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Betulaceae family

Gray Alder Alnus incana

Native
Also known as: Grey Alder

Gray Alder is a perennial tree native to Alaska, Canada, and the lower 48 states. It grows to 25 ft and blooms Apr in full sun – part shade, with brown fruit. A keystone plant for native insects and the food web.

More about this plant

Alnus incana, the grey alder, tag alder or speckled alder, is a species of multi-stemmed, shrubby tree in the birch family, with a wide range across the cooler parts of the Northern Hemisphere. Tolerant of wetter soils, it can slowly spread with runners and is a common sight in swamps and wetlands. Wikipedia →

Growing & care

USDA PLANTS · TRY
Conditions
Sources · Conditions
USDA PLANTS — Sun · Soil & moisture
USDA — SoilPH — Soil pH
USDA PLANTS (DwCA, Zenodo 17903503) — Fertility Requirement / Soil Adaptation — Fertility need · Adapts to
USDA — Temp-Min °F — Hardiness
Niinemets & Valladares 2006 (CC0) — Drought tolerance · Shade tolerance · Wet-soil tolerance
Sun
Full sun – part shade
Soil & moisture
High moisture
Soil pH
5–7
Fertility need
Medium
Adapts to
Coarse (sandy), Medium (loam), Fine (clay)
Hardiness
USDA zone 4+
Drought tolerance
Low
Shade tolerance
Low
Wet-soil tolerance
Moderate waterlogging
Size & form
Sources · Size & form
USDA PLANTS (DwCA, Zenodo 17903503) — Height, Mature — Height
Tallo — Jucker et al. 2025 (CC BY 4.0) — wild crown width — Mature width
USDA PLANTS (DwCA, Zenodo 17903503) — Planting Density (per acre) — Spacing
USDA PLANTS (DwCA, Zenodo 17903503) — Growth Rate / Spread / Growth Form / Lifespan / Active Growth Period / Seed Period — Spread · Growth rate · Growth form · Active growth
USDA PLANTS — Lifespan
TRY Plant Trait Database (CC BY 4.0) · USDA PLANTS — Foliage
USDA PLANTS (DwCA, Zenodo 17903503) — Fruit/Seed Color / Fruit Persistence — Fruit
Lifespan: USDA’s lifespan scale for trees: short = under ~100 years · moderate = 100–250 · long = over 250. Balsam fir and other short-lived conifers are genuinely short for a tree, not short like an annual.
Height
25 ft
Mature width
≈ 23 ft wide wild/forest-grown — open-grown specimens spread wider
Spacing
6–12 ft apart from USDA planting density
Spread
Slow
Growth rate
Rapid
Growth form
Thicket-forming
Lifespan
Perennial · short-lived
Foliage
Deciduous broadleaf · coarse texture
Active growth
Spring & summer
Fruit
Brown
In the garden
Sources · In the garden
USDA PLANTS (DwCA, Zenodo 17903503) — Propagation Method / Commercial Availability — Propagate by · In the trade
USDA PLANTS (DwCA, Zenodo 17903503) — Germination (cold stratification) — Seed starting
USDA PLANTS — Seed Period — Seeds ripen
USDA PLANTS (DwCA, Zenodo 17903503) — Foliage Texture / Browse Palatability / Resprout — Deer browsing · Resprouts if cut
Propagate by
Seed, Bare root, Container
Seed starting
Needs cold stratification a cold-moist spell before it germinates
Seeds ripen
Spring – Summer seed-collection / harvest window
In the trade
Routinely available
Deer browsing
Medium moderately palatable
Resprouts if cut
No
Sub-canopy / understory layer — Sits in the understory of a layered food forest or polyculture.Open guide →
Nitrogen fixer — Fixes atmospheric nitrogen through root-nodule bacteria — builds soil fertility for itself and its neighbours, cutting fertiliser need.Open guide →
High-nutrient foliage — Above-average measured leaf nitrogen — its prunings make rich compost and mulch (the measured stand-in for folklore "dynamic accumulator" lists).Open guide →
Rain garden / bioswale — Rated FACW on the wetland list — tolerates wet feet, so it suits the soggy zone of a rain garden or bioswale that catches and filters runoff.Open guide →
derived roles
The garden year bloom → fruit → fall colour
J
F
M
A
M
J
J
A
S
O
N
D
Bloom
Fall colour
Bloom (the flower's colour) Fall colour
Bloom · Apr — USDA PLANTS (DwCA, Zenodo 17903503) — BloomPeriod
Fall colour · 1,061 obs · USA-NPN observed “coloured leaves” — timing varies by site & year
When to sow · for your area

Sow timing keys off your local last- and first-frost dates.

Species characteristics from USDA PLANTS (public domain) + TRY (CC BY) — general guidance, not a guarantee for your exact site. Deer "browsing" is documented palatability, not a deer-proof claim.

Wildlife & pollinators

How pollinator value is scored →
❧ Caterpillar hosts ~255 caterpillar species · keystone genus

Alnus supports ~255 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 powerhouse genus.

