To plant the seed means to
1) plant a male seed (a colony of sperm) into the female counterpart.
2) to drop a flossy move on a girl to show you are interested in her. AKA to start a relationship without any commitment.
I heard Jim planted the seed with his swizzle stick last night....Better look into plan B jim....
The seed is usually planted on a Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.
If the bad influencer has succeeded in persuading the group to 'sack off' their responsibilities, i.e. lectures, deadlines, group meetings, one may exclaim 'The seed has grown into a tree'
Laura: I want to go to Oceana to seek out some Spice Boys
Mel: DON'T PLANT THE SEED!
Tash: It's already grown into a tree, don't fight it.