My parents have a house with a plot of land with a garden on it.

Every year, my mother dive deep intofocuses on the garden.

Her preparation for the season starts in spring when she cultivates the seedlings of different plants.

But the hottesHowever, the most difficult part starts in May when she gets drownimmerses herself in ther garden routine.

She goes out at sunrise and grets back byurns at sunset.

When my grandma was alive, she always helped my mom around with everythingin the garden.

But in the last year grandma got unwell andLast year when my grandmother was unwell, she spent most of the time she was just watching.

She was just chillwatching alongside athe company of her cat.

This sentence is too informal for the text

My parents have a house with a plot with agarden (or garden plot) on it.

Don't have to say "plot" as "garden" is good enough.

Every year my mother dives deep into the garden.

dives = one/singular; dive = more than one/plural. In this case, mother is one person. Therefore, "dives".

But the hottbusiest part starts in May when she gets drown in the garden routine.

hottest = temperature (e.g. weather) or popular (e.g. this new band is the hottest topic right now).

When my grandma was alive, she always helped my mom around (around is okay, but can also use out, e.g. helped out my mum) with everything.

Every year my mother dives deep into the garden.

But the hottbusiest part starts in May when she gets drown in the garden routine.

Since you're talking about gardening the "hottest" part is usually referent to temperature, so using hottest here could mean "the height of summer".

When my grandma was alive, she always helped my mom around with everything.