Recorded feeding on Alnus in North America, including:

Achatia distincta · Distinct Quaker ✦ Acleris braunana · leafroller moth ✦ Acleris caliginosana · leafroller moth ✦ Acleris cornana · leafroller moth ✦ Acleris fuscana · Small Aspen Leaftier ✦ Acleris logiana · leafroller moth ✦
+ 8 more species →
Acleris maccana · leafroller moth ✦ Acrobasis rubrifasciella · Alder Tubemaker Moth ✦ Acronicta americana · American Dagger Moth ✦ Acronicta dactylina · Fingered Dagger ✦ Acronicta fragilis · Fragile Dagger Moth ✦ Acronicta grisea · Gray Dagger ✦ Acronicta hesperida · owlet moth ✦ Acronicta impleta · Yellow-Haired Dagger ✦
Keystone count (genus-level) from Warren II 2026 (CC0) · Tallamy host-use records. Named species (a documented Nearctic sample, not exhaustive) from NHM HOSTS (CC0). ✦ = recorded on this exact species.
✦ Bees 2 bee visitors

2 native & managed bee species are documented visiting Gray Alder :

Bombus terrestris · Buff-Tailed Bumblebee Melitta eickworti · melittid bee
Visitor records (observations, not exhaustive) from Noori et al. 2026 (CC BY 4.0) · GloBI →
Wildlife & visitors 3 birds · 5 nectaring

Open records of who else uses Gray Alder — a generalist food-web signal, kept separate from the keystone Ecological Value.

5 adult butterfly & moth species are recorded nectaring at its flowers:

Lophocampa maculata · Spotted Tussock Moth Acronicta psi · Grey Dagger Hyphantria cunea Diurnea fagella Limenitis arthemis · White Admiral
Interaction records (observations, not exhaustive) from GloBI → (CC0). Counts are distinct species; names are the most-recorded. Common names from Wikidata (CC0).
Species thumbnails re-hosted from iNaturalist — Creative Commons, credited per image (hover for credit). Click any species to see it on iNaturalist. Not exhaustive; many taxa have no openly-licensed photo yet.
Frequently grows with

Across 211 U.S. vegetation-survey plots that recorded Gray Alder, these catalog species turned up most often in the same plot — a real field co-occurrence signal, not a planting prescription.

Field Horsetail · 46% Kentucky Bluegrass · 41% Bluejoint · 39% Starry False Lily Of The Valley · 34% Redosier Dogwood · 33% Woods' Rose · 33% Common Yarrow · 32% Fragrant Bedstraw · 32%
By region — what it grows with near you
"%" = share of this plant's plots that also held the species. From sPlotOpen → (Sabatini et al. 2021, CC BY 4.0) — open vegetation plots: wild community composition, not garden design.
How we know this (3) Methods & honest limits
Leaf build Direct fact

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.

Díaz, S. et al. (2022) The global spectrum of plant form and function: enhanced species-level trait dataset. Scientific Data 9:755.
Photosynthesis Direct 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.

Kattge, J. et al. TRY plant trait database — Categorical Traits Dataset (2012).
Climate niche (heat tolerance & native rainfall) Derived

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.

Realized-niche / climate-envelope approach (Pearson & Dawson 2003; Soberón 2007). Climate: NOAA NCEI nClimDiv county normals (1991–2020).
Derived values are computed from open data by a published method — labelled, cited, and never shown as a direct observation. Full methodology →
Sources for this entry (42) Open & cited
[01] Scientific name & family — USDA PLANTS (DwCA, Zenodo 17903503)
[02] Growth habit & duration — USDA PLANTS (DwCA, Zenodo 17903503)
[03] Native status & distribution — USDA PLANTS (DwCA, Zenodo 17903503)
[04] Common name — USDA PLANTS (via GBIF)
[18] Invasive / introduced status — USDA PLANTS (DwCA, Zenodo 17903503) — native status
[19] Wetland indicator — USACE National Wetland Plant List (2022)
[21] Description — Wikipedia (CC BY-SA)
[22] Ecological value — Warren II 2026 (CC0) · Tallamy host-use counts
[24] Conservation rank — NatureServe Explorer (CC BY)
[26] Functional traits — TRY Plant Trait Database (CC BY 4.0)
[27] Photosynthetic pathway — TRY Plant Trait Database (CC BY 4.0)
[28] Climate niche — Derived — realized climate niche from U.S. county occurrences (GBIF) × NOAA NCEI county climate normals 1991–2020 (public domain)
[29] Wildlife & visitors — GloBI — Global Biotic Interactions (CC0)
[31] Nitrogen fixation — Werner et al. 2014 (Dryad, CC0)
[35] Other common names — Wikidata (CC0)
[36] Stress tolerance — Niinemets & Valladares 2006 (CC0)
[38] Flower-visitor value (derived) — Noori et al. 2026 (CC BY 4.0) · GloBI
[39] Caterpillar host count — Warren II 2026 (Dryad, CC0) · Tallamy host-use counts
[40] Caterpillar species — NHM HOSTS (CC0)
[41] County range — GBIF Open Data — open-licensed occurrences (CC0/CC-BY) → county
Spot an error or have a better photo? Every field links to its source, and the whole dataset is open.
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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