I assume these are seedlings. That would be cutworms. Wrap a bit of aluminum foil around the bottom of the seedlings, going below the soil line. That will stop them.

Another trick is to take several toothpicks or small twigs and insert them at least an inch into the soil right up against the stem of the plants. I usually do 2-3. Cutworms encircle the stem to sever it, and this thwarts them.

To find them - tho this may be difficult - scratch around in loose soil or mulch nearby. They rest during the day, curled up under mulch or dry soil.

Thank you. I'll see what I can find tonight to protect the plants. Each of these plants was about 12 inches tall with decent diameter stems.

Sultry what a great idea about the cans!

I've done the can thing as well. Be sure you check that no cutworms are already in the soil around the base of the seedlings/plants before you put them on.