Very best way is to treat them in late fall. Dandelion is a winter perennial - it's basic germination is when temps cool down, late Sept in our area, probably later by you. Here basic dandelion control is late Oct to mid-Nov, and lately even into Dec, if temps stay up.
Easiest time to control any problem is when it is juvenile, and growing aggressively. Trying to control dandelion in the spring is not very effective. They will usually flower before they are susceptable to weedkiller. Most spring apps will just burn the leaves off, and will not control the root, so they grow back. You will not have much real success in controlling any weed if it is in it's flowering or seeding stage.
North central Indiana
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