If you haven't foraged sorrel before, or if you simply haven't spotted any around Edinburgh yet, June  is the perfect time to find it:  During the midsummer months, Rumex acetosa, the common or meadow sorrel, grows to an impressive meter tall , sending up beautiful and very visible red and green flower spikes, which make spotting the plants, even amid tall grass, easy.  If you are new to sorrel, these flowers also help to distinguish the delicious Rumex acetosa (and the much smaller, but equally tasty Rumex acetosalla/sheep sorrel) from the closely related, but far less tasty docks.  Rumex crispus, the curled dock, has much bigger  oval  leaves, and yellow -green flower spikes.   It  shares meadow sorrel's  wet grassland habitat (though dock isn't picky and grows almost anywhere) and can be found growing right next to it.  Dock leaves are wider, almost frilly at the edges, appear less neat and even, and often have reddish tones especially along the stems.  They taste less tart and much more bitter and are best reserved for external use. 

The neat, narrow, arrowhead shaped leaves of sorrel could  also resemble very young Arum maculatum (Lords and ladies/Cuckoo pint) leaves, which are similarly arrowhead shaped lwith a point on top and two points down either side of the stem.  Arum contains oxalates which will cause painful irritation of the mouth and throat if consumed, but chance of confusing the two species is slim, since Arum doesn't share the grassland habitat of meadow sorrel, favouring damp woodlands, and is an earlier species.  It will have already wider, triangular leaves by March when sorrel leaves are still tiny and delicate.

Sorrel is a member of the Polygonaceae, the knotweed family, and like many other familiar edible relatives (buckwheat, rhubarb, and japanese knotweed, for instance), it contains oxalic acid.  Oxalic acid gives sorrel it's deliciously tart, lemony flavour, and is quite healthy consumed  in moderation, however, in excess it becomes a toxin leading to kidney stones and eventually kidney damage, so, as with all things, moderation is indicated.  

Rumex acetosa is widespread and abundant, and has been a part of our diet for millennia.  There are records of Roman soldiers appreciating the thirst quenching tartness of the little leaves and it used to be cultivated for use as a leafy green in many British gardens since the early middle ages at least.  While it is no longer a common garden crop here, French, German, and eastern European gardeners, cooks,  and foragers still enjoy the lemony green in soups and sauces and salads and vegetables.  If you like the flavour of wood sorrel (Oxalis acetosella), you will love meadow sorrel also : in both cases the mouth watering, grape skin flavour is due to oxalic acid, and meadow sorrel is a far more substantial leaf (I have found some  which were over twenty cm long)  and therefore lends itself to more applications than just cute edible decoration.  It's a wonderful native lemon substitute, and pairs beautifully with fish, but really can be used anywhere some sharp tart flavour is required.  While you can find and harvest sorrel leaves almost year around, the basal rosettes-which have the tastiest leaves- are lushest and largest in late spring and then again in late summer to early autumn.  Around September, when there aren't as many leafy greens as  there are in the springtime, the basal leaves are biggest and most abundant and many of the surrounding grasses are yellow and dry , making the lush green meadow sorrel rosettes easy to find, making late summer an optimal time to harvest this old time kitchen favourite.