Pennsylvania Smartweed (Polygonum Pensyvanicum)
Ladysthumb (Polygonum Persicaria)
Swamp Smartweed (Polygonum coccineum)
Smartweed Species-Group
Smartweed Family

Pennsylvania smartweed and ladysthumb are difficult to differentiate. In Iowa, we have almost all Pennsylvania smartweed, so that part is easy. Many are taught that ladysthumb has the purple "ladysthumb" spot on the leaf, but I have seen so many Pennyslvania smartweed plants (using all the other features) with the spot that I wouldn't rely on that feature. This is a demonstration of the first principle of weeds, biodiversity. It might also indicate cross-fertilization, I am not sure. The best way to separate these two species is by hairs on the ochrea.

Here (left) is Pennsylvania smartweed in flower interfering in Michigan barley field.
Another robust Pennsylvania smartweed:

A young ladysthumb plant with the distinctive purple thumb mark (left).

Biodiversity in weeds leads to many forms of a plant and plant parts. On the left are leaves of ladysthumb, on the right leaves of Pennsylvania smartweed. Notice the different sizes. This could be due to growth conditions (Plasticity: more water and light and nutrients, the bigger the leaf), or it could be developmental leaf differences encoded in the plant genes (somatic polymorphism, a big word for many forms of a plant).

Ochrea & Stems (remember this from the broadleaf weed ID toolkit?)

ladysthumb Pennsylvania smartweed

Seed & Seedlings

Pennsylvania smartweed Seed
Notice the small seedlings on the right side of the right picture (photo credit to Dr. David Staniforth).

Pennsylvania smartweed Seedlings

Flower color in both varies from white to pink to rose to purplish. The color of the variant within the species or species-group is not determined by environment, and is a good example of polymorphism within a species or species-group. If different colored flowers occur within the same genotype (probably less likely) it would be somatic polymorphism. More likely, the different colored flowers are a reflection of genetic polymorphism with a species or species-group.

ladysthumb
Pennsylvania smartweed

Swamp smartweed:

Seedhead, with purple flowers:

©jdekker-1998