The fourth experiment was established to examine the effects of growth stages on HA to the seedlings. Cucumber, marigold, and geranium were grown from germination paper and HA solutions were added at various growth stages. For all three species, along with the delay of HA application, i.e., from stage 1 to stage 4, there was a continuing decline of root fresh weight, root dry weight, and taproot length. Finally, a histology study was conducted on HA treated cucumber root tissues. Stained cucumber taproot slides were examined. The diameter of HA-treated roots was significantly larger than those treated by NC and DI. Also, HA treated tissue appeared more active in cell division.A comprehensive study composed of five experiments was conducted to examine the effects of humic acids (HA) on root development of ornamental seedlings. In the first experiment, cucumber, marigold, geranium, impatiens, and pansy were examined on two types of germination paper system drenched with various concentrations of HA, nutrient control solutions (NC), or deionized water (DI). Among cucumber, geranium, and marigold, root fresh weight, root dry weight, taproot length, lateral roots number and lateral root length were significantly greater for HA treatments than those for control solutions and deionized water. However, significance was not found with impatiens and pansy growth parameters. In the second experiment, the effects of HA as a substrate drench was addressed. Optimal concentrations of HA that were acquired from the first experiment were used in this experiment. Root fresh weights were significantly higher for HA treatments than those for NC and DI among all five species. Shoot fresh and dry weight responses varied while the majority of the species didn't have a significant difference between HA and NC. In the third experiment, cucumber, marigold, and geranium were sprayed with a foliar application using HA and NC at 200 and 400 mgà ·1⁻à ¹, and DI. All three species showed a significant response in root fresh weight and shoot fresh weight, while only cucumber showed a significant response in plant shoot weight for HA treatment.