When does