Answer: Earthworms like cool, moist places and that's probably why they're congregating beneath your containers. I suspect that they can easily crawl into the soil through the drainage holes in your containers without any assistance from you, but if you move the containers and find them just hanging out there, you can hose them into a flower bed or onto the lawn adjacent to the concrete. They'll be perfectly happy in either place. I wouldn't try to place them into the containers - it's possible that the soil within the containers gets too hot for them and they escape beneath the containers to find a cooler spot.
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