LEARN THE HISTORY OF USES AND HEALTH BENEFITS OF LADY'S THUMB
This plant is a native of Europe and the British Isles, and was introduced to North America where it has become an invasive weed. It can grow to heights of three and a half feet but is more normally about two and a half feet tall. The Polygonum family gets their name from the Greek words, poly, meaning many and gonus meaning knee or joint, referring to the nodes on the stem of the plants.
Despite their weed status and their introduction to the North American continent the Native Americans found uses for these plants and soaked legs and feet that were rheumatic in a strong decoction of the arssmart with flour in a thick paste for pain relief. They also used the leaves crushed, on poison ivy rashes. The leaves and flowering tops were used in an infusion for stomach problems and when boiled can get rid of gravel in the organs.
Culpeper in the 17th Century had this to say of arssmart’s medicinal properties:-
He said arssmart heals ulcers in humans and animals, kills worms, the juice can help with swellings and dissolve congealed blood of bruises by strokes and falls. A piece of the root or some smashed seeds held to a sore tooth can take the pain away. The juice in the ear can kill worms in the ear. You can toss the leaves of the plant in animal stables to kill all the fleas. If you put the juice from the plant on cattle’s sores, the scent will drive away flies. The frontier people would put a handful of arssmart under the horse saddle to make the horse travel better. The plant is good to heal wounds in the early stages. Keep in mind, this is the wisdom of a 17th century herbalist.
The whole plant makes a yellow dye when used with an alum mordant. The leaves, flowers, shoots, and seeds are edible, but should not be eaten in large quantities.  Lady's Thumb is considered a medicinal plant. Native Americans rubbed the plant on their horses as an insect repellant.  The plant has been found to have anti-fungal properties, bearing out its traditional use in Argentina for vaginal diseases and skin ailments.  This plant is also used for heart trouble. 
If you rub off the pink papery seed cover, you discover the very small, black and shiny seed. If viewed through a hand lens, you may see the surface of the seed has three gently rounded ridges running lengthwise. 
Also, the small seeds are edible either raw or cooked which are very hot and can be made into a condiment and substituted for pepper.  This plant is best characterized by its dense, spike-like, elongated cluster of small dark pink or purple grain-like flowers. These flowers are arranged along a thin, vibrant green, fibrously robust stem. Lady's Thumb leaves are long and lanceolate having the same vibrant green color of the stem.
Flowers are approximately 4 mm. in length and are dark pink or purple in color. Each bloom has 4-6 sepals but are completely absent of petals forming slightly ovate grain-like structures. Flowers are arranged in elongated clusters approximately 1.5-5 cm. in width and varying in height.
The seed-like fruit are small, glossy black and three-sided. Lady’s Thumb grows best in damp clearings, along roadsides, and is also cultivated in gardens.
When it comes to foraging for edibles, lady's thumb is one of the most easy to identify plants, making it especially suitable for the novice gatherer. The thumbprint mark on the leaf of lady's thumb is indeed the best way to distinguish it from plants similar in appearance. At the center of each lance-shaped leaf is a distinct smudge that looks as if an ink-dipped thumb had grasped the top of the foliage. Lady's thumb is found throughout the continental United States, thriving as a perennial in U.S. Department of Agriculture plant hardiness zones 2 to 10.
Along with the smudged mark on each leaf, other features make lady's thumb readily identifiable. The jointed stem often turns red as the season progresses, inspiring its other common name, redleg. Because other members of the Polygonum genus feature jointed stems and pinkish flower spikes, the thumbprint remains the best way to distinguish lady's thumb from its near relations.
Closely related plants also in the Polygonum genus are often known as smartweeds. Those common to the West Coast include wild buckwheat and pale smartweed. All are edible, with the exception of lady's thumb which is best used sparingly. As always, make a careful identification of any wild plant you've found and cross-check it with a reputable source before attempting to eat it.
The leaves and flower spikes are the edible parts of the lady's thumb plant. Because both are so mild-tasting, they are suitable for fresh eating. I personally find the plant to be slightly spicy, and I enjoy the zesty flavor. Add leaves, whole or chopped, to green salads, and use the colorful flowers as edible garnish to green salads or other cold salads. They can also be used like spinach as a cooked side dish, or as a green vegetable in quiche, soups or stir-fries. Boil leaves for 5 to 10 minutes when using lady's thumb as a spinach substitute.
As an edible plant, the main value of lady's thumb may be for its ability to add bulk to a salad or vegetable dish when you are low on greens. Lady's thumb contains natural fibers, sugars, fats and tannins, and has some minor nutritional value. Some Polygonum species cause a sensitivity to the sun in some people. Some side effects of Lady’s Thumb are associated with oxalic acid, which, as with the oxalic acid in rhubarb, can cause health issues if eaten in large quantities. 
However, the oxalic acid may be removed by cooking the Smartweed. 
Some people develop a skin sensitivity from contact with smartweed leaf—so beware if you are wading into a stand of smartweed in shorts for the first time until you know if you are sensitive.
Seeds have no risks indicated.