Rhododendron occidentale, Fresno County, CA, USA

Rhododendron occidentale (Torr. & A.Gray) A.Gray

California azalea, Sonoma azalea, Western Azalea, Western azalea

Climate known 80 photographs Temperate garden plant

Every figure on this page is derived from public data by a stated rule and says its source; nothing here is written by a person or a model, except the Wikipedia passage, which is quoted and marked as such. How it is made.

Summary

Rhododendron occidentale, the western azalea or California azalea, is one of two deciduous Rhododendron species native to western North America. The western azalea is known to occur as far north as Lincoln and Douglas Counties in Oregon and as far south as the mountains of San Diego county. Typically found in the coastal ranges of western North America, it also grows in the Cascade and Sierra Nevada mountain ranges, but is not known east of them.

Text from Wikipedia, “Rhododendron occidentale”, CC BY-SA 4.0, quoted as written. Kept separate from everything derived here.

About the genus · Rhododendron

Rhododendron, from Ancient Greek ῥόδον (rhódon), meaning "rose", and δένδρον (déndron), meaning "tree", is a very large genus of about 1,024 species of woody plants in the heath family (Ericaceae). They can be either evergreen or deciduous. Most species are native to eastern Asia and the Himalayan region, but smaller numbers occur elsewhere in Asia, and in North America, Europe and Australia.

Text from Wikipedia, “Rhododendron”, CC BY-SA 4.0, quoted as written.

Family

Ericaceae

Described by

(Torr. & A.Gray) A.Gray

Native to

Oregon, California

Wild records

1462 in range· 240 open

At a glance

Warmest month

28°C

Jul, mean day; nights 13 °C (CHELSA)

Rain

1404mm/yr

8 wet months · peak Dec 256 mm (CHELSA)

Light

12–52DLI

mol/m²/day, winter to summer, open sky (CHELSA shortwave)

In short condensed by rule from the cultivation cards · not written by a person

Grouped as a temperate garden plant by the genus Rhododendron, which is reliably one kind of plant (archetype table). Rain rule: a winter growing season, November to March in the northern hemisphere (November to March at the habitat, northern). Habitat rain 1404 mm a year, wettest December at 256 mm, driest August at 7 mm (habitat calendar, northern hemisphere). Open sky over the habitat: 12 to 52 mol/m²/day. Cold floor -6.9 °C (1st-percentile habitat night, NASA POWER).

Each sentence is one card's own one-line form, written by the same rule as the card (Its year, Rain, Light, Temperature); the note cannot say what a card does not. Months are given for the northern hemisphere (set your site in Settings to change this), and the habitat's own alongside. The cards · the figures.

Also known as Azalea californica (Hook.) Torr. & A.Gray; Azalea californica (Hook.) Torr. & A.Gray ex Durand; Azalea californica (Hook.fil.) Kuntze; Azalea nudiflora var. ciliata Kellogg; Azalea occidentalis Torr. & A.Gray and 5 more.

Cultivation

Grouped as a temperate garden plant by the genus Rhododendron, which is reliably one kind of plant (archetype table). The table holds no figure for this group, which spans too much for one minimum; the cold floor is the habitat's alone, and no prose comes from the table.

Its year Rain rule: a winter growing season, November to March in the northern hemisphere (November to March at the habitat, northern).

This species’ habitat figures, and what two fixed rules read from them.

The rain rule reads a winter growing season: 70% of the year's rain (1098 mm of 1404 mm) falls in November to March, cooler than the year (wet-season mean 5.5 °C against a yearly mean of 11.1 °C). Months are the habitat's, northern hemisphere; shifted six months for a southern-hemisphere collection: May to September.

Read from the median year of CHELSA monthly rainfall and mean temperature across the envelope cells: 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 gives a season to reverse. A reading of the curves, not an observation of the plant.

Rain Habitat rain 1404 mm a year, wettest December at 256 mm, driest August at 7 mm (habitat calendar, northern hemisphere).

This species’ habitat figures, with their source.

1404 mm a year at the habitat, 1098 mm of it November to March. Wettest month December at 256 mm, driest August at 7 mm. Across the envelope cells the year's total runs 818 mm to 2592 mm (10th to 90th percentile of each cell's own year) (median year across the envelope cells, CHELSA; habitat calendar, northern hemisphere).

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

Light Open sky over the habitat: 12 to 52 mol/m²/day.

This species’ habitat figures, with their source.

Open sky over the habitat: 12 to 52 mol/m²/day across the year (daily light integral, median year across the envelope cells, CHELSA; the lowest 10th-percentile month across the cells is 9, the highest 90th-percentile month 53).

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.

Warmth and air Cold floor -6.9 °C (1st-percentile habitat night, NASA POWER).

This species’ habitat figures, with their source.

