•We construct an index measuring the strength of IP protection for plant varieties.•IP strength for plant varieties has been increasing in all countries.•Determinants of IP level are not equal for DCs and LDCs pointing to non-linear relations.•The TRIPS Agreement drives the process towards stronger levels of IP protection.•The effect of IPRs on agricultural product is not significant for LDCs. In this paper, we construct a new index measuring the strength of intellectual property (IP) protection for plant varieties in 69 countries over the period 1961–2011. We examine the statistical properties of the index and compare it with other indicators of IP protection. We conclude that the index provides a reasonable synthetic assessment of the relative strength of IP protection in plant varieties across countries. In addition, we study the main determinants of the evolution of the index and examine the patterns of correlation between the index and agricultural production.