Root hair development is orchestrated by nutritional factors and plant hormones. We investigated the action of ammonium ($NH{_4}^+$) and its interactions with methyl jasmonate (MeJA) and ethylene in Arabidopsis root hair growth. The formation of root hair branches was dramatically stimulated in media containing 1.25 to 20 mM $NH{_4}^+$ at pH values of 4.0 to 6.5. The $NH{_4}^+$-treated root hairs showed a very short tip growth stage and swells on the sides that indicated the emergence of branches. MeJA (0.08 to 10 ${mu}M$) worked in synergism with $NH{_4}^+$ to enhance hair branching. In contrast, ethylene had an antagonistic effect; the stimulation of hair branching by $NH{_4}^+$ was suppressed by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and was diminished in ethylene-overproducing mutant eto1-1 seedlings. Moreover, the application of $Ag^+$, an ethylene inhibitor, reduced the ACC-induced inhibition of $NH{_4}^+$- stimulated hair branching and restored $NH{_4}^+$-stimulated hair branching in eto1-1 seedlings. Thus, the actions of jasmonate and ethylene appear to be dependent on nutritional conditions such as available nitrogen.