Apply herbicides when weeds are actively growing, on moist soil, and application should only be in infected areas (use a post-emergent). Don’t apply if hot or windy, don’t mow for 2 days.
Post-emergent herbicides include: contact (kills weeds above ground), systemic (kills weed roots and is most effective), selective (kills certain weeds), non-selective (kills broad and narrowleaf weeds).
Pre-emergent herbicides don’t give the weeds a chance to grow, but might kill a newly seeded lawn.
Then there are the herbicides that will kill everything including your grass (Roundup).
Common Weeds in Lawns
Crabgrass spreading shoots with broad leaves, grows rampantly in neglected lawns.
Dandelions spring/fall, yellow flower turns into a white fluffball, grows even from a leftover root piece.
Spurge light pink to white flowers, spreading habit.
Chickweed small leaves, sometimes hairy, with small white flowers.
Knotweed likes hard beaten paths, tiny white flowers on small-leafed shoots.
Oxalis upright perennial, clover-like, small yellow flowers, cucumber shaped seed pods.
Mushrooms appear with wet weather and can form a ring.
Others like Purslane, Tall Fescue, Canada & Musk Thistle, Bindweed, Bent & Quack Grass, Puncture Vine, Mallow, Clover, Violets, Bluebell, Annual Bluegrass, Tree Suckers (use Sucker Stopper)