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Ceanothus arboreus; photograph (c) Trevor Van Loon, some rights reserved (CC BY) CC BY(c) Trevor Van Loon, some rights reserved (CC BY) · observed growing wild · 2024-03-09

Ceanothus arboreus Greene

Felt-leaf ceanothus; Catalina mountain-lilac; Catalina ceanothus; Island ceanothus

Rhamnaceae · California, Mexican Pacific Is.

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Ceanothus arboreus, photograph (c) Matt Berger, some rights reserved (CC BY)Ceanothus arboreus, photograph (c) dorian, some rights reserved (CC BY)Ceanothus arboreus, photograph (c) dorian, some rights reserved (CC BY)Ceanothus arboreus, photograph (c) dorian, some rights reserved (CC BY)Ceanothus arboreus, photograph (c) dorian, some rights reserved (CC BY)Ceanothus arboreus, photograph (c) dorian, some rights reserved (CC BY) 90 photographs ›
Habitat figures
Coldest month, mean nightly low
11.4°C
Feb at the habitat, not a floor; extremes not used (sea cell) CHELSA
Warmest month, mean daily high
19°C
Jul, Aug and Sep at the habitat CHELSA
Rain a year (median across the range)
501mm
6 months of 25 mm or more in the median year CHELSA
Open-sky light
18–52DLI
Lowest to highest month (DLI) CHELSA
The year in your months, northern hemisphere
  1. Jan
  2. Feb
  3. Mar
  4. Apr
  5. May
  6. Jun
  7. Jul
  8. Aug
  9. Sep
  10. Oct
  11. Nov
  12. Dec
Wet winters: the rain comes December to March, read as a winter growing season. rain rule, CHELSA
Northern hemisphere unless you set your site. The chart is in the habitat's months.

Summary

Ceanothus arboreus is a species of perennial shrub to small tree in the family Rhamnaceae, commonly known as the feltleaf ceanothus, island ceanothus, and island mountain lilac. It is the largest member of the California lilacs (the Ceanothus genus), and is characterized with glossy, dark green foliage that is adorned by pale blue to white flowers in bloom. It is endemic to the Channel Islands of California and Guadalupe Island in Mexico, only being re-discovered on Guadalupe Island after the elimination of feral goats.

Quoted from Wikipedia, "Ceanothus arboreus", CC BY-SA 4.0.

About the genus · Ceanothus

Ceanothus is a genus of about 50–60 species of nitrogen-fixing shrubs and small trees in the buckthorn family (Rhamnaceae). Common names for members of this genus are buckbrush, California lilac, soap bush, or just ceanothus. The genus is native to North America, ranging from Canada and the United States south through Mexico to Panama, with the center of diversity in the California floristic province of the western United States and Mexico, where around ~42 species are endemic out of the roughly ~58 species in the genus. More on Wikipedia ›

Quoted from Wikipedia, "Ceanothus", CC BY-SA 4.0.

Cultivation

Seasons

The rain rule reads a winter growing season: 70% of the median year's rain (385 mm of 503 mm) falls in December to March, cooler than the year (wet-season mean 12.9 °C against a yearly mean of 15.4 °C). Months are the habitat's, northern hemisphere; shifted six months for a southern-hemisphere collection: June to September.

Rain

503 mm in the median year at the habitat, 385 mm of it December to March. Wettest month February at 111 mm, driest month July at 1 mm; 3 months under 5 mm. Across the grid cells of the range the year's total runs 312 mm to 821 mm, the 10th to 90th percentile of each cell's own year. The median of the cells' own yearly totals, 501 mm, is the year's rain at the top of the page; the 503 mm here is the median year's, the twelve monthly medians added, which the rain rule reads. Habitat calendar, northern hemisphere; median year across the grid cells of the range, CHELSA.

Light

Open sky over the habitat: 18 to 52 mol/m²/day across the year (daily light integral, median year across the grid cells of the range, CHELSA; the lowest 10th-percentile month across the cells is 14, the highest 90th-percentile month 53).

