По сравнению с орхидеей кактус крепче и не вянет, даже когда почва сухая.

Because the phrase по сравнению с requires the instrumental case.

So:

• орхидея = nominative singular
• орхидеей = instrumental singular

This is a fixed pattern:

• по сравнению с братом = compared with the brother
• по сравнению с прошлым годом = compared with last year
• по сравнению с орхидеей = compared with an orchid

So the ending -ей is there because орхидея has to be in the instrumental after с in this expression.

По сравнению с means in comparison with or compared with.

It is similar in meaning to using a comparative with чем, but the structure is different.

For example:

• Кактус крепче, чем орхидея. = The cactus is tougher than the orchid.
• По сравнению с орхидеей кактус крепче. = Compared with the orchid, the cactus is tougher.

The version with по сравнению с feels a bit more explicit and sometimes a bit more formal or explanatory. It sets up the comparison first, almost like giving the frame:

• As compared with the orchid...

So both are possible in many contexts, but they are not grammatically interchangeable word-for-word.

Крепче is the simple comparative form of крепкий.

• крепкий = strong, sturdy, tough
• крепче = stronger, sturdier, tougher

Russian often prefers this short comparative form when it exists. It is usually more natural than более + adjective.

So:

• кактус крепче sounds natural
• кактус более крепкий is possible, but less idiomatic here

This is similar to English preferring stronger over more strong.

Not necessarily literally muscular or physically strong. In this sentence, крепче is better understood as:

• hardier
• more resilient
• tougher
• more robust

With plants, крепкий often suggests that something is sturdy and can handle difficult conditions well. So here it means the cactus is better able to survive dryness than the orchid.

In Russian, the verb to be is normally omitted in the present tense.

So where English says:

• The cactus is tougher

Russian simply says:

• Кактус крепче

This is completely normal.

Russian does use forms of to be in the past and future:

• Кактус был крепче = The cactus was tougher
• Кактус будет крепче = The cactus will be tougher

But in the present, you usually do not say есть in this kind of sentence.

Вянет is:

• 3rd person singular
• present tense
• imperfective
• from the verb вянуть = to wilt

So не вянет means does not wilt or doesn’t wilt.

The imperfective is used because the sentence describes a general characteristic, not a single completed event. It means the cactus is the kind of plant that does not wilt under these conditions.

If you used a perfective verb such as завянуть, that would point more toward a single completed result:

• не завянет = will not wilt

That would change the meaning.

Даже когда means even when. It refers to a condition that is presented as real or regularly occurring.

So:

• даже когда почва сухая = even when the soil is dry

This fits the meaning well: dry soil is treated as an actual condition under which the cactus still does not wilt.

Даже если means even if, which is usually more hypothetical:

• даже если почва сухая = even if the soil is dry

That is also possible in some contexts, but it gives a slightly different feel. In this sentence, когда is better because it describes a real, typical situation rather than just a hypothetical one.

Because сухая has to agree with почва.

• почва is feminine
• singular
• nominative here

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• сухая

Compare:

• сухой воздух = dry air, masculine
• сухая почва = dry soil, feminine
• сухое лето = dry summer, neuter

Because когда почва сухая is a subordinate clause.

Russian normally separates subordinate clauses with a comma:

• ..., когда ...
• ..., если ...
• ..., потому что ...

So the comma marks the beginning of the clause when the soil is dry.

The word даже is just modifying когда, giving the meaning even when, but the clause still needs a comma.

Yes, the word order is natural.

По сравнению с орхидеей is placed first to set up the comparison right away. That gives the sentence a topic-first structure:

• Compared with the orchid, the cactus...

This is very normal in Russian.

You could also say:

• Кактус крепче и не вянет по сравнению с орхидеей, даже когда почва сухая.

But that sounds less smooth and less natural in many contexts.

Or:

• Кактус по сравнению с орхидеей крепче и не вянет, даже когда почва сухая.

This is also possible, but the original version is clearer and more elegant because it introduces the comparison frame first.