Control of hay-scented and New York ferns with Oust XP® herbicide: revisiting rate and timing required in mixed oak and northern hardwood stands.


Todd Ristau

Todd E Ristau, USDA Forest Service, Northern Research Station, PO Box 267, Irvine, PA 16329

Abstract – Dense rhizomatous fern layers compete with desirable tree seedlings for light causing suppressed development or even seedling mortality. Sulfometuron methyl (Oust XP®) herbicide can be safely and effectively used to control ferns. Previous research showed that as little as 2 oz. of Oust XP® per acre controlled ferns, while hardwood tree seedlings growing beneath survived, depending on application timing. It was recently suggested that some sites may require higher rates due to soils differences. The current study is a re-evaluation of rates and timing of Oust application using a wider variety of site conditions than in previous studies. I examined the efficacy of 2, 3, or 4 oz. Oust XP® herbicide per acre applied in July, August, or September at controlling Dennstaedtia punctilobula and Thelypteris nove-boracensis (hayscented and New York ferns) in mixed oak or northern hardwood forests. Square 10 by 10 foot plots were treated and monitored for fern control and impacts on non-targeted species within 10 stands across northcentral and northwestern Pennsylvania. Ferns were at least partially controlled with all rates and times of application, though there were more surviving fern fronds in the plots treated in September. Hardwood seedlings survived well, even at the highest rate though some herbicide effects were noticed on individual seedlings. Rubus (blackberry) was killed by Oust XP® as were some grasses, but most other herbs were unaffected. Oust XP® continues to be an effective tool for controlling ferns at a variety of sites while allowing seedlings to remain in place.