Warmth and air
Temperature

Monthly means: the coldest month's mean nightly low 11.4 °C in February (across the grid cells of the range 5.8 °C to 12.6 °C), the warmest month's mean daily high 19.4 °C in August and September (median year across the grid cells of the range, CHELSA). Cold floor: none read. The nearest figure is 11.4 °C, the coldest month's mean nightly low, February, in the median year (CHELSA); a month's mean of its nightly lows is warmer than the nights a floor is read from. The daily extremes were read at a weather cell that is mostly sea and are not used.

Humidity

Relative humidity at the habitat: 57 to 65% across the year (monthly means, median year across the grid cells of the range, CHELSA). A figure about the air, saying nothing about how the plant takes water.

How this section is made

Each card states this species' habitat figures with their sources, and what two fixed rules read from them; nothing here says what the plant does, wants or tolerates. The rules · the terms.

Seasons. Read from the median year of CHELSA monthly rainfall and mean temperature across the grid cells of the range: the rain rule takes the smallest set of months carrying 70% of the rain and calls it winter when its mean is more than half a degree below the year's, summer when more than that above, and neither in between; the temperature rule, used only under 120 mm, names the cooler six months when they are at least a degree cooler than the other six. Months are shifted for the other hemisphere only where the temperature curve moves 4 °C or more and the habitat lies 10° or more from the equator. A reading of the curves, not an observation of the plant.

Rain. CHELSA monthly precipitation, median year across the grid cells of the range. The rain that falls where the species is recorded; nothing about how the plant takes water.

Light. CHELSA shortwave radiation at each envelope cell, a daily-mean flux taken to a daily total and converted at 2.07 mol of PAR per MJ. The sky over the habitat, measured; what reaches a plant under a rock, a shrub or a shade cloth is not.

Temperature. CHELSA monthly means, median year across the grid cells of the range. No cold floor is read without a daily extremes series; the coldest month's mean nightly low named in its sentence is not one. Not a measured survival limit for any plant in a pot.

Humidity. CHELSA relative humidity, median year across the grid cells of the range.

Climate across the habitat

Habitat months, northern hemisphere

Habitat climate of Ceanothus arboreus through the yearWarmest months July, August and September, mean daily high 19 °C; coldest month February, mean nightly low 11.4 °C; 503 mm of rain in the median year (the twelve monthly medians added). Medians across the range, from CHELSA. Habitat months, northern hemisphere.0°5°10°15°20°25°30°35°°C · day and nightcold quarter050100150200mm rainDLI 18–52RH 57–65%JanFebMarAprMayJunJulAugSepOctNovDec
day night rain medians and 10th–90th percentile across 55 habitat cells, CHELSA cold quarter: the three months around the coldest month's mean nightly low DLI, mol/m²/dayRH %each on its own scale
Figures by month
JanFebMarAprMayJunJulAugSepOctNovDec
Day °C14 / 13–1714 / 13–1814 / 13–1915 / 14–2216 / 14–2517 / 15–2819 / 17–3119 / 17–3219 / 18–3018 / 17–2517 / 15–2115 / 13–17
Night °C12 / 5–1311 / 6–1312 / 7–1312 / 8–1413 / 10–1515 / 12–1617 / 14–1817 / 14–1817 / 13–1815 / 11–1714 / 8–1512 / 5–13
Rain mm109 / 65–167111 / 74–15480 / 49–12532 / 18–549 / 5–212 / 2–61 / 0–22 / 1–35 / 4–820 / 13–4247 / 29–10085 / 48–151
DLI20 / 16–2225 / 21–2735 / 31–3644 / 40–4549 / 46–5052 / 50–5350 / 49–5247 / 44–4839 / 37–4030 / 27–3222 / 18–2418 / 14–20
RH %57 / 52–6359 / 55–6361 / 56–6560 / 54–6663 / 54–7164 / 53–7565 / 50–8264 / 51–8264 / 51–7759 / 51–6757 / 51–6257 / 52–62

The daily extremes were read at a NASA POWER cell that is 24% land: neither it nor the next candidates whose coldest month's mean nightly low is within 2 °C of the median across cells sit in a POWER cell that is mostly land, and the rule reads a cold floor only from a cell that is mostly land, so none is read for this species. Nor is the coldest month's mean nightly low taken as one: it is a mean of a month's lows, not its coldest night. What that cell gave over 44 years: absolute minimum 4.0 °C, 1st-percentile night 8.3 °C, 99th-percentile day 23.2 °C.

