This paper presents a literature review and case study on constraints to smallholder's tree planting in northern mountainous areas of Viet Nam. It reveals that existing literature primarily highlights constraints within the ‘input domain’ that relates to inputs to tree system establishment such as land availability, or ‘output domain’ associated with market of tree system products, but sparsely report constraints within ‘knowledge domain’ namely knowledge in tree management practice. Our case study exploring farmer's perspectives also finds those related to input and output domain but identified lack of knowledge as the prime constraint, especially to poor farmers and ethnic minorities. We recommend that the dissemination of knowledge on tree system management, farmer's skill capacity building, and improvement in the national extension system as well as its linkage with research and education, be placed in the forefront of policy to sustainably expand and intensify tree planting in the country.