Persicaria hydropiper (Water Pepper)

Persicaria hydropiper (Water Pepper) on 9-4-19, #623-24.

Water Pepper, Marsh Pepper, Knotweed

Persicaria hydropiper

per-sis-KAR-ee-uh    hy-droh-PYE-per

Synonyms of Persicaria hydropiper (73) (Updated on 1-8-23 from Plants of the World Online): Hmmm… The list of synonyms increased from 44 on the last update on 12-10-21. Since then, the list has increased to 73. I didn’t want to include the whole list this time around, but you can click HERE to view the list on POWO.

Persicaria hydropiper (L.) Delarbre is the accepted scientific name for this species of Persicaria. It was named and described as such by Antione Delarbre in the second edition of Flore d’Auvergne in 1800. The species was first named and described as Polygonum hydropiper by Carl Linnaeus in Species Plantarum in the first edition of Species Plantarum in 1753. Plants of the World Online lists nine synonyms of Persicaria hydropiper.

The genus, Persicaria Mill., was named and described as such by Philip Miller in the fourth edition of The Gardeners Dictionary in 1754. 

As of 1-8-23 when this page was last updated, Plants of the World Online lists 130 species in the Persicaria genus. It is a member of the plant family Polygonaceae with 56 genera. Those numbers could change as updates are made by POWO.

Distribution map for Persicaria hydropiper from Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet; Retrieved 10-9-19.

The above map is from Plants of the World Online and shows where Persicaria hydropiper is native (green) and where it has been introduced (purple). The map on the USDA Plants Database for the United States and Canada is similar.

The map on iNaturalist shows where members have made observations. Anyone can join and it is a great website to confirm and share your observations. The maps on iNaturalist are continually updated as members post new observations.

TOP PHOTO: The small colony of Smartweed in the pasture behind the lagoon was VERY perplexing for a while. It has red stems while the other clumps near it have green stems or near-red. Also, its racemes of flowers were very pendulous while the others were more erect and only drooping at the top. Even the larger colony a few feet away in the rock pile had green stems with racemes that go every which direction. As it turned out, all those characteristics are true for Persicaria hydropiper, the Water Pepper. I checked seed in this entire area, both from plants with red stems and green stems, and their seeds were all dull (not shiny) and black to brownish. The above photo was taken on September 4 and the racemes of this colony weren’t that long yet because it had been mowed off previously.  


Persicaria hydropiper (Water Pepper) on 6-19-19, #633-9.

This very large colony of Persicaria hydropiper with mainly green stems is growing next to the rock pile behind the lagoon. Well, not really a pile of rocks so much as large pieces of the old concrete foundation from an old barn. The barn used to be where the lagoon is and was one that my grandpa (mom’s dad) and his brother-in-law (Uncle Arthur) tore down and rebuilt here around 1960 or a few years later. They rebuilt the barn here and used the original square nails to rebuild it. I have a lot of memories of that barn, and not all good. The barn was VERY OLD and not all that sturdy. You had to be very careful walking around in the loft because there were a lot of holes in the floor. One time I fell through all the way to the ground. 🙂 Even though it was very old, it was also very neat. I always loved old barns…

Getting back to the above photo… You can see how the racemes of flowers are kind of growing in every direction.

Persicaria hydropiper (Water Pepper) stems on 8-30-19, #618-44.

Persicaria hydropiper can typically grow to around 36″ tall, or long. They are mainly decumbent unless they can lean on other plants. Missouri Plants says: “To 1m tall, herbaceous, glabrous or with some pubescence above, typically green or reddish, erect to spreading, multiple or single from base, simple to few-branching.”

Most Smartweeds are decumbent, which means they sprawl but turn upward toward the end. They root at their leaf nodes which allows them to spread quite readily. Even though these plants may appear to be only around 2′ tall (more or less), if you pull them will see the entire plant is much longer and has a lot of stems doing the same thing… Branching out… Some species grow more upright than others especially if they can lean on something.

Persicaria hydropiper (Water Pepper) on 9-4-19, #623-28.

This is a very interesting photo. If you are randomly observing this plant or taking photos without knowing what you are looking at, you would say, “OH, that is pretty cool”.

