Autotoxicity in some ornamentals was investigated. The plants were grown by hydroponics with or without the addition of activated charcoal (AC) to the nutrient solution. The AC was used to trap the exuded organics from roots. Among the 37 plants under study, growth of lily, prairie gentian, corn poppy, farewell-to-spring, rocket larkspur, and carnation was drastically reduced in the absence of AC compared with those in the presence of AC in the nutrient solution. Root exudates of some plants were analyzed and several organic compounds were detected. The strong growth inhibitors such as lactic acid in pot marigold, benzoic and p-hydroxybenzoic acid in lily, o-hydroxyphenylacetic acid in rocket larkspur, benzoic and p-hydroxybenzoic acid in sweet pea, and maleic and benzoic acid in prairie gentian were detected in the root exudates. The reduced growth of prairie gentian after prolonged cultivation in a field might be avoided by amending the soil with AC at a rate of 60 kg·10a−1.