Hi Wendy!

Fire ants fortunately do not overwinter in MD, so on rare occasion they arrive in the state (usually by way of contaminated soil in nursery stock shipped from southern locations), they do not become established. Several ant species form mounds around their nest entrances, not just fire ants. 

If you disturb the mound and ants rush out to defend the nest, see if they readily run up vertical surfaces. According to a fire ant information page by Texas A&M, this is a behavior they are known for, though it does not necessarily exclude all other species. Other features not unique to fire ants, but collectively can help identify them, include a variation in worker size (though under a quarter-inch), no central nest entrance in the mound, and a blister-like skin reaction to the venom in their sting. (The latter phenomenon is purportedly more diagnostic, though certainly one which someone would hope to avoid using.)

Here's an interesting article written by our retired entomologist: https://bugoftheweek.com/blog/2013/12/9/in-a-warming-world-here-comes-the-red-imported-fire-ant-isolenopsis-invicatai

Given that the ants were found in the community garden, chemical control should be avoided. You can prevent ants from biting by wearing closed shoes rather than sandals while gardening. Tuck your pant legs into your socks to prevent ants from moving beneath clothing and up your legs to sting. Colonies can be eliminated by treating them with very hot water if necessary. 

Jamie