As part of your kumquat tree care, you should keep the soil moist around young trees, but not wet or soggy. Once the tree is established, water during dry spells. Withhold fertilizer for the first two or three months. Use a fertilizer designed for citrus trees thereafter, following the label instructions. Use a layer of mulch over the root zone to help the soil hold moisture and inhibit weeds that compete with the tree for moisture and nutrients. Pull the mulch back several inches (7.5 to 12.5 cm.) from the trunk of the tree.