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Rhododendron tomentosum; photograph author not stated, no rights reserved CC0author not stated, no rights reserved · observed growing wild · 2026-09-21

Rhododendron tomentosum Harmaja

Marsh Labrador tea; Northern Labrador tea; Crystal-tea; Dwarf Labrador tea

Ericaceae · Finland, Norway +46 · listed by its genus as a temperate garden plant

Cited as Kew's World Checklist cites it, Rhododendron tomentosum Harmaja: GBIF's backbone key for this page carries the name with another author, (Stokes) Harmaja.

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Rhododendron tomentosum, photograph (c) Полина Полежанкина, some rights reserved (CC BY)Rhododendron tomentosum, photograph (c) Matt Berger, some rights reserved (CC BY)Rhododendron tomentosum, photograph (c) Igor Balashov, some rights reserved (CC BY)Rhododendron tomentosum, photograph (c) Igor Balashov, some rights reserved (CC BY)Rhododendron tomentosum, photograph (c) Igor Balashov, some rights reserved (CC BY)Rhododendron tomentosum, photograph (c) Igor Balashov, some rights reserved (CC BY) 53 photographs ›
Habitat figures
Cold floor (1 night in 100)
-19.3°C
Record low -32.0 °C in 44 years, at a typical spot in the range NASA POWER
Warmest month, mean daily high
21°C
Jul at the habitat CHELSA
Rain a year (median across the range)
691mm
12 months of 25 mm or more in the median year CHELSA
Open-sky light
1–35DLI
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 summers: the rain comes June to December, read as a summer growing season. rain rule, CHELSA
Northern hemisphere unless you set your site. The chart is in the habitat's months.

Summary

Rhododendron tomentosum (syn. Ledum palustre), commonly known as marsh Labrador tea, northern Labrador tea, marsh rosemary or wild rosemary, is a flowering plant in the subsection Ledum of the large genus Rhododendron in the family Ericaceae.

Quoted from Wikipedia, "Rhododendron tomentosum", CC BY-SA 4.0.

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.

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

Cultivation

Grouped as a temperate garden plant, listed under the genus Rhododendron in the archetype table, and Kew describes it as “subshrub or shrub”, “temperate”.

Seasons

The rain rule reads a summer growing season: 70% of the median year's rain (484 mm of 691 mm) falls in June to December, warmer than the year (wet-season mean 8.8 °C against a yearly mean of 5.4 °C). Months are the habitat's, northern hemisphere; shifted six months for a southern-hemisphere collection: December to June.

Rain

691 mm in the median year at the habitat, 484 mm of it June to December. Wettest month July at 83 mm, driest months (2 at 36 mm). Across the grid cells of the range the year's total runs 592 mm to 796 mm, the 10th to 90th percentile of each cell's own year. The median of the cells' own yearly totals, 691 mm, is the year's rain at the top of the page; the 691 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: 1 to 35 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 0, the highest 90th-percentile month 37).

Warmth and air
Temperature

Coldest: the 1st-percentile night over 44 years at a typical spot in the range is -19.3 °C, the absolute minimum -32.0 °C, 6,192 frost nights in 44 years, 140.7 a year (NASA POWER daily minima). Warmest: the 99th-percentile day is 25.8 °C. Monthly means: the coldest month's mean nightly low -8.3 °C in February (across the grid cells of the range -13.5 °C to -4.1 °C), the warmest month's mean daily high 21.0 °C in July (median year across the grid cells of the range, CHELSA). Cold floor: -19.3 °C, which is the 1st-percentile night over 44 years at a typical spot in the range (NASA POWER).

Humidity

Relative humidity at the habitat: 57 to 71% 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. NASA POWER daily minima and maxima over 44 years at a typical spot in the range, lapse-corrected -25 m to its elevation; CHELSA monthly means, median year across the grid cells of the range. The cold floor is the 1st-percentile night named in its sentence. Not a measured survival limit for any plant in a pot.

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

Group. Listed under the genus Rhododendron in the archetype table, and Kew describes it as “subshrub or shrub”, “temperate”. A group is taken only where Kew's description of the species agrees with the table's listing. The table gives no figure and no prose; the cold floor is the habitat's alone.

