St. Augustine grass is invading my tall fescue lawn. Will Roundup kill the St. Augustine grass or is there some selective herbicide which will kill the St. Augustine without killing the tall fescue?

Before you use an herbicide you should make sure that the grass invading your lawn is actually St. Augustine.  The grass could be kikuyu grass which has a similar appearance.  Both types of grass have thick runners about 1/4 inch diameter which spread on top of the soil.  You can tell which grass you have by looking at the leaf tip. Kikuyu grass has a pointed leaf tip and St. Augustine grass has a rounded blunt tip.  St. Augustine grass does not produce seed and must be propagated vegetatively.  Kikuyu grass produces seed which remains in the soil and can sprout and re-infest an area after the parent grass is removed.  The herbicide glyphosate (Roundup) is a non-selective herbicide which will kill either St. Augustine or kikuyu grass but it will also kill your tall fescue turf grass.  If you use glyphosate you will have to replant the treated area with tall fescue.  If you kill existing kikuyu grass it will probably grow back from seed the following year. Kikuyu grass growing in a tall fescue lawn can be controlled with the selective herbicide triclopyr (Monterey Turflon Ester).  More than one application may be required to control established kikuyu grass.  Turflon Ester is not registered to control St. Augustine grass; however, the herbicide will damage the grass according to the product label.  Bermuda grass and zoysia grass are also damaged by the herbicide.  For more information see UC Pest Note 7458 – Kikuyugrass:

By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension,

September 2012