Genju of the Fens — ruling 1

Activating the ability that turns it into a creature while it's already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.

Genju of the Fens — ruling 2

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Genju of the Fens : règles, fonctionnement et interactions (MTG)

Genju of the Fens is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Enchant Swamp : Until end of turn, enchanted Swamp becomes a 2/2 black Spirit creature with ": This creature gets +1/+1 until end of turn." It's still a land. When enchanted Swamp is put into a graveyard, you may return this card from your graveyard to your hand.

How it works : Genju of the Fens

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
Activating the ability that turns it into a creature while it's already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
No other card is named in this ruling.