The effects of humic acid on the growth of azalea seedlings

Azaleas, belonging to genus Rhododendron, are highly valued landscape plants, and the major propagation method to meet the large requirements of their commercial production is vegetative stem cutting. However, seedlings are important for azalea interspecific crossing, which can expand the morphological variations among cultivars. Azalea seeds are tiny, and plants grow slowly during the juvenile phase. Thus, promoting plant growth and/or shortening the juvenile phase is important. Humic acid is reported to promote the biomass and root growth in herbaceous plants, and to improve the qualitative and quantitative fruit traits. Whether it could accelerate the growth in woody perennials is still unknown. In this study, we used 5-month-old seedlings of Rhododendron pulchrum Sweet to explore if humic acid could promote the early growth via drenching to the seedling root system. We applied 0, 50, 100, 200, 400, 600, 800, and 1,000 mg L^‑1 humic acid at 7-day interval for 110 days. The data showed that the application of an appropriate humic acid concentration could increase the plant growth, in terms of plant height, leaf area, shoot diameter, dry matter content, root activity, and ionic absorption. The optimal concentration ranged from 400 to 600 mg L^‑1. Taken together, humic acid application to the root system can be an effective way of promoting seedling growth in the juvenile phase.