Woodlots have become the most important investment opportunity among smallholders of Mufindi district in the southern highlands of Tanzania. Smallholder woodlots are also a major source of wood supply contributing to narrow the supply gap which in 2015 was reported to be 19.5 million m3 per year, where the main wood consumption sectors being construction and domestic heating energy. However, inadequate information about smallholder woodlots, supporting organizations, their linkages and impacts on woodlots performance derail its sustainable development and potential contribution for wood supply and poverty alleviation. The present study therefore specifically explored the tree grower’s motivations, knowledge base and challenges to woodlots farming; assessed woodlot tree species, products, and performance; analyzed the linkages of tree growers with support organizations and evaluated their impacts on the performance of woodlots. Both survey and case study approaches were used to collect data in the three villages namely: Igowole, Mninga, and Nundwe, both in Mufindi district, Tanzania. Mufindi district was purposively selected because of advanced smallholder tree growing. In all the three villages, a total of 93 actors were approached, including 72 tree grower households, 24 from each village, 14 nursery operators and 9 support organizations by snowball sampling. Then, an in-depth interview was conducted to all 72 sampled households. Of which 48 woodlots, 12 - from organized and the other 12 from unorganized tree growers in each village for Igowole and Nundwe, were assessed by rapid appraisal (RA) approach and their performance compared. While 24 woodlots were assessed from Mninga village, and all were from unorganized tree growers. Quantitative data were analyzed using SPSS version 20 and the results summarized in tables and graphs in excel. Woodlots performance and social network data were analyzed using R – software. Based on the study respondents, the results revealed that tree growers were motivated to plant and manage trees mainly for economic reasons (48%, 45%, and 51%) and land security reasons (37%, 30% and 31%) for Igowole, Mninga and Nundwe respectively. About the knowledge base, most tree growers (75% – 100%) in all the three villages had the knowledge on land preparation, nursery management, planting, weeding, pruning, and fire protection. But in all the villages, respondents did not have knowledge on forest growth principles and dynamics, on objectives for the product of the plantations and influence of tree spacing on such desired products. Again, other analyses revealed that; fire, inadequate knowledge, inadequate capital, lack of improved seeds and low timber/tree prices were the main challenges constraining farmers to plant and manage trees in woodlots in the three study villages. And, the main tree species in the study area were Pinus patula and Eucalyptus sp. Organized tree growers where much more supported by organizations than the non-organized ones. Logistic regression analysis performed in R (P = 0.05) revealed significant difference in woodlots performance among organized farmers based on gaps (P = 0.00216), growth condition (P = 0.04478) and planting space (P = 0.02013) criteria. That means, woodlots from organized farmers were generally performing better than those from unorganized farmers. The better performing woodlots of organized tree growers were contributed by social capital through networks and the role of collective action of the farmers in TGAs. Nursery operator farmers were the main source of tree seedlings for unorganized tree growers, while organized tree growers obtained most of the resources including knowledge, seeds and planting materials as well as funds from tree grower associations (TGAs), which were supported by organizations. Thus, for future planning, nursery operator farmers should be supported for improved seeds and planting materials to benefit the nonorganized farmers. Nursery operator farmers should be encouraged to join TGAs, and TGAs should generally be adopted as an effective smallholder tree grower’s support platform in the study area.