Pre-Emergent Herbicides For Roadside Flowers Most pre-emergent herbicides can only be applied to well established plantings. They work well for perennial roadside flowers, not for annuals. If you rely on natural re-seeding do not use these chemicals. Dimension Gallery Pennant Pendulum Surflan Snapshot Treflan Plateau herbicide - A pre- and post- emergent herbicide that can be used with a variety of roadside flowers. Plateau can be used at the time of sowing roadside flowers. This is a growth regulator, thus temperature and soil moisture can influence the effect of this chemical. Stunting of roadside flowers may occur. Measure the chemical precisely to avoid problems. We suggest that roadside managers use this chemical with caution until they have a better feel for the chemicals potential effects. More information can be found at prairiesource.com/pages/plateau