The ocrea, sometimes spelled ochrea, is the “sheath” surrounding the stem at the node where a leaf emerges. After a while, a branch may grow from this same node. Some species only branch out at the lower nodes of the plant. The ocrea on Persicaria species is nearly translucent and is formed by the fusion of two stipules. One word seems to lead to another… A stipule is formed at the base of a petiole. GEEZ! A petiole is the “stem-like” gizmo between the stem and base of a leaf. A gizmo is what you call it when you don’t know what else to call it. Many species, maybe all, have cilia (bristles) growing from their ocrea but fall off fairly soon so they don’t become an ID issue. Persicaria longiseta have bristles on their cilia that do not fall off.

Typical leaf of the Persicaria hydropiper (Water Pepper) on 8-30-19, #618-46.

Their alternate leaves have short petioles, sort of olive green in color, lanceolate to linear, and are around 3 1/2″ long x 3/4″ wide, smooth, and normally hairless. So, if you see a colony of white-flowered Persicaria and some of the leaves are 4″ or longer, they are likely not P. hydropiper and more likely to be P. punctata.

I need to point out “hairless” in the Persicaria sense when it comes to their leaves. I haven’t noticed any “hair” on any Persicaria leaves although some feel slightly rough. Very slightly.

Persicaria hydropiper Water Pepper) on 9-4-19, #623-26.

As with all Persicaria species, P. hydropiper stems end with a raceme of flowers. P. hydropiper racemes are very slender, are pendulous or droop sideways. Their flowers are sparsely placed along the raceme.

raceme is an elongated inflorescence with pedicellate flowers. An inflorescence is the part of the plant that contains the flowers. Umm… A pedicel is a flower stalk with a single flower. The stem part the flowers are on has a specific name but I forgot. So, the part I forgot with the pedicles of flowers and everything that goes along with it is the raceme. Of course, the flowers themselves have many parts but that is for another time. Nevermind that!

Persicaria hydropiper (Water Pepper) flowers on 9-22-19, #635-5.

Now, about their flowers… It took me a while to get a fairly good close-up photo of the flowers of P. hydropiper. I took A LOT of photos and none were as good as the photo above. I will keep trying so I can replace this photo with a better one at some point.

Anyway, Persicaria hydropiper flowers are greenish-white have 5 sepals, 2-3 styles, 4-6 stamens, and no petals. As with P. punctata, the flowers are covered with “punctate glandular dots” which you will only notice with magnification. The “glands” turn brown when the outer sepals dry out. The outer sepals are greenish, as with P. punctata, where other species are not. If you do not have a magnifying glass and the Persicaria you are trying to identify have white flowers, you likely have Persicaria hydropiper, P. hydropiperoides, or P. punctata (at least if you are in the U.S.). Both P. hydropiper and P. punctata have glandular dots. I don’t know much about P. hydropiperoides because I don’t have them here. With some magnification, you can see the green sepals are kind of weird and rough looking, like small bumps. The seeds of P. hydropiper are dull, black or brownish. Seeds of P. punctata are black and shiny.

Persicaria hydropiper (Water Pepper) on 9-14-19, #631-2.

With Persicaria hydropiper and a few other species, there are a few flowers that develop at their leaf nodes. These are called “axillary racemes”.  Hmmm… A little spider is defending her territory.

Persicaria hydropiper (Water Pepper) on 9-16-19, #633-2.

Several species have this “zig-zag” effect on their stems, but not on all their stems.

Persicaria hydropiper (Water Pepper) on 9-16-19, #633-3.

Another neat photo showing new stems coming from a leaf node.

Persicaria hydropiper (Water Pepper) on 9-16-19, #633-5.

This photo shows several racemes of flowers growing from lower nodes. Very common with P. hydropiper. The flowers on many Persicaria species are shy to open, so I was surprised to see a few flowers opened up on the Persicaria hydropiper on September 16.

Without going into all the technical terminology, Persicaria species are self-pollinating and can pollinate without their flowers opening.

Persicaria hydropiper (Water Pepper) seeds on 9-16-19.

Here you can see the seeds of Persicaria hydropiper that are dull, not shiny, and are black to brownish color. The seeds are one a few ways to really tell P. hydropiper from P. punctata.

