Vi brukte fiberduken selv når plantene var herdet.

Because fiberduken is the definite singular form: the fiber cloth / the row cover.

• en fiberduk = a fiber cloth / a row cover
• fiberduken = the fiber cloth / the row cover

In Norwegian, definiteness is often added as an ending on the noun:

• en plante = a plant
• planten = the plant

So Vi brukte fiberduken means We used the fiber cloth / the row cover, referring to a specific one that is known from the context.

Brukte is the preterite (simple past) of å bruke = to use.

• jeg bruker = I use / am using
• jeg brukte = I used

So:

• Vi brukte fiberduken = We used the row cover

The sentence is talking about something that happened in the past, so the simple past is natural here.

Selv når means even when.

It adds emphasis and shows that something was true despite a condition or stage where you might expect otherwise.

So in this sentence:

• Vi brukte fiberduken selv når plantene var herdet
• We used the row cover even when the plants were hardened off

The idea is: using the fiber cloth continued even at a point when someone might think it was no longer necessary.

This is a very common question.

In Norwegian:

• når is often used for general, repeated, or less specific "when"
• da is used for one specific occasion in the past

Examples:

• Når jeg er trøtt, drikker jeg kaffe.
  When I am tired, I drink coffee.
• Da jeg kom hjem, var det mørkt.
  When I came home, it was dark.

In your sentence, selv når plantene var herdet is not mainly pointing to one single dramatic moment. It refers more to the stage/condition when the plants were hardened off. That is why når works well.

A learner should also know that in real Norwegian, the line between når and da is not always perfectly simple in every context, but the basic rule above is the one to remember.

Var herdet is:

• var = past tense of å være (to be)
• herdet = past participle of å herde (to harden)

Together, var herdet can mean something like:

• was hardened
• had been hardened off
• was in a hardened/hardened-off state

In gardening contexts, herdet often refers to plants being hardened off, meaning they have been gradually adapted to outdoor conditions.

So this is not just a literal made hard. In context, it is a standard gardening idea.

Not exactly.

• herdet usually means hardened or hardened off
• hardy in English often means naturally able to survive cold or difficult conditions

So:

• plantene var herdet = the plants had been hardened off / were acclimatized
• hardføre planter would be closer to hardy plants

That is an important difference. Herdet often describes a process the plants have gone through, not just an inborn quality.

Because this is a subordinate clause introduced by når.

In Norwegian subordinate clauses, the normal order is:

subject + verb

So:

• når plantene var herdet

not:

• når var plantene herdet

This is different from main clauses, where Norwegian often uses verb-second (V2) word order.

Compare:

• Plantene var herdet. = main clause
• ... når plantene var herdet. = subordinate clause

This is a very important pattern in Norwegian grammar.

Yes, you possibly could, but the meaning shifts slightly.

• selv når = even when
• selv om = even though / although

So:

• selv når plantene var herdet focuses on the time/stage/condition
• selv om plantene var herdet focuses more on contrast

In practice, both can sound natural depending on context, but they are not identical.

Here, selv når suggests: at the stage when the plants had already been hardened off, we still used the row cover

While selv om would suggest: despite the fact that the plants were hardened off, we still used the row cover

This is more about meaning and emphasis than grammar.

The sentence is highlighting that they continued using the fiber cloth even after the plants had reached a stage where you might expect it to stop being necessary.

So the point is:

• We used it anyway
• We still used it
• Even then, we used it

That is exactly why selv når is there: it creates a contrast with the listener's expectation.

Yes, fiberduk is a real and common gardening/agricultural word in Norwegian.

In this context, it usually means a light protective fabric placed over plants, often called things like:

• row cover
• garden fleece
• horticultural fleece

It helps protect plants from:

• cold
• wind
• insects
• harsh weather

So fiberduken in this sentence is not just any random cloth. It is a specific type of protective gardening material.

It can often suggest a general past practice, not just one single event.

• Vi brukte fiberduken ... can mean we used the row cover
• depending on context, it may also imply we would use the row cover / we used to use the row cover

Because of the combination of past tense and når, many readers may understand it as describing what they did in that kind of situation.

So the sentence could describe:

• one specific past case, or
• a general gardening practice in the past

The wider context would decide that more clearly.