Studies were conducted in 2007 and 2008 to evaluate herbicides having both PRE and POST activity for selective weed control in bald cypress plantings. Five herbicides were applied at two or three rates at two different timings. The first timing was to dormant seedlings without foliage and prior to weed emergence (i.e., PRE). The second timing was to foliated seedlings with established weed seedlings present (i.e., POST). Herbicide treatments included aminopyralid at 70 and 120 g ae ha⁻¹, hexazinone at 420 and 560 g ai ha⁻¹, imazapyr at 140 and 210 g ae ha⁻¹, sulfometuron methyl at 110, 160, and 210 g ai ha⁻¹, and flumioxazin at 290 and 430 g ai ha⁻¹. Herbicide rate had little effect on vegetation control. PRE-applied sulfometuron methyl was most effective, providing nearly complete control of graminoids and broadleaves at 60 d after treatment (DAT). POST-applied treatments were generally less effective, though in the 2008 study imazapyr and sulfometuron methyl resulted in approximately 60% bare ground at 60 DAT. Growth of bald cypress seedlings was enhanced by both PRE- and POST-applied sulfometuron methyl, flumioxazin, or hexazinone and by PRE imazapyr. The best bald cypress growth response followed POST-applied sulfometuron methyl at 210 g ha⁻¹, which resulted in 63 cm³ mean volume index, more than fivefold greater than the nontreated check. Aminopyralid caused severe and lasting seedling injury. POST-applied imazapyr resulted in fasciculation and no growth benefit, despite providing the most efficacious weed control among POST treatments. Survival and growth of bald cypress can be greatly enhanced with a single selective herbicide treatment using sulfometuron methyl, flumioxazin, or hexazinone applied before or following foliation in the spring.

Nomenclature: Aminopyralid; flumioxazin; hexazinone; imazapyr; sulfometuron methyl; bald cypress, Taxodium distichum (L.) Rich var. distichum.