Hairy Arnica Arnica mollis
Hairy Arnica is a perennial wildflower native to Alaska, Canada, and the lower 48 states. It grows to 2 ft and blooms Jul in full sun – part shade, with brown fruit.
More about this plant
Arnica mollis, the soft arnica or hairy arnica, is a perennial herbaceous flowering plant in the family Asteraceae. It is native to Canada and the United States. There may be a disjunct population of this species in Coös County, New Hampshire. The species grows in subalpine mountain habitat such as meadows and streambanks. Wikipedia →
Growing & care
USDA PLANTS · TRY- Sun
- Full sun – part shade
- Soil & moisture
- High moisture
- Soil pH
- 5–7
- Fertility need
- Low
- Adapts to
- Coarse (sandy), Medium (loam)
- Hardiness
- USDA zone 4+
- Height
- 2 ft
- Mature width
- ~ 1 ft wide DERIVED derived estimate — open-grown crown allometry (height × growth-form ratio); not a measurement
- Spacing
- 1.5–2 ft apart from USDA planting density
- Spread
- None — clumping
- Growth rate
- Slow
- Growth form
- Single stem
- Lifespan
- Perennial · moderate
- Foliage
- Broadleaf · medium texture
- Active growth
- Spring & summer
- Fruit
- Brown
- Flower colour
- Yellow AI AI image-analysis of community-science photos (~87% expert agreement) — not a botanical record
- Propagate by
- Seed, Bare root
- Seed starting
- No stratification needed
- Seeds ripen
- Autumn 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 Documented caterpillar host
Recorded feeding on Arnica in North America, including:
✦ Bees 12 bee visitors
12 native & managed bee species are documented visiting Hairy Arnica :
Wildlife & visitors 30 nectaring
Open records of who else uses Hairy Arnica — a generalist food-web signal, kept separate from the keystone Ecological Value.
30 adult butterfly & moth species are recorded nectaring at its flowers — the most-recorded:
Across 37 U.S. vegetation-survey plots that recorded Hairy Arnica, these catalog species turned up most often in the same plot — a real field co-occurrence signal, not a planting prescription.
Related plants
More Arnica →
How we know this (4) 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 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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