When the mustard plant is fully grown it is the largest of the common garden crops and can reach a height of up to three meters. In that case it is not unnatural to speak of it as a tree. A tree is a familiar image for a mighty realm that shelters its dependents (Ezekiel 17:23, Ezekiel 31:6; Daniel 4:12, Daniel 4:21).1
32 It is the smallest of all seeds, but when it has grown it is larger than all the garden plants and becomes a tree, so that the birds of the air come and make nests in its branches.”