People often get worried when they see weed seedlings emerge after applying a preemergent herbicide.  However, not all preemergent herbicides kill weeds before they emerge. None of them kill seeds. Some are absorbed only by roots, some only by shoots, and some by both roots and shoots, but in all cases, the weed seed must first germinate. Those herbicides that are absorbed only by roots ordinarily kill weed seedlings before they emerge from the soil. These include trifluralin (Treflan), pendimethalin (Prowl) and benefin (Balan). Those that are absorbed only by shoots often kill weed seedlings soon after they emerge from the soil. These include DCPA (Dacthal) and EPTC (Eptam). Pronamide (Kerb) is absorbed by both roots and leaves and occasionally will not kill some weed species until very early after they have emerged. When used preemergent, some contact herbicides such as oxyfluorfen (Goal) and flumioxyzin (Chateau) are absorbed by seedling weeds only after they germinate and are emerging through the soil where they pick up the herbicide.