When does White Horehound bloom in California?

Most often in May. Across 320 dated, research-grade observations of Marrubium vulgare in California, the flowering season runs roughly April to May.

Peak May In flower 320 Examined 824 State California

Flowering 320 in flower of 824 examined

10%

71%

87%

64%

61%

63%

31%

19%

Proportion of examined Marrubium vulgare in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	4	90	4%	2% to 11%
Feb	1	51	2%	0% to 10%
Mar	9	86	10%	6% to 19%
Apr	92	129	71%	63% to 78%
May	90	104	87%	79% to 92%
Jun	55	86	64%	53% to 73%
Jul	23	38	61%	45% to 74%
Aug	20	32	63%	45% to 77%
Sep	10	32	31%	18% to 49%
Oct	5	26	19%	9% to 38%
Nov	6	65	9%	4% to 19%
Dec	5	85	6%	3% to 13%

Peak flowering in May. Each bar is the share of Marrubium vulgare in California observations in which someone actually recorded the reproductive state and found the plant in flower, not the raw number of flowering records. That distinction matters: people observe plants far more in spring than in winter, so a bare count of flowering records partly measures when people go outside. Dividing by the number examined removes that. 320 of 824 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. This is still a global aggregate and not a forecast for your garden: the same species flowers on different dates in different hemispheres. Where a species has fewer than 30 flowering records we do not draw this chart at all. Computed from 10.15468/dl.wt38fd.

What this is, and what it is not

This is a record of when people in California found Marrubium vulgare in flower, not a forecast. It is computed only from observations made in California, so it is not the species’ global average dragged onto a map: the same plant flowers on different dates in different places, and that is the entire point of the page.

It will not tell you what your particular plant will do this year. Bloom time moves with the season, with altitude, and with the weather, and a warm February pulls everything forward. We publish the distribution and the sample size, and we refuse to draw a month that too few people examined.

The plant

Marrubium vulgare

White Horehound

Sourcesevery claim on this page

GBIF (iNaturalist Research-grade Observations). Dated flowering annotations in California. Every record achieved iNaturalist quality grade Research, which is applied upstream at export. 10.15468/dl.wt38fd. Retrieved 2026-07-14.
World Flora Online Plant List. The accepted name. CC0. Retrieved 2026-07-12.