Is it possible for an IPEN-garden to have a bilateral limited-period-agreement (e.g. 5 years) with a non-IPEN garden without violating the IPEN regulations?
It is possible but it does not make much sense. The non-IPEN garden must accept the IPEN conditions either by signing the IPEN Material Supply Agreement for any plant transfer or by a bilateral agreement that complies with the CBD and with IPEN. In the last case it would be easier to become an IPEN member.