Temperature

Coldest: the 1st-percentile night over 44 years at the typical cell is -6.9 °C, the absolute minimum -14.5 °C, about 58 frost nights a year (NASA POWER daily minima). Warmest: the 99th-percentile day is 38.5 °C. Monthly means: coldest night 0.2 °C in December (across the envelope cells -5.2 °C to 6.7 °C), warmest day 27.5 °C in July (median year across the envelope cells, CHELSA). Cold floor: -6.9 °C, which is the 1st-percentile night over 44 years at the typical cell (NASA POWER).

NASA POWER daily minima and maxima 1981–2024 at the typical cell, lapse-corrected to its elevation; CHELSA monthly means, median year across the envelope cells, CHELSA. The cold floor is the figure named in its sentence. Not a measured survival limit for any plant in a pot.

Humidity

Relative humidity at the habitat: 48 to 59% across the year (monthly means, median year across the envelope cells, CHELSA). A figure about the air, saying nothing about how the plant takes water.

CHELSA relative humidity, median year across the envelope cells.

The figures the cards read from are in Climate below, and where they came from in Natural habitat.

Climate across the habitat

day night rain 10th–90th percentile across 505 habitat cells cold quarter: the three months around the coldest night extremes over 44 years at the typical cell, marked at the edge: undated DLI, mol/m²/dayRH %each on its own scale

Figures by month

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Day °C	9 / 4–13	10 / 5–14	12 / 7–16	14 / 11–19	18 / 14–24	23 / 18–29	28 / 19–32	27 / 20–32	24 / 19–29	19 / 15–23	12 / 8–17	8 / 4–13
Night °C	0 / -5–7	1 / -5–7	2 / -4–8	3 / -2–8	7 / 2–10	10 / 6–13	13 / 10–16	13 / 10–16	11 / 7–14	7 / 3–11	3 / -2–9	0 / -5–7
Rain mm	248 / 154–406	220 / 137–342	195 / 110–329	104 / 53–196	60 / 24–114	23 / 6–53	8 / 1–13	7 / 2–18	23 / 8–41	81 / 43–167	179 / 104–386	256 / 159–521
DLI	14 / 11–19	20 / 16–25	29 / 25–34	39 / 34–43	46 / 41–49	51 / 47–53	52 / 49–53	46 / 44–48	37 / 34–40	26 / 22–30	16 / 12–21	12 / 9–17
RH %	59 / 54–65	59 / 56–64	59 / 56–64	57 / 53–63	54 / 50–63	50 / 46–63	48 / 42–62	48 / 42–63	48 / 44–60	52 / 49–61	58 / 52–65	59 / 53–65

Each figure is the median across the 505 grid cells holding the 1400 in-range records, with the 10th–90th percentile span across those cells after the slash where it differs. Extremes and elevation were read at the typical cell 1004:1171 (39.775, -121.425). Over 44 years there: absolute minimum -14.5 °C, 1st-percentile night -6.9 °C, 99th-percentile day 38.5 °C. Normals: CHELSA V2.1 1981–2010 climatology, 0.05° cells (each the mean of ~36 1 km pixels). Envelope: median and 10th–90th percentile of each month across the 505 distinct grid cells holding the 1400 in-range records; extremes and elevation at the typical cell 1004:1171 (its coldest month's mean night nearest the median across cells); its position is the cell centre. Extremes: NASA POWER (MERRA-2) daily 1981–2024, cell 40.00_-121.250, lapse-corrected -355 m at 6.5 °C/km. Elevation: ETOPO 2022, 1001 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 every in-range record's cell, not at the marker.

Openly licensed records inside the range, framed on where they fall.

NativeOregon, California

Kew's description“shrub or tree” · “temperate”· quoted from WCVP, RBG Kew (CC BY 4.0); not derived here

Map marker38.342, -122.434 · in the densest population, 1048 records (72% of those in range)

Evidence usedThe map marker and the climate envelope rest on all 1462 georeferenced records inside the native range; the map shows only the 240 openly licensed ones, which alone would put the marker 15 km away. 3 records outside the range (gardens, roadsides, misidentifications) ignored. 62 in-range records are placed to worse than 10 km and stay on the map but off the climate.

The map markerthe map marker: the openly licensed record nearest the middle of the densest 1° population inside the densest 2° block of the 1462 in-range records; it decides nothing.

Photographs

Photographs: (c) Brian Finzel, some rights reserved (CC BY-SA) (4); (c) desertnaturalist, some rights reserved (CC BY); (c) Aaron H, some rights reserved (CC BY) (3).

Photographs: iNaturalist did not answer when this page was built; what is shown came from the sources that did.

Papers naming this species

Works whose title or abstract names Rhododendron occidentale, most cited first, from OpenAlex (CC0). A mention, not a cultivation source: nothing on this page is drawn from them.

Infectivity and sporulation potential of Phytophthora kernoviae to select North American native plants

Elizabeth J. Fichtner, David M. Rizzo, Susan Kirk, Joan Webber (2011) · Plant Pathology

Rhododendron occidentale (Western azalea)

CABI (2019) · CABI Compendium

Occurrence Download