KAREN'S SORREL

maybe Common Sheep Sorrel (Rumex acetosella), non-native
https://www.minnesotawildflowers.info/flower/common-sheep-sorrel
"It resembles the related Green Sorrel (Rumex acetosa), a larger plant with flowers and fruits about twice the size, the tepals with a broad wing around the edge, and much larger leaves with basal lobes that point downward (sagitate) rather than outward (hastate)."

-Rumex: Latin name for plants with acid leaves

This is likely not Heartwing Sorrel, a native plant.
-named "heartwing" for the shape of the seeds, image below
-unfortunately Flora of Delaware doesn't list R. hastatulus for the state, another check against Heartwing.
http://www.wra.udel.edu/de-flora/

scientific descriptions of
-hastatulus
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242417184
-acetosella
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242414256

here are a few nice examples of several Rumex spp:
http://www.marylandbiodiversity.com/viewChecklist.php?genus=Rumex

Tap root/no tap root
http://www.eattheweeds.com/sorrel-not-a-sheepish-rumex/comment-page-1/
R. acetosella doesn’t have a tap root and is more mat growing rarely getting above 18-inches high.

This plant is growing in gravel on top of a base of landscaping cloth, not exactly conducive to tap root development. Am not sure, either, that the picture below showing roots shows something that qualifies as tap roots.

Waiting for the plant to produce seeds is one way to make sure.