Persicaria hydropiper (Water Pepper) on 8-3-19, #610-3.

I take a lot of photos over the summer months to help with identification and to show how the plants progress and change over time. I do the same thing with the plants in flower beds and pots. As you can see with the above photo, some colonies have stems that are kind of a greenish-red and darker red at the nodes.

Persicaria hydropiper (Water Pepper) on 8-30-19, #618-43.

I took a lot of photos. More than I have words for…

Persicaria hydropiper (Water Pepper) on 8-30-19, #618-45.


Persicaria hydropiper (Water Pepper) on 9-4-19, #623-25.

Typical pendulous racemes. Some racemes of some colonies just go in every direction and may not be as pendulous.

Persicaria hydropiper (Water Pepper) on 9-4-19, #623-27.


Persicaria hydropiper (Water Pepper) on 9-4-19, #623-30.


Persicaria hydropiper (Water Pepper) with a few Persicaria longiseta (Oriental Lady’s Thumb) on 9-14-19, #631-1.


Persicaria hydropiper (Water Pepper) on 9-14-19, #631-3.

As you can see with this large colony, the stems are greener and the flowers are more upright but nodding at the top. They are going in all directions. I checked the seeds and they are dull in this colony so they are P. hydropiper. Their leaves are shorter than those of P. punctata as well.

Persicaria hydropiper (Water Pepper) on 916-19, 633-1.


Persicaria hydropiper (Water Pepper) on 9-16-19, #633-4.

A few open flowers…

Persicaria hydropiper (Water Pepper) on 9-16-19, #633-8.


Persicaria hydropiper (Water Pepper) on 9-16-19, #633-9.

Another photo of the large colony of P. hydropiper with the greener stems behind the lagoon.

Persicaria hydropiper (Water Pepper) on 9-16-19, #633-10.


Persicaria hydropiper (Water Pepper) on 9-16-19, #633-11.


Persicaria hydropiper (Water Pepper) on 9-22-19, #635-4.

Well, no doubt about it, Persicaria hydropiper was king of the territory in the pasture behind the lagoon south of the barn. They are also growing south of the pond in several small colonies and a few individual plants are among the P. punctata behind the chicken house.

One thing I might add is that the leaves are edible. I ran across an article on a website called FORAGER/CHEF that talks about eating its leaves. The taste of the leaves is another way to tell this species from Persicaria hydropiperoides. Persicaria hydropiper and P. punctata have a very hot, peppery taste whereas P. hydropiperoides does not. Some information, however, says not to eat it because it will make your mouth burn and swell. I could live without that experience.

The link below from Hindawi (Evidence-Based Complementary and Alternative Medicine) has A LOT of information…

I enjoyed the experience of identifying the seven species of Persicaria growing on the farm. You can read the post about them all by clicking HERE where you will also find links to their own pages.

I have enjoyed photographing and learning about the many wildflowers growing on the family farm and in other areas. I have grown over 500 different plants and identified over 250 species of wildflowers (most have pages listed on the right side of the blog). I am not an expert, botanist, or horticulturalist. I just like growing, photographing, and writing about my experience. I rely on several websites for ID and a horticulturalist I contact if I cannot figure them out. Wildflowers can be somewhat variable from location to location, so sometimes it gets a bit confusing. If you see I have made an error, please let me know so I can correct what I have written.

I hope you found this page useful and be sure to check the links below for more information. They were written by experts and provide much more information. If you can, I would appreciate it if you would click on the “Like” below and leave a comment. It helps us bloggers stay motivated. You can also send an email to me at I would enjoy hearing from you.


NOTE: The data (figures, maps, accepted names, etc.) may not match on these websites. It depends on when and how they make updates and when their sources make updates. Some websites have hundreds and even many thousands of species to keep up with. Accepted scientific names change periodically and it can be hard to keep with as well. Some of the links may use a name that is a synonym on other sites. In my opinion, Plants of the World Online by Kew is the most reliable and up-to-date plant database and they make updates on a regular basis. I make updates “at least” once a year and when I write new pages or add new photos but I do get behind. We are all a work in progress. 🙂


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