Blue Cohosh (Caulophyllum thalictroides)
[kawl-oh-FIL-lum thah-lik-TROY-deez]

Deciduous Perennial Upright, Unbranched in the Berberidaceae Family
A native woodland perennial with striking blue-green foliage, quiet spring flowers, and showy (but poisonous) blue seeds.

Caulophyllum thalictroides, commonly called blue cohosh, is a native North American woodland perennial prized for its unusual blue-green, compound foliage and quiet spring interest. A single unbranched stalk rises 2 to 3 feet, topped by a large, three-parted leaf that resembles meadow-rue (hence the species name thalictroides). Small, greenish-yellow to purplish flowers appear in early-to-mid spring before the tree canopy leafs out, followed in summer by showy, berry-like blue seeds. All parts of the plant, especially the seeds, contain alkaloids that are toxic if ingested, which also makes it unappealing to deer and rabbits.

Pros

  • Unusual blue-green foliage adds texture to shade gardens
  • Reliably deer and rabbit resistant
  • Long-lived once established, slowly forming colonies
  • Good native alternative for rich, moist woodland sites

Cons

  • All parts, especially the seeds, are toxic if ingested
  • Slow to establish and slow to spread
  • Inconspicuous flowers provide little ornamental color
  • Needs consistently moist soil; will not tolerate drought
Why Like As a true native of eastern and midwestern woodlands -- including scattered sites across Illinois -- blue cohosh is part of the rich spring ephemeral layer of mature deciduous forests. Its early, inconspicuous flowers are visited by small bees, flies, and parasitoid wasps drawn to its nectar glands, and its bright blue, berry-like seeds are eaten and dispersed by woodland birds and mice later in summer. Its toxic alkaloids made it historically important to Indigenous peoples and early American herbalists, though it is no longer considered safe for medicinal use.
Highlights
  • Rare true blue-green foliage color among woodland perennials
  • Early spring bloomer, active before the tree canopy fills in
  • Low-maintenance once established in the right site
 

Plant Data

Mature Size 24 - 36 inches tall. 12 - 24 inches wide.
Growth Rate Slow
Sun Exposure Full Shade - Partial Shade
Moisture Tolerance Medium - Wet: Needs consistently moist soil and does not tolerate drought; avoid sites that dry out between rains.
Zones 3a - 8b (Usually hardy to -40° F)
Tags Attracts Bees, Attracts Beneficial Insects, Attracts Birds, Attracts Pollinators, Attracts Wildlife, Deciduous, Deer Resistant, Full Shade, Illinois Native, Medium, Native to North America, Partial Shade, Perennial, Potentially Toxic, Rabbit Resistant, Shade Tolerant, Slow Growth Rate, Wet, Woodland Garden
Bloom Period March to May
Flower Color Yellow-Green to Purple-Green
Foliage Color Blue-Green
 

More Info from Experts
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Pricing and Availability History

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Past Availability
2025 : 04/03 - 09/20
2024 : 05/01 - 05/04
2023 : 05/04 - 07/22

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Field-Grown vs. Greenhouse-Grown Plants

Plants which are well-adapted to our local climate are most often field-grown (outside). Field-grown plants are generally cheaper and have the advantage of already somewhat acclimated to our cold winters, but that means they’re not artificially far along in the spring and tend to bloom at the normal time in our area.

Spring annuals and tender perennials are typically grown in Greenhouses so they can be ready and luxurious exactly when customers want them. Some perennials are also “forced” into early bloom in greenhouses. In May, there can be a very big difference between field-grown and greenhouse-grown plants of the same type. The latter typically look good right away (so they’re a great choice where that’s important), but we typically pay a premium for it.


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No Pricing or Availability Right Now

We order from a rotating cast of the best nurseries in the Great Lakes region. It looks like we've offered this plant in the past, but the nurseries we're working with this week don't appear to have it in stock at the moment.

Our goal is to bring as many plants together under "one roof" as possible, so we'll try hard to make it available again in the future!