Climate across the habitat

Habitat months, northern hemisphere

Habitat climate of Rhododendron tomentosum through the yearWarmest month July, mean daily high 21 °C; coldest month February, mean nightly low -8.3 °C; 691 mm of rain in the median year (the twelve monthly medians added). Medians across the range, from CHELSA; the extremes at the edge from NASA POWER. Habitat months, northern hemisphere.-40°-30°-20°-10°0°10°20°30°frost25.8° 99th-percentile day, NASA POWER-32.0° lowest night in 44 years, NASA POWER°C · day and nightcold quarter0255075100125mm rainDLI 1–35RH 57–71%JanFebMarAprMayJunJulAugSepOctNovDec
day night rain medians and 10th–90th percentile across 605 habitat cells, CHELSA cold quarter: the three months around the coldest month's mean nightly low extremes over 44 years at a typical spot in the range, from NASA POWER, marked at the edge, in no month DLI, mol/m²/dayRH %each on its own scale
Figures by month
JanFebMarAprMayJunJulAugSepOctNovDec
Day °C-3 / -9–1-3 / -8–11 / -3–59 / 3–1215 / 10–1818 / 15–2121 / 19–2320 / 16–2214 / 11–178 / 4–112 / -3–5-1 / -7–2
Night °C-8 / -14–-4-8 / -14–-4-5 / -9–-21 / -4–36 / 3–811 / 8–1214 / 12–1513 / 10–148 / 5–104 / -1–6-1 / -8–1-6 / -13–-2
Rain mm47 / 36–5836 / 30–4438 / 31–4736 / 31–4450 / 39–6375 / 60–8883 / 69–10181 / 66–9764 / 55–7667 / 45–8360 / 44–7354 / 41–65
DLI2 / 1–57 / 4–1015 / 12–1825 / 24–2733 / 31–3535 / 33–3733 / 31–3526 / 22–2816 / 12–198 / 5–113 / 1–51 / 0–3
RH %71 / 68–7569 / 66–7364 / 62–6659 / 57–6157 / 55–5959 / 57–6060 / 59–6162 / 60–6363 / 62–6667 / 64–7070 / 67–7371 / 68–75
How this section is made
Each figure is the median across the 605 grid cells on land that hold in-range records (807 records in those cells; 3 more cells hold records but no land by the elevation layer, so those records sit at sea and the cells are left out), 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 606:4164 (59.675, 28.225). Over 44 years there: absolute minimum -32.0 °C, 1st-percentile night -19.3 °C, 99th-percentile day 25.8 °C. 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 605 distinct grid cells holding 807 of the 810 in-range records (3 further cells with records at sea, by the elevation layer, left out); extremes and elevation at the typical cell 606:4164 (its coldest month's mean nightly low nearest the median across cells); its position is the cell centre. Extremes: NASA POWER (MERRA-2) daily 1981–2024, cell 59.50_28.125, lapse-corrected -25 m at 6.5 °C/km. Elevation: ETOPO 2022, 3 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.
NativeFinland, Norway, Sweden, Austria, Czechoslovakia, Germany, Poland, Belarus, Baltic States, Central European Russia, East European Russia, North European Russia, Northwest European Russia, Ukraine, Altay, Buryatiya, Chita, Irkutsk, Krasnoyarsk, Tuva, West Siberia, Yakutskiya, Amur, Kamchatka, Khabarovsk, Kuril Is., Magadan, Primorye, Sakhalin, Kazakhstan, Inner Mongolia, Manchuria, Mongolia, Japan, Korea, Aleutian Is., Alaska, Greenland, Nunavut, Northwest Territories, Yukon, Alberta, British Columbia, Manitoba, Saskatchewan, Labrador, Ontario, Québec · 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 marker59.102, 17.406 · in the densest population, 352 records (42% of those in range)
Extinct inRomania · recorded as extinct by WCVP: history, not habitat; no records are tested against these regions
Kew's description“subshrub or shrub” · “temperate” · quoted from WCVP, RBG Kew (CC BY 4.0); not derived here
Evidence usedThe map marker rests on all 831 georeferenced records inside the native range; the climate across the range on the 807 of them in the land cells it reads, of 810 placed to within 10 km; the map shows only the 284 openly licensed ones, which alone would put the marker 32 km away. 2 records outside the WCVP native range not used. 21 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 inside the densest 4° block of the 831 in-range records; it decides nothing. Records outside the native range, living specimens and records marked introduced are not used; which records count.

