Es probable que la hierba adventicia compita con las plantas aromáticas.

Because es probable que is an expression of probability, not certainty. In Spanish, expressions like es posible que, es probable que, es dudoso que, and puede que normally trigger the subjunctive.

So:

• Es probable que la hierba adventicia compita... = the speaker sees it as likely, but not as a confirmed fact.

If the speaker presented it as a fact, Spanish would normally use the indicative instead.

Compita is the present subjunctive of competir.

Here is the present subjunctive:

• yo compita
• tú compitas
• él/ella/usted compita
• nosotros/nosotras compitamos
• vosotros/vosotras compitáis
• ellos/ellas/ustedes compitan

It comes from the verb competir = to compete.

Because competir is an e → i stem-changing verb in many present forms.

Compare:

• indicative: compito
• subjunctive: compita

Like many -ir verbs, the stem vowel changes:

• pedir → pida
• servir → sirva
• competir → compita

So compita is the expected irregular stem form.

In standard Spanish, no. After es probable que, you normally need the subjunctive, so compita is the correct choice.

• Correct: Es probable que ... compita
• Not standard here: Es probable que ... compite

If you want the indicative, you would usually need a structure expressing certainty or fact, such as:

• Es cierto que...
• Está claro que...

It is a more technical or agricultural way to say weed or unwanted wild plant growing among crops.

In everyday Spanish, many people would simply say:

• mala hierba
• maleza (depending on context)

But hierba adventicia sounds more formal, botanical, or agronomic.

Yes, hierba is feminine, so it takes la:

• la hierba
• la hierba adventicia

A learner may wonder about nouns like agua, which use el in the singular even though they are feminine. But that special pattern happens with feminine nouns beginning with a stressed a sound. Hierba does not follow that pattern, so it stays la hierba.

Because in Spanish, descriptive adjectives are very often placed after the noun.

So these are the normal orders:

• hierba adventicia
• plantas aromáticas

This is especially common in technical, scientific, and neutral descriptions. Putting the adjective before the noun is sometimes possible in Spanish, but it often changes the tone, emphasis, or meaning.

Spanish uses the definite article much more often than English, especially when talking about a whole class of things in a general sense.

So:

• las plantas aromáticas can mean aromatic plants in general, or the aromatic plants being discussed in context.

This is very natural in Spanish. English often drops the article where Spanish keeps it.

Because competir con is the most natural and standard way to say to compete with.

• competir con alguien/algo = to compete with someone/something

Contra is possible in some contexts, especially where there is a strong sense of opposition, rivalry, or a direct contest, but con is the usual choice here. In an agricultural sentence, compite con sounds much more natural.

Literally, yes, it means aromatic plants. But in real use, it often refers to plants grown for their scent, flavour, or essential oils, including many culinary and medicinal herbs.

Depending on context, it could refer to plants such as:

• rosemary
• thyme
• mint
• lavender

So it can be broader than just plants that smell nice.

Both are possible in Spanish, but they create slightly different structures.

This sentence uses an impersonal pattern:

• Es probable que + subjunctive

Another option would be:

• Probablemente, la hierba adventicia compite...

So Spanish can express probability either with:

1. an impersonal expression: Es probable que...
2. an adverb: Probablemente...

In your sentence, the speaker chose the first pattern, which naturally leads to the subjunctive.

Que introduces the subordinate clause after es probable.

So the structure is:

• Es probable = it is likely
• que la hierba adventicia compita con las plantas aromáticas = that the weed competes / may compete with the aromatic plants

In other words, que works like that in English, although in English that is often omitted.

Spanish often uses an impersonal construction without a separate word for it.

So:

• Es probable que... literally looks like Is likely that...
• but the real meaning is It is likely that...

Spanish does not need a dummy subject like English it in this kind of sentence.