You know the shop that sells seeds and other items (by the way, make sure u have 22 coins)?

Go to seeds, then buy poison ivy. Plant, it then let it grow, then when the top has fallen off there should be 2 poison ivy seeds on the ground.

Plant them, and then its just like times two each time, so you get more and more money.

When you sell the top of the poison ivy you get 100 coins, the seeds get 20 coins.