Photographs

Rhododendron tomentosum, photograph 1did not loadRhododendron tomentosum, photograph 2did not loadRhododendron tomentosum, photograph 3did not loadRhododendron tomentosum, photograph 4did not loadRhododendron tomentosum, photograph 5did not loadRhododendron tomentosum, photograph 6did not load

Photographs: (c) Полина Полежанкина, some rights reserved (CC BY); (c) Matt Berger, some rights reserved (CC BY); (c) Igor Balashov, some rights reserved (CC BY) (4).

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

Anthoxanthum australe Anthoxanthum australe Southern Holy-grass Photo: CC BY, LjažPotentilla norvegica Potentilla norvegica Ternate-leaved cinquefoil Photo: CC BY, Syd CanningsCarex cespitosa Carex cespitosa Turfy sedge Photo: CC BY, Константин СеливёрстовGaleopsis speciosa Galeopsis speciosa Large-flowered hemp-nettle Photo: CC BY, Andreas StillerRubus saxatilis Rubus saxatilis Stone bramble Photo: CC BY, Сергей ГолубевMatteuccia struthiopteris Matteuccia struthiopteris Ostrich fern Photo: CC0, author not stated

Other Rhododendron 574 in the reference

Rhododendron aberconwayi Rhododendron aberconwayi Ericaceae Photo: Public domain, Daderot Rhododendron abietifolium Ericaceae Rhododendron acrophilum Ericaceae Rhododendron adamsii Rhododendron adamsii Sagan Dalya Photo: CC BY, Valentin KosterinRhododendron adenogynum Rhododendron adenogynum Ericaceae Photo: CC BY-SA, Kurt StüberRhododendron adenopodum Rhododendron adenopodum Ericaceae Photo: CC BY-SA, Ulf EliassonRhododendron adenosum Rhododendron adenosum Ericaceae Photo: CC0, DaderotRhododendron aeruginosum Rhododendron aeruginosum Ericaceae Photo: CC BY, Siddarth MachadoRhododendron aganniphum Rhododendron aganniphum Ericaceae Photo: CC0, Daderot Rhododendron agathodaemonis Ericaceae Rhododendron alabamense Rhododendron alabamense Alabama azalea Photo: CC BY, Rob JohnRhododendron albiflorum Rhododendron albiflorum White rhododendron Photo: CC0, author not stated all 575 ›

Names & registers

Also known as Ledum decumbens; Ledum dilatatum; Ledum graveolens; Ledum groenlandicum f. denudatum; Ledum maximum and 25 more. (GBIF Backbone)

GBIFPOWOIPNIWFOWikidataWikipediaiNaturalist

The record

Family
Ericaceae
Described by
Harmaja
Native to
Finland, Norway, Sweden +45
Wild records
831 in range · 284 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 2026-10-22
GBIF species record gbif.species answeredasked 2026-10-22GBIF's key 6400569 is Rhododendron tomentosum (Stokes) Harmaja; the page links Kew's Rhododendron tomentosum Harmaja, and the name is cited as Kew cites it
GBIF synonyms gbif.synonyms answeredasked 2026-10-22
GBIF common names gbif.vernacular answeredasked 2026-10-22names fetched 2026-10-22; "Ledum decumbens" not shown: a scientific name Kew or this reference holds
WCVP native range wcvp.distribution answeredas of 2026-06-04
GBIF occurrence records gbif.occurrences answeredasked 2026-10-22GBIF occurrence search API; 7 records within 1 km of a botanic garden, herbarium or zoo left out (7 at a botanic garden or arboretum; Wikidata's list)
Habitat climate climate answeredasked 2026-10-22
Daily extremes (NASA POWER) climate.extremes answeredasked 2026-10-22NASA POWER (MERRA-2) daily 1981–2024, cell 59.50_28.125, lapse-corrected -25 m at 6.5 °C/km
Wikidata wikidata answeredasked 2026-10-22IPNI and POWO linked by Kew's number for Rhododendron tomentosum Harmaja (the item's: IPNI 30378322-2, POWO 961622-1)
Wikipedia wikipedia answeredasked 2026-10-22
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 2026-10-22
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 7 datasets via GBIF.org; each carries its own licence.

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