Blue Cohosh Caulophyllum thalictroides
Blue Cohosh is a perennial wildflower native to Canada and the lower 48 states. It grows to 2.5 ft and blooms Jul in full sun, with brown fruit.
More about this plant
Caulophyllum thalictroides, the blue cohosh, is a species of flowering plant in the Berberidaceae (barberry) family. It is a medium-tall perennial with blue berry-like fruits and bluish-green foliage. The common name cohosh is probably from an Algonquian word meaning "rough". The Greek-derived genus name Caulophyllum signifies "stem-leaf", while the specific name thalictroides references the similarity between the large highly divided, multiple-compound leaves of meadow-rues and those of blue cohosh. Wikipedia →
Growing & care
USDA PLANTS · TRY- Sun
- Full sun
- Soil & moisture
- Medium moisture
- Soil pH
- 4.5–7
- Fertility need
- High
- Adapts to
- Medium (loam)
- Hardiness
- USDA zone 4+
- Height
- 2.5 ft
- Mature width
- ~ 2 ft wide DERIVED derived estimate — open-grown crown allometry (height × growth-form ratio); not a measurement
- Spacing
- 2–3 ft apart from USDA planting density
- Spread
- None — clumping
- Growth rate
- Moderate
- Growth form
- Multiple stems
- Lifespan
- Perennial · moderate
- Foliage
- Broadleaf · medium texture
- Active growth
- Spring & summer
- Fruit
- Brown
- Flower colour
- Green AI AI image-analysis of community-science photos (~87% expert agreement) — not a botanical record
- Propagate by
- Bare root, Container, Sprigs, Corms
- Seed starting
- Needs cold stratification a cold-moist spell before it germinates
- Seeds ripen
- Spring – Summer seed-collection / harvest window
- In the trade
- No known commercial source
- Deer browsing
- Low often deer-resistant
- Resprouts if cut
- No
Sow timing keys off your local last- and first-frost dates.
Wildlife & pollinators
How pollinator value is scored →❧ Caterpillar hosts ~3 caterpillar species
Caulophyllum supports ~3 caterpillar species.
Native butterfly & moth caterpillars are the base of the terrestrial food web — most songbirds rear their young almost entirely on them. As a host for native Lepidoptera this is a modest genus.
Recorded feeding on Caulophyllum in North America, including:
Across 86 U.S. vegetation-survey plots that recorded Blue Cohosh, these catalog species turned up most often in the same plot — a real field co-occurrence signal, not a planting prescription.
Related plants
More Caulophyllum →
How we know this (6) Methods & honest limits
We read each species’ measured specific leaf area (leaf area per unit dry weight) from the Global Spectrum dataset and band it: thin & fast (high SLA), medium, or thick & tough (low SLA). The global median is about 10 mm²/mg.
Honest limits: A species-mean from pooled measurements — individual plants vary with light and site. A broad strategy signal, not a precise per-plant figure.
We place each species on Grime’s competitor–stress-tolerator–ruderal (C–S–R) triangle using the globally-calibrated “StrateFy” method: leaf size drives the competitor score, dense low-area leaves the stress-tolerator score, and thin high-area leaves the ruderal score. The result is a C/S/R percentage mix and one of 19 strategy classes; we show it in plain words and keep the percentages for the curious.
Honest limits: A species-mean strategy from pooled global leaf measurements — a broad ecological signal, not a precise per-plant or per-site value. Derived, never a measured fact.
A recorded categorical fact: each species is tagged C3 (standard), C4 (heat/water-efficient) or CAM (succulent, night-time CO₂ uptake) — or a facultative combination. We only show a trait card for the noteworthy C4/CAM cases; C3 is the unremarkable majority, kept in the data but not surfaced as a card.
We take the plant’s U.S. county range and look up each county’s long-term climate, then summarise: native rainfall = the median annual rainfall across its counties; heat tolerance = the warm end (90th percentile) of average temperature across its range (mirroring how our cold-hardiness floor uses an extreme). Counties + medians blunt the roadside/observer bias that makes point-level climate unreliable. Needs at least five placeable counties.
Honest limits: A realized, sampling-biased niche (where it has been recorded, not its physiological optimum), and county climate is coarse — large Western counties span deserts and mountains. Derived guide, never a measured fact.
For woody plants that have a height but no measured crown, we estimate width = height × a crown-to-height ratio fit for that plant’s form (conifers narrower than broadleaf trees, shrubs widest), calibrated on our measured open-grown crowns and capped at the largest one ever measured. A measured crown always wins; herbaceous plants get nothing (no anchor).
Honest limits: A coarse class-median estimate for garden-scale spacing, not a measurement; woody single/multi-stem forms only.
McKenzie et al. assigned each species a flower colour with a GPT-4V vision model over iNaturalist photos. We use the confident tier plus a separately-labelled lower-confidence top-up (mostly inconspicuous green/brown flowers), and render it as a small tint — never as an asserted fact, and kept out of the written synopsis.
Honest limits: AI image inference (~87% expert agreement on the confident tier); a decorative, confidence-tiered indicator, not a measured trait.
Sources for this entry (36) Open & cited
Cite this page Open data, please attribute
PlantKey’s data is open under CC BY-SA 4.0 — free to reuse and adapt, with attribution and the same licence. Photos keep their own per-image licence + credit (see Sources above).
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