In some tournaments premium plants do have an advantage but I'd say most of the time it's just a slight advantage. You can easily compete in Jade League without them. I myself don't have any plants I paid money for and have now problem competing in Arena (I do have Imitator on my level 1 profile since it was available for gems once). All you really need is a good F2P strategy. You can look up strategies in the Mid Level Mini Forum.
i have been in Jade league with F2P plants for a long time.. until i brought premium. But with strategy you can cope well.
