Generally, herbicide should be applied when milkweed is in the late-bud to flowering stage and actively growing. The herbicide Tordon will leave a residue for several years, and may injure crops grown in successive years. However, applications made to small patches of milkweed may be justified because only a small percentage of the field will be applied. It will only get worse, assured Zollinger. Pappus on seeds allows long-distance travel and can assist small patches in taking over complete fields.
Seedlings become perennial within a few weeks after emergence.
One shoot can soon become a large patch with ample subsoil moisture to feed underground roots. If herbicide application is not feasible, clip or cut the flowering parts. Patch spraying covers only a fraction of the area of a broadcast application, and allows use of higher herbicide rates, which means getting milkweed under control sooner and with less expense than broadcast spraying. Research has shown substantial root kill the spring after application from Roundup and Tordon. Apply at late bud to flowering.