How this section is made
Each figure is the median across the 55 grid cells holding the 322 in-range records, with the 10th–90th percentile span across those cells after the slash where it differs. A dash is a month one or more of those cells has no figure for in the grid (a variable CHELSA does not carry there), so no median is taken rather than one over fewer cells. Extremes and elevation were read at a typical spot in the range, cell 1120:1198 (33.975, -120.075). Normals: CHELSA V2.1 1981–2010 climatology, 0.05° cells (each the mean of ~36 1 km pixels). Across the range: median and 10th–90th percentile of each month across the 55 distinct grid cells holding 322 of the 322 in-range records; extremes and elevation at the typical cell 1120:1198 (its coldest month's mean nightly low nearest the median across cells, though its NASA POWER cell is only 24% land and none of the 11 next candidates within 2 °C of the median was mostly land); its position is the cell centre. Extremes: NASA POWER (MERRA-2) daily 1981–2024, cell 34.00_-120.000, lapse-corrected +180 m at 6.5 °C/km. Elevation: ETOPO 2022, 226 m (cell mean).

Natural habitat

Native range as published by WCVP; the marker is where the records are densest and decides nothing: the climate was read across the cells of the in-range records placed well enough to read one, not at the marker.
Openly licensed records inside the range, framed on where they fall.
NativeCalifornia, Mexican Pacific Is. · regions are the level-3 units of the World Geographical Scheme for Recording Plant Distributions, as Kew's World Checklist records ranges, some under historical names such as Yugoslavia or Czechoslovakia (how).
Map marker34.029, -119.701 · in the densest population, 356 records (91% of those in range)
Kew's description“shrub or tree” · “subtropical” · quoted from WCVP, RBG Kew (CC BY 4.0); not derived here
Evidence usedThe map marker rests on all 390 georeferenced records inside the native range; the climate across the range on the 322 of them placed to within 10 km; the map shows only the 115 openly licensed ones, which alone would put the marker 2 km away. 5 records outside the WCVP native range not used. 68 in-range records are placed to worse than 10 km, or with no stated accuracy (admitted only as a person's observation to three decimals), and stay on the map but off the climate.
How this section is made
The map marker: the openly licensed record nearest the middle of the densest 1° population of the 390 in-range records; it decides nothing. Records outside the native range, living specimens and records marked introduced are not used; which records count.

Photographs

Ceanothus arboreus, photograph 1did not loadCeanothus arboreus, photograph 2did not loadCeanothus arboreus, photograph 3did not loadCeanothus arboreus, photograph 4did not loadCeanothus arboreus, photograph 5did not loadCeanothus arboreus, photograph 6did not load

Photographs: (c) Matt Berger, some rights reserved (CC BY); (c) dorian, some rights reserved (CC BY) (5).

Similar habitat climate The 6 nearest by mean daily high, mean nightly low and rain, month by month. How.

Lyonothamnus floribundus Lyonothamnus floribundus Catalina ironwood Photo: CC BY, Casey H. RichartDendromecon harfordii Dendromecon harfordii Channel Island tree-poppy Photo: CC BY, Aaron KreisbergQuercus tomentella Quercus tomentella Island oak Photo: CC BY, Devon BradleyQuercus × macdonaldii Quercus × macdonaldii MacDonald's oak Photo: CC BY-SA, paulexcoffEriogonum arborescens Eriogonum arborescens Island buckwheat Photo: CC BY, Matt BergerEriogonum grande Eriogonum grande Redflower buckwheat Photo: CC BY, Melanie J. Davis

