Yellow spots on the lower leaves of your weed plant could be a couple of things. It might be a nutrient deficiency, like a lack of nitrogen. Or, it could be a sign of overwatering, which can cause root problems.
To figure out what's going on, you'll want to take a close look at your plant. If the leaves are also curling or have brown edges, that's a sign of underwatering. If the leaves are yellowing between the veins but the veins are still green, that's more likely a nutrient issue.