Late May may not be too late for most summer annuals. All depends upon what weeds/rainfall/temps/cultural practices/hardiness zone, etc. Sure -- you're gunna miss out on knotweed control, but you easilly can kill knotweed later on with esters.
We offer pre-emergent as late as mid July for new customers that need it. One example: New sod laid in early July = tons of foxtail grass "unless a pre-emergent is applied" a.s.a.p.
Late apps of Pre can prevent crabgrass, purslane, foxtail, spurge, and more. Just sayin'
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