Root system development in perennial crops is critical for the long-term survival of trees. Long lived coarse roots form the rigid structure that supports the dynamics of the absorptive fine root system. The woody root scaffold developed early in a tree’s life can shape lasting aspects of mature root system structure and function, including the extent of soil exploration. Research on early root scaffold development under different propagation and field management conditions has untapped potential for orchard system improvement. Rootstock propagation method significantly influences root system architecture (RSA). Seed propagation in Prunus species produces a vertically oriented taproot at germination, with lateral roots developing along this main axis. In contrast, clonally propagated rootstocks (e.g., cuttings, tissue culture) develop adventitious roots from a wound site or callus, often resulting in shallower, clustered root systems emerging from a concentrated point of origin. While clonal propagation exhibits trait uniformity and a high rate of initial root production, excessive root crowding and limited vertical distribution could lead to structural deformities, increased susceptibility to soil-borne disease, and reduced soil exploration for water and nutrients as trees mature. Studies have shown that a greater number of roots increases initial transplanting success, but a profusion of scaffold roots may result in competition for space, leading to permanent changes in RSA detrimental to long-term tree health. This study examines RSA traits (the spatial configuration of roots, including distribution, morphology, and topology) for coarse roots in commercial almond rootstock cultivars that were either seed or clonally propagated and subjected to full or deficit irrigation regimes. We evaluate how propagation method influences root angle (deep water access), scion growth (survival and vigor), root crowding (potential disease risk), and root distribution (soil exploration) in this 3-year common garden experiment. We found that rootstock significantly influenced traits like mean root diameter and convex hull (p < 0.05), suggesting genotypic differences in root vigor and extension into the soil profile. Propagation method did not significantly affect root angle after the first season, but prolonged exposure to deficit irrigation conditions in succeeding years will be important to understanding the adaptive capacity of these traits over time. As climate variability intensifies, understanding how rootstock propagation impacts RSA traits–particularly those relevant for accessing water stores, such as deeper root systems, elevated root vigor, and greater root length density at depth–will be essential for improving almond production in drought conditions, suboptimal soils, and other marginal environments.
