Developing countries have recognised the need to strengthen their agricultural extension services as an engine for improving productivity, reaching marginalised, poor and female farmers and addressing new challenges, such as environmental degradation and climate change. However, structuring effective and viable extension systems remains a major challenge in most places. This paper studies the Plantwise extension programme in Kenya, an initiative that supports and increases collaboration among actors in the national plant health system, establishes and maintains a network of plant clinics - a physical interface between farmers and crop protection experts - to address the needs of smallholders, and collects key information in the process that can be used for informing policy and for monitoring and evaluation purposes. To assess the programme in Kenya, we conducted a longitudinal mixed-methods evaluation of the programme from 2014 to 2018. Our results show that Plantwise has altered the way the Government of Kenya addresses crop protection. The programme is innovative and improves knowledge at multiple levels. At the farm level, Plantwise has contributed to improvements in the use of agricultural practices, inputs and maize productivity, a major staple crop. The results show that the Plantwise approach is a reasonable alternative to other agricultural extension systems that support smallholder farmers.