As with aphids on any plant, just wipe them off with your fingers. Easy in this case. For bigger plants you can just hose or spritz them off with plain water.

Dave

Wow, never saw aphids on sorrel, I grow sorrel for many years. Are they kept inside? Sometime aphids attack the plants they would not normally do if they are only source of food. For example, I had terrible aphids infestation once on my pepper starters inside and once outside under tulle cover. Aphids disappeared as soon as I took them outside in first case and removed the cover in second case.

Your sorrel appears to still be in a small peat pot. It needs to be either in a much larger container or in the ground. Also is it indoors? Aphids will go after weakened plants and the small pot and (possibly) indoor conditions will only encourage them. I have never seen aphids on sorrel either. Mine just grows in the grass alongside the veg plot. It's tough and totally hardy.

Yes, they're kept inside. I moved them out after seeing the aphids to keep them from moving to one of my houseplants. I'm thinking of putting them in the ground. Will they come back after winter? I'm in zone 7.

Sorrel is bone hardy (down to z3) and truly perennial (ie many years). If you plant it in the ground it will be absolutely fine. By keeping it inside and in those peat pots you are making it very hard for yourself.... and for your plant.