Botanical Research

I have the exact same problem too....~Mithra

Same here, but I wonder if the quest gets broken if you accept one of the missions that turns the Garden into Nightfallen Garden. ~AC

I followed the arrow, and I can't get through the gate at all. I'm not as far along with the missions as you 2 are obviously. The quest also doesn't get advanced when you try to take the Tihark Orchard mission. ~Why?

Remember, you need to have Tahlkora along to get through the gate just outside of Tihark Orchard - she's royal blood.

Yes, take Tahlkora with you and take the West exit (not the east as Rud mentioned below.) out of Tihark Orchard. The entrance to the Explorable Area of Garden of Seborhin is right outside; two steps and you're there. Talk with Seborhin Protector Zuor and he'll let you in! I did this right after completing the Tihark Orchard mission. ~Broken0007

I Found a "Work around" on this Quest
Take Tahlkora and a single player that has not done the Tihark Orchard Mission. Exit to the East in Tihark Orchard with a full party and talk to the gate guard. Tahlkora is the "KEY" to enter the closed gates. The player who allowed you to enter can now map out. From there you can do the Quest and Explore the area. ~Rud

For me Worked this way
Once i finished the game, the guard let me in as now I'm the hero of Elona and at the end of the game the garden has back to normal. ~TyFricko