Various parts of plants can have these allelopathic properties, from the foliage and flowers to the roots, bark, soil, and mulch. Most all allelopathic plants store their protective chemicals within their leaves, especially during fall. As leaves drop to the ground and decompose, these toxins can affect nearby plants. Some plants also release toxins through their roots, which are then absorbed by other plants and trees.
Read more at Gardening Know How: Allelopathy In Plants: What Plants Suppress Other Plants http://www.gardeningknowhow.com/garden-how-to/info/allelopathic-plants.htm