Plant with purple flowers? 25 common ones and how to tell them apart

Which second clue narrows it fastest

Purple tells you very little on its own. Three questions, in this order, cut the table down to two or three rows.

1. Is the stem square? Roll it between finger and thumb. Four corners plus leaves in opposite pairs puts you in the mint family: lavender, catmint, salvia, purple dead-nettle, henbit and ground ivy. NC State describes square stems for lavender, catmint, woodland sage and both dead-nettles. Purple loosestrife also has a square stem, but separate five- to six-petaled flowers instead of two-lipped ones.

2. What month is it? A purple shrub in May is likely lilac; the same color in August is likely butterfly bush. Wisteria flowers in spring, often before its leaves are fully out. Ironweed and New England aster carry purple into September and October, when most garden plants have finished.

3. Is it a single flower or a head of many? Thistles, knapweed, ironweed, asters and coneflowers carry dozens of tiny flowers packed into one head. Iris, clematis and African violet carry real single flowers. Spikes and chains (lavender, salvia, loosestrife, foxglove, wisteria) are a third build.

The chart puts names on these builds. Thistles, knapweed and asters carry a flower head; lavender, salvia and loosestrife carry a spike. Raceme, panicle, umbel, compound umbel, corymb and cyme cover the other ways flowers branch from a stem, and the arrangement stays the same whatever shade of purple the flowers turn.

Look-alikes: same color, different plant

Lavender, catmint and salvia. All three are aromatic mint-family plants with square stems and violet spikes. The leaves split them: lavender leaves are narrow, under an inch long and white-felted gray-green; catmint leaves are 1 to 2 inch gray-green ovals with rounded teeth; woodland sage leaves are 3 to 4 inches long, green, wrinkled and toothed, and its flower spikes carry reddish-purple bracts (NC State). Crush a leaf: lavender's scent is unmistakable once you know it.

Lilac and butterfly bush. Butterfly bush is sometimes called summer lilac. NC State gives butterfly bush lance-shaped leaves 4 to 10 inches long with white soft hairs underneath and lilac flowers with an orange throat from June to September. Common lilac flowers in spring and has heart-shaped, smooth leaves. NC State also notes butterfly bush can be weedy, so check local guidance before you plant more.

Wisteria and clematis. Both climb fences and pergolas. Wisteria has pea-shaped flowers in hanging chains and compound leaves of 7 to 19 leaflets. Clematis has flat, open flowers made of colored sepals and climbs by twisting its leaf stalks. Our wisteria page covers Chinese, Japanese and American wisteria.

Bull thistle and common knapweed. Both make purple shaving-brush heads in summer pastures. Bull thistle's bracts and leaves are spiny; Plantlife describes knapweed as "somewhat thistle-like" but with a spineless, black-brown knob of triangular bracts under each head, which gave it the name hardheads.

Ironweed and New England aster. Both are tall, purple, late-season natives of eastern North America. NC State says ironweed's heads are fluffy disc flowers only, with no rays, while New England aster has up to 100 purple rays around a yellow center.

Henbit and purple dead-nettle. Both are small spring lawn weeds with square stems and two-lipped flowers. NC State gives purple dead-nettle crowded, overlapping heart-shaped upper leaves on short stalks, tinted purple at the tip; henbit's upper leaves have no stalk and clasp the stem.

Devil's-bit scabious and its relatives. Plantlife says devil's-bit, small scabious and field scabious all carry rounded heads of many tiny flowers, usually blue and sometimes purple; devil's-bit has long oval leaves and prefers damp ground.

Purple shamrock and wood sorrel. On a windowsill, purple shamrock looks purple from across the room, but the color is in its maroon leaflets; its flowers are white to pale pink. Wild yellow wood sorrel has green leaves and yellow flowers.

Purple flowers listed as toxic

These listings come from the sources named; a plant missing from this list is not a sign that it is harmless.

• Wisteria, clematis, lavender, iris, foxglove and oxalis (shamrock plant) are each listed as toxic to dogs, cats and horses by the ASPCA. The ASPCA wisteria entry names lectin and wisterin glycoside, with vomiting and diarrhea as signs; the shamrock plant entry names soluble calcium oxalates.
• Foxglove is rated high-severity in the NC State Plant Toolbox, which lists effects on the heart among its symptoms.
• Bittersweet nightshade is rated high-severity by NC State: its plant page says it is highly toxic and may be fatal if eaten, and every part, including the red berries, is listed as poisonous.

If a pet has chewed one of these, call your vet or the ASPCA Animal Poison Control Center at (888) 426-4435. If a person has swallowed part of a plant, call Poison Control at 1-800-222-1222 in the US, or NHS 111 in the UK. This site gives no edibility advice for any purple flower.

How a photo settles it

Purple is exactly where a photo app earns its place, because the color that confuses you is the one clue a model weighs least. Take the picture in open shade rather than full sun, which bleaches violet toward pink and blue. Fill the frame with one open flower, then take a second photo that shows the stem and a leaf, so the square stem or the compound leaf is visible.

Plant Identifier names the plant from one photo, taken with the camera or picked from your library, and gives the common and scientific names. It shows one answer with no confidence score and no list of alternatives, so check that answer against the field mark in the table above: if it says catmint, the leaves should be gray-green ovals with rounded teeth. For a plant a dog or cat may chew, the Toxicology tool in Plant Identifier writes notes for people, dogs and cats, which you should check against the ASPCA list before you rely on them. If you meet a purple wildflower on a walk and want to come back when it is in seed, Plant Identifier's Plant Map pins the find when you allow location.