Polygonum persicaria

or Lady's Thumb

Lady's Thumb (Polygonum persicaria)

Originally a native to Europe and Asia, Polygonum persicaria was introduced in the mid-1800's to the Great Lakes regions and has spread throughout the United States. Other names outside of Lady's Thumb include Oxheart, Heartweed, Redshank, Willow Weed, Lover's Pride, Spotted Smartweed and Spotted Knotweed.

Quite adaptable to growing environments, this summer annual and broadleaf weed can be found growing in moist soils, especially along streams, marshes and lakes.

It can also grow in rich soils, acidic soils (soils deficient in lime), peaty-loam and dry soils. Click for more about soils.

Because of it's generic appearance and soil preferences, Lady's Thumb is often confused with it's very close cousin (who also prefers these soils), Pennsylvania Smartweed (Polygonum pennsylvanicum). Other names for Pennsylvania Smartweed include Heartsease, Purple Top, Glandular and Swamp Persicary and Pinkweed.

These can be viewed as a wildflower or one of many flowering weeds. It is important to understand that whether one chooses to place Polygonum persicaria in different garden types, it can become invasive if not managed.

Because both Lady's Thumb and Pennsylvania Smartweed are found growing together they are considered one in the same.

Lady's Thumb (Polygonum persicaria):

Lady's Thumb stems are branched, wavy and can be reddish in color and can grow up to 3-1/2 feet tall. The lower stem joints, or nodes,
are swollen at bottom where roots initiate.

A thin papery sheath called an ocrea (also ochrea) wraps around the stem at each leaf base that grows off of it. These sheaths are tipped with short, fringed hairs growing off the top.

Photo: Diane Miller

Seeds germinate from April to June and shortly after the cotyledons emerge. Cotyledons, or the first leaves sprouting from seedling, are elliptical to lance-shaped. They are up to 1/2 inch long and up to 1/10 inch wide. There are hairs on the edges close to the base and leaf undersides that are reddish in color. Stems below the small leaves can be reddish to brownish-green and as weed develops cotyledons will die away.

As Polygonum persicaria grows, leaves alternate along stems. They are slender and pointed, somewhat lance-shaped and grow anywhere between 2 to 6 inches long and 1-1/4 inches wide. Leaves are hairy on the top side and as plant ages, the older leaves become slightly hairy. There are black, brown or purplish color marks on leaves with a thumb-like appearance.

Flowers are dense terminal clusters at the top of stems and can bloom as early as May depending on location. Blooms remain until September, sometimes October depending on when first frost kills them off. Flowers are about one inch long and range in colors from white to pink to red, overall giving the cluster a rose colored appearance.

Individual flowers are small with five sepals, or the outside ring of green leaves that are found at the bottom of the bloom, each individually is a sepal. Calyx is the term used collectively for all the sepals. Flowers self-pollinate, but insects including butterflies, carry pollen as well.

Seeds are a shiny brown to black when mature and are flat to three sided in appearance. Seeds that don't germinate can remain dormant for over 45 years.

Differences of Pennsylvania Smartweed (Polygonum pennsylvanicum) and Lady's Thumb:

Pennsylvania Smartweed is also an annual. It grows up to 4 feet tall with leaves up to 7 inches long. Like Lady's Thumb, leaves are also lance-shaped but are not spotted.

Photo: Melinda Fawver

Pennsylvania smartweed ochrea, or papery sheath, is less prominent than Lady's Thumb and also does not have the short fringed hairs.

Except for stems having sticky hair on them, overall Smartweed does not have hair anywhere else.

Flower colors vary in colors of white to pink, like Polygonum persicaria,
but can also be described as grain-like, as buckwheat is found within the Smartweed family.

Pennsylvania Smartweed stands more erect than Lady's Thumb and if growing in shaded area, stems will lean toward the light. Lady's thumb overall is bigger, more branched and more likely to lie down forming large clumps. With Lady's Thumb the flower clusters are not as erect and can sometimes bend downward and the weed can grow in sun or shade well.

Pennsylvania Smartweed can also grow in the same kinds of soils as Lady's Thumb, but this weed, Polygonum pennsylvanicum, is more tolerant to drought than many other smartweeds.

Eradicating:

When handling Smartweed don't place hands close to eyes - it burns!

Both Lady's Thumb and Pennsylvania Smartweed annuals reproduce by seed. Each plant can produce a few seeds up to many thousands. They grow easily in moist, shaded areas. If introduced to a shade garden it can become more invasive than gardens with sunlight.

As with any sprouting weeds, it is best to pull these two weeds when they are young and soil is wet to ensure getting entire small root system.

Other manual removal methods are to hoe before seeds spread and cut back early when weed is growing low to the ground. If hand pulling a mature plant be aware that there can be a tough taproot to hand pull and it is likely seeds will fall from plant to the ground. Use gardening tools that expose the root and help to pry and twist root up when pulling.

Seeds begin to ripen mid-summer until late fall. Solarizing black plastic mulch will kill weed seeds. Other more organic weed control methods are using a propane torch to burn weed seeds and acetic acid also is very effective.

Pre emergent herbicides, especially with the chemical Dichlobenil, are effective. Use on-going for up to one year. Systemic herbicides to kill the fibrous roots can also work well. Other options for herbicide control are non-selective, contact herbicides. With Lady's Thumb and Pennsylvania Smartweed products containing dicamba work best over others containing 2,4-D and glyphosate.

Once weeds are eradicated use a mulch to cover the soil to prevent any more germination of seeds.

FYI:

Be aware - It is important to note that if importing topsoil, Polygonum persicaria is one of the first weeds to germinate from it.
Although both weeds are edible, there are no distinguishing flavors to Polygonum persicaria and it is considered a highly nutritious vegetable. Lady's Thumb is known for it's many medicinal purposes, however. Polygonum "persicaria" contains persicarin which is used in medicines for diarrhea and infections.

Pennsylvania Smartweed is quite the opposite as far as having distinguishing flavors. It has a highly acidic, peppery flavor. Digesting the plant as usual, any foraging animals quickly learn of it's taste and don't eat it again. It is because of this acid-pepper why keeping hands away from face is important after touching it.
Lady's Thumb is especially favored by birds as the fruits providing nectar. But butterflies and other insects visit frequently and the weed Polygonum persicaria itself provides a butterfly host plant to the Purplish Copper and Gray Hairstreak. Occasionally these two caterpillar types can be found on Smartweeds, but not as often.
Other relatives to Polygonum persicaria and pennsylvanicum are often confused with:

    * Green Smartweed (Polygonum scabrum) - Similar but
     green in color and it is also a perennial.

    * Pale Smartweed (Persicaria lapathifolium) - Doesn't have
     the purplish leaf blotch and same ocrea found on
     Lady's Thumb.

    * Polygonum minus - Common names include Knotweed
     or Knotgrass, which have narrower leaves. Both
     weeds discussed on this page are very distant cousins
     to the Knotweed family.