Грибница растёт под мхом у опушки.

Грибница means mycelium: the underground network of fungal threads that mushrooms grow from.

It is a feminine noun, which is why the verb is растёт and not a masculine or neuter form.

• грибница = feminine singular
• so: грибница растёт = the mycelium grows

Yes. In normal Russian writing, ё is often replaced by е, so you may see растет instead of растёт.

But the pronunciation and stress are really растёт:

• растёт
• stress on -ёт

This matters for learners because е and ё are pronounced differently. If you are learning pronunciation, it is best to think of this word as растёт.

Растёт is present tense, 3rd person singular, from расти (to grow).

So it means:

• it grows
• it is growing

In this sentence, it usually expresses a general fact or natural process, not necessarily something happening only right this second.

Because под can take different cases depending on the meaning:

• под + instrumental = location: under, beneath
• под + accusative = movement to a position under something

Here there is no movement. The mycelium is located there, so Russian uses the instrumental:

• под мхом = under the moss

Compare:

• Грибница растёт под мхом. = The mycelium grows under the moss.
• Положи это под мох. = Put it under the moss.

It does look unusual at first. Мох means moss, and its instrumental singular form is мхом.

Its singular forms are:

• nominative: мох
• genitive: мха
• dative: мху
• instrumental: мхом
• prepositional: мхе

So in под мхом, the noun is simply in the instrumental case after под with the meaning of location.

У normally takes the genitive case.

So:

• опушка = nominative singular
• у опушки = genitive singular

In this sentence, у means by, near, or at:

• у опушки = near the forest edge / by the edge of the woods

No. У often has two common uses:

1. location near something

   • у дома = by the house
   • у опушки = by the forest edge

2. possession constructions

   • у меня есть книга = I have a book

So in this sentence, у is the location meaning, not possession.

Опушка means the edge of a forest/woodland, especially where the trees meet an open area.

So у опушки suggests a place near the edge of the woods, not deep inside the forest.

They give two pieces of location information:

• под мхом = where exactly? under the moss
• у опушки = in what area? near the forest edge

Together, they mean something like:

• the mycelium grows under moss, near the forest edge

Russian often stacks location phrases like this very naturally.

Russian word order is flexible, because the case endings show the relationships between the words.

The neutral order here is:

• Грибница растёт под мхом у опушки.

But other orders are possible, for example:

• Под мхом у опушки растёт грибница.
• У опушки грибница растёт под мхом.

These can change emphasis or sound more literary or context-driven, but the basic meaning stays the same.

A helpful stress pattern is:

• грибнИца растЁт под мхом у опУшки

Approximate pronunciation:

• greeb-NEE-tsa ras-TYOT pot mhom oo a-POOSH-kee

A few notes:

• грибница has stress on -ни-
• растёт has stress on -ёт
• опушки here is the genitive singular of опушка, and the stress stays on у: опУшки

Russian has no articles like a or the.

So грибница can mean:

• mycelium
• the mycelium
• a mycelium (depending on context, though in English this is less common)

Russian leaves that kind of definiteness to context, word order, and the situation being described.