It's only enforcable against the parties to the contract. If a party receiving the seeds then passes them (against the terms of the contract) to another entity

It's only enforcable against the parties to the contract. If a party receiving the seeds then passes them (against the terms of the contract) to another entity without the contract attached, that ultimately-receiving entity has not accepted the contract and therefore isn't bound by it. An action can be taken against the initial party for breach of contract, of course, for whatever good that does, but that the seeds are now free of the contractual obligations.