Au printemps, ma tante achète du terreau pour les pots sur la terrasse.

Au is the contraction of à + le. In this expression, au printemps means in the spring / in spring.

So:

• au printemps = in spring
• literally: at the springtime

This is just the normal French expression. You would not usually say en printemps.

A useful comparison:

• au printemps
• en été
• en automne
• en hiver

So spring behaves a little differently from the other seasons here.

The phrase Au printemps has been moved to the front of the sentence to set the time first.

• Au printemps, ma tante achète du terreau...
• Ma tante achète du terreau... au printemps.

Both are possible. The comma helps separate that opening time expression from the rest of the sentence. It is very natural in writing.

Ma is a possessive adjective, meaning my. In French, a possessive adjective normally replaces the article.

So you say:

• ma tante = my aunt

Not:

• la ma tante

And la tante would mean the aunt, not my aunt.

The infinitive is acheter = to buy.

In the present tense, this verb often changes its spelling in some forms:

• j’achète
• tu achètes
• il/elle achète
• nous achetons
• vous achetez
• ils/elles achètent

So in your sentence, ma tante is she, so the verb form is achète.

This is a common stem change in French: the e becomes è in several forms.

Achète is in the present tense.

In a sentence like this, the present tense often describes a habit or something that happens regularly, not necessarily right now.

So the idea is more like:

• In spring, my aunt buys potting soil...
• or Every spring, my aunt buys potting soil...

The time phrase Au printemps makes that habitual meaning very natural.

Du here is a partitive article. It is used for an unspecified quantity of something that is usually not counted as separate items.

So:

• du terreau = some potting soil

You use du because terreau is being treated as a material/substance, like:

• du pain = some bread
• du café = some coffee

It is not referring to all potting soil in general, just some amount of it.

Les pots means the pots, so these are understood as specific pots, probably the ones already known in the situation.

• pour les pots = for the pots

Using des pots would mean for some pots or for pots in general, which is less specific.

Because the sentence continues with sur la terrasse, the most natural reading is that these are the pots on the terrace.

In this sentence, the most natural meaning is that sur la terrasse describes the pots:

• les pots sur la terrasse = the pots on the terrace

So the sentence is understood as: your aunt buys potting soil for the pots that are on the terrace.

Grammatically, word order can sometimes leave a little room for ambiguity, but meaning and common sense make this interpretation the obvious one here. If someone wanted to make it completely explicit, they could say:

• pour les pots qui sont sur la terrasse

Terrasse is a feminine noun, so its singular definite article is la.

• la terrasse = the terrace

French nouns have grammatical gender, so you have to learn them with their article:

• le pot
• le terreau
• la tante
• la terrasse

Yes. That word order is also correct.

• Au printemps, ma tante achète du terreau pour les pots sur la terrasse.
• Ma tante achète du terreau pour les pots sur la terrasse au printemps.

The first version emphasizes the time right away. The second sounds a bit more neutral. Both are natural French.