Other Ceanothus 38 in the reference

Ceanothus americanus Ceanothus americanus New Jersey tea Photo: CC BY, WF_MICeanothus caeruleus Ceanothus caeruleus Rhamnaceae Photo: CC BY, Neptalí Ramírez MarcialCeanothus cordulatus Ceanothus cordulatus Mountain whitethorn Photo: CC BY-SA, Nancy OverholtzCeanothus crassifolius Ceanothus crassifolius Hoary-leaf ceanothus Photo: CC0, author not statedCeanothus cuneatus Ceanothus cuneatus Buckbrush Photo: CC BY, Devon BradleyCeanothus cyaneus Ceanothus cyaneus Lakeside ceanothus Photo: CC BY, Matt BergerCeanothus dentatus Ceanothus dentatus Sandscrub ceanothus Photo: CC BY, Ryan DonnellyCeanothus divergens Ceanothus divergens Calistoga ceanothus Photo: CC BY, Peter WarnerCeanothus fendleri Ceanothus fendleri Fendler buckbrush Photo: CC BY, Matt BergerCeanothus foliosus Ceanothus foliosus Wavyleaf ceanothus Photo: CC BY, jbradburyCeanothus gloriosus Ceanothus gloriosus Point Reyes ceanothus Photo: CC BY, Peter BrastowCeanothus griseus Ceanothus griseus Rhamnaceae Photo: CC BY, Irina Mitine all 39 ›

Names & registers

Also known as Ceanothus arboreus var. glaber; Ceanothus arboreus var. glabra; Ceanothus sorediatus; Ceanothus velutinus var. arboreus. (GBIF Backbone)

GBIFPOWOIPNIWFOWikidataWikipediaiNaturalist

The record

Family
Rhamnaceae
Described by
Greene
Native to
California, Mexican Pacific Is.
Wild records
390 in range · 115 open

Every figure here is derived from public data by a stated rule and names its source; nothing about this species is written for its page by a person or by AI, apart from the marked, credited quotations. How →

Where this page came from built 2026-10-21
GBIF species record gbif.species answeredasked 2026-10-21
GBIF synonyms gbif.synonyms answeredasked 2026-10-21
GBIF common names gbif.vernacular answeredasked 2026-10-21names fetched 2026-10-21
WCVP native range wcvp.distribution answeredas of 2026-06-04
GBIF occurrence records gbif.occurrences answeredas of 2026-10-21GBIF occurrence download https://doi.org/10.15468/dl.hbdw64, of 2026-10-21; 36 records within 1 km of a botanic garden, herbarium or zoo left out (34 at a botanic garden or arboretum, 1 at a herbarium, 1 at a zoo; Wikidata's list)
Habitat climate climate answeredasked 2026-10-21
Daily extremes (NASA POWER) climate.extremes answeredasked 2026-10-21NASA POWER (MERRA-2) daily 1981–2024, cell 34.00_-120.000, lapse-corrected +180 m at 6.5 °C/km; read, and set aside: NASA POWER's cell is mostly sea (24% land), so no cold floor is shown
Wikidata wikidata answeredasked 2026-10-21
Wikipedia wikipedia answeredasked 2026-10-21
iNaturalist photographs, wild inat.photos.wild answeredasked 2026-10-22from iNaturalist's open data of 2026-09-27: research-grade observations with an open-licence photograph, the 24 whose photographs iNaturalist received last (the open data holds no votes); filled 2026-10-22
iNaturalist photographs, cultivated inat.photos.cultivated not asked
GBIF media gbif.media answeredasked 2026-10-21
OpenAlex papers openalex not askednot yet asked for this reference; a later fill takes the papers from OpenAlex's snapshot

Every source is recorded as having answered, reported nothing, refused, failed, not been asked, or answered twice where one answer is taken (“not taken: two items answered”), and, for a source asked when this page was built, when: the figures are as the source held them that day; a source read from a file (Kew's World Checklist, a GBIF download) says "as of" the file's day instead; a row carried from an earlier build of the page names that build instead. Only “nothing to report” is ever shown as an absence. Records came from 41 datasets via GBIF.org; each carries its own licence.

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