plant parts, and characteristics, Russian often omits есть.

So the version without есть is more natural here.

The dictionary form is росток.

It means sprout, shoot, or seedling, depending on context.

In the sentence, it appears as ростка because it is in the genitive singular after у:

• dictionary form: росток
• after у: у ростка

With the demonstrative:

• этот росток = this sprout
• у этого ростка = this sprout’s / this sprout has

Yes. Because there is no explicit tense marker and no verb like grew or became, the sentence is simply stating a characteristic of the sprout.

It reads like a descriptive fact:

This sprout has a thin stem, but a strong root.

That could be used in:

• a biology description
• a gardening comment
• a comparison of plant development

So the sentence is not about an action; it is about traits.