Birdsrape mustard (Brassica rapa) and wild mustard (Brassica kaber) both flower simultaneously in early spring.  Wild mustard is typically more dense with foliage than birdsrape mustard.  They both grow to approximately the same height, with similar flowers.  The easiest way to distinguish them is by looking at how the leaves attach to the stem.  Wild mustard has petioles, while birdsrape mustard leaves clasp the stem.

Wild mustard (Brassica kaber)

Birdsrape mustard (Brassica rapa)

Wild mustard (Brassica kaber)
Birdsrape mustard (Brassica rapa)
Foliage of B. kaber is larger than B. rapa.  Most distinguishable however, is that B. kaber has petioles.
Upper foliage of B. rapa does not have petioles.  This type of foliage is called clasping.
Wild mustard (Brassica kaber)
Birdsrape mustard (Brassica rapa)
Flowers of B. kaber are generally similar to B. rapa.  Petals are broader and more overlapping.
Petals of B. rapa are more separated than those of B. kaber.  Petals of B. rapa look more like a cross than (which is how this family got its old name of Cruciferae).

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