As its name implies, butterfly weed (Asclepias tuberosa) is a magnet for butterflies and hosts their larvae, caterpillars. While it’s not uncommon to see the leaves mostly (or completely) chewed, yellowing of the leaves may indicate a cultural problem.
Plants require nitrogen and other trace elements to produce chlorophyll, the green pigment in plant leaves. If the soil is exceptionally poor, lack of nitrogen, iron and other elements may result in yellowing of leaves. If soil is extremely alkaline, butterfly weed may not be able to absorb nutrients, even if they are present. Neutralizers such as compost or acidifiers such as ammonium sulphate may be helpful.
While butterfly weed doesn’t display outstanding fall colors, the leaves may turn yellow before they drop in late fall. Ideally, though, the leaves shouldn’t be present at all by the time cold weather arrives. Butterfly larvae normally consume all the leaves long before then. This defoliation does not kill butterfly weed. It is part of its natural cycle.
Lack of sufficient drainage at the planting site may leave the taproot of the butterfly weed sitting in water. Water pushes oxygen away from the fine root hairs, preventing them from taking up nutrients. The lack of food will cause leaves to yellow and, if there is still no oxygen, eventually die.