Q: I was told by a garden center person that if I aerate after applying a pre-emergent it would be less effective. Is that true?
A: One research source says that aerating does not lower the effectiveness of a pre-emergent applied previously. However, it makes sense to me to avoid disturbing things after applying a pre-emergent. Your goal is to have a continuous layer of chemical dissolved in the top half-inch of soil where weed seeds lay in wait.
UGA weed scientist Dr Tim Murphy says:
“Core aeration generally has not been recommended or practiced following a preemergence herbicide application. Core aeration was believed to disrupt the herbicide barrier in the soil and stimulate weed emergence.
“B. J. Johnson reported in 1987 that core aeration immediately prior to or one, two, three, or four months after applications of benefin, bensulide, DCPA, and bensulide + oxadiazon to common bermudagrass did not stimulate large crabgrass emergence. Aeration at one or two months after application increased large crabgrass cover 5% for oxadiazon at 2.0 lbs. ai/acre, but not at 4.0 lbs. ai/acre.
“In a related Georgia study, it was shown that core aeration at one, two, or three months after an application of oxadiazon did not decrease goosegrass control on a ‘Tifgreen’ bermudagrass putting green.
“In Michigan, core aeration, or vertical mowing, immediately or one month after an application of benefin, bensulide, or DCPA did not affect large crabgrass control in annual bluegrass.
“A study conducted in North Carolina showed that aeration did not affect the activity of several preemergence herbicides in controlling crabgrass species in either ‘Tifgreen’ or common bermudagrass. However, in creeping bentgrass, significantly greater amounts of crabgrass occurred in aerified plots with the cores returned than in plots not aerified, or aerified plots with the cores removed.
“While most herbicide labels do not recommend aeration after preemergence herbicide application, university-conducted research has not shown an adverse effect on crabgrass control. Results can vary between research plots and commercial turfgrass sites and there may be situations where core aeration after preemergence herbicide application could stimulate crabgrass and goosegrass emergence. But, if the site requires aeration to encourage turfgrass growth and development, then it should be done. If crabgrass or goosegrass emerges, there are excellent postemergence herbicides that can be used.”