Conyza canadensis (L.) Cronquist
Horseweed, Canada horseweed
Horseweed is a native weed that is widespread in North America, now also found in Europe, Asia, Africa, and South America. It is a serious problem in some regions, competing actively with crops.
Plants: 1½–6½′ (50–200 cm) in height, with a single erect stem that has bristly hairs on it. Although horseweed is big and common, you are likely to overlook it completely unless it starts muscling out your vegetables—it just isn’t a showy plant.
Leaves: Alternately from the stem in a pattern that looks whorled, but isn’t. They are all about the same length and orientation, making the plant resemble a green cylinder. Leaves are narrowly lanceolate or oblanceolate, 3–4 ⨉ ½″ (7.6–10 ⨉ 1.3 cm). They are mostly untoothed, but have a few teeth toward their tips, and they have small white hairs along their edges.
Flowers: The top of the plant branches into a shape like the top of an ice cream cone, which becomes covered with a profusion of little flowers. The flowers are greenish-white, but only ⅛″ (3.2 mm) in size. They are composite flowers, producing tiny white ray flowers as well as itsy-bitsy yellow disk flowers, but their small size and coloring overall is barely noticeable. Flowers appear from June to October.
Fruits: A cypsela tipped with dirty white down.
Habitats: A common weed in disturbed soils.
Edibility: Young leaves and seedlings are edible when cooked. Dried leaves have a flavor said to be similar to that of tarragon. An oil extracted from the leaves is used as a flavoring agent.
Medical: Plants produce dermatitis in some people. The common name sneezeweed derives from its use as a snuff to induce sneezing in herbal medicine, in hopes of lessening cold symptoms. Its smoke has been used as an insect repellant. Like many common plants, it has a host of other purported medical uses, among them treatment of gastrointestinal problems, rheumatism, dysentery, internal haemorrhages, gonorrhoea, and menstrual problems.
Online References:
7/31/2012 · Bemis Rd Conservation Area, Pepperell, Massachusetts · ≈ 1 × 1½′ (34 × 48 cm) 
ID is uncertain 
8/2/2012 · Nashua River Rail Trail, near Nashoba Hospital, Ayer, Massachusetts · ≈ 6 × 9″ (14 × 22 cm) 
8/20/2012 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 3½ × 5″ (9.2 × 13 cm) 
8/20/2012 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 6 × 9″ (15 × 23 cm) 
8/20/2012 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 4½ × 7″ (11 × 17 cm) 
8/18/2009 · Nashua River Rail Trail, Groton, Massachusetts · ≈ 10 × 17″ (26 × 42 cm) ID is uncertain 
8/17/2013 · Hopkinton State Park, Hopkinton, Massachusetts · ≈ 6 × 8″ (15 × 20 cm) 
7/20/2013 · Wooden Bridge, East Pepperell, Massachusetts · ≈ 7 × 10″ (18 × 25 cm) 
7/31/2012 · Bemis Rd Conservation Area, Pepperell, Massachusetts · ≈ 7 × 8″ (18 × 20 cm) ID is uncertain 
9/11/2025 · Waldoboro, Maine · ≈ 1½ × 2½′ (55 × 83 cm) 
8/20/2012 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 10 × 7″ (25 × 18 cm) 
7/20/2013 · Wooden Bridge, East Pepperell, Massachusetts · ≈ 4 × 4″ (10 × 10 cm) 
7/20/2013 · Wooden Bridge, East Pepperell, Massachusetts · ≈ 8 × 12″ (21 × 31 cm) 
7/31/2012 · Bemis Rd Conservation Area, Pepperell, Massachusetts · ≈ 1 × 1½′ (34 × 52 cm) ID is uncertain 
8/16/2013 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 7 × 10″ (16 × 25 cm) 
8/16/2013 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 7 × 7″ (17 × 16 cm) 
8/16/2013 · Nashua River Rail Trail, Groton Center, Groton, Massachusetts · ≈ 7 × 7″ (18 × 16 cm) 
8/22/2013 · Beaver Brook Conservation Area, Hollis, New Hampshire · ≈ 7 × 10″ (16 × 25 cm) 
| Kingdom | Plantae | Plants, but not fungi, lichens, or algae (from Stearn's Botanical Latin) |
| Subkingdom | Tracheobionta | Vascular plants—plants with a “circulatory system” for delivering water and nutrients |
| Division | Magnoliophyta | Flowering plants, also known as angiosperms |
| Class | Magnoliopsida | Dicotyledons—plants with two initial seed leaves |
| Subclass | Asteridae | A large class that encompasses asters |
| Order | Asterales | Flowering plants with a central disk flower and surrounding petals, like daisies |
| Family | Asteraceae | The aster family, which also includes daisies and sunflowers; from the Greek ἀστήρ, “star,” for the star-shaped flowers |
| Genus | Conyza | From Greek konops, “flea,” for the use of the leaves in beds as a flea repellant |
| Species | canadensis | “From Canada” |
Conyza canadensis description by Thomas H. Kent, not updated.
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