Summon Naturia Pumpkin when your opponent has a monster in the field, its effect will activate which lets you summon a Naturia monster from your hand. You can then Special Summon Naturia Stag Beetle since it has a pretty good attack when special summoned and it has a good effect. Due to the activation of Naturia Pumpkin's effect, Naturia Hydrangea's effect activates which is you can Special Summon it from your hand. This makes you have 3 monsters in the field in 1 turn.
Special Summons itself / Special Summons from your hand / Special Summons itself from your hand
Apparently, you cannot SS this with pineapple effect (return from graveyard) or stag beetle's effect (enemy activate card/effect). Probably the reason is Hydrangea only can be SS by its effect during YOUR MAIN TURN.
It work before battle phase. Its effect is not triggered by Naturia effect but is a condition itself.
If it said "If the effect of a "Naturia" monster you control was activated during this turn, Special Summon this card from your hand", then you can trigger its effect to Special summon itself, similar to Kidmodo Dragon.
i cant special summon when i use pineapple's effect not sure why