Polygonum Persicaria is a weed which is found all over North America except in the north. It's commonly known as The Lady's Thumb. It has pink spikes which are slender and shorter than the spikes of the Pink Knotweed. Most of the leaves have lunar or triangular marks next to the centre. The leaves are found in close proximity to the stem, in fact they are almost seated on it.
Doorweed is a small little plant that is found all over America, Asia and Europe. It is also known as the Knot-grass. Scientifically it's called Polygonum aviculare. The plant has smooth jointed stems which often trail over the ground. It can also be feebly erect at times. It flowers from June to October. The flowers are either white with pink edges or greenish in color. The leaves are oblong in shape and bluish green in color. The leaves are not more than an inch in length. The plant doesn't secrete much nectar and thus fails to attract insects.

Water Persicaria or Polygonum amphibium blooms in the month of July and August. It produces rose colored flowers which may be oblong or ovoid shaped. The flowers are not more than an inch in length. The plant grows in lakes and ponds. Water protects it from crawling pilferers. The stems and leaves of this plant have sticky hairs on their epidermis which are protective. These glandular hairs are smooth when the flower is in water and they become sticky when the water dries up. This way they protect the plant. Some insects like might pilfer stick to it. The stem can be twenty feet in length. The stem may float or it may be completely immersed in water. The leaves are rounded and elliptical in shape. They can be two to four inches in length. The flower is found in all regions between Quebec and. New Jersey. In the west it extends till the Pacific.

Mild Water Pepper or Polygonum hydropiperoides flowers from June to September though it can be inconspicuous sometimes. It is a variety of the pink persicaria but it is more slender and paler. The plant grows well in moist places, swamps and shallow water. The plant can be up to three feet in length and is found in whole of the Northern United States. Its erect and narrow flowers are pink or green colored. They bloom in the summer months. The stems have a cylindrical sheath around their swollen joints. The sheath has long bristles which serve as an identifying feature of this plant. Another related Water pepper is the Smart weed. It is known as Polygonum hydropiper because it produces biting, acrid juice.