yea that's a good idea but I am not sure about the purple on normal bagweed. The black maybe....does it look like its dying on the bottom? Do you give it too much fertilizer, or do you even give any? And I think that also could be from not enough water or tooooooo much water
the little shit looks very healthy,no ferts, the top of the leaves are starting to get black lines down the middle. i have never seen this before!
check overgrow.com/faq there EVERYTHING u need to know there, hehe....they have pictures of everything that can go wrong and explain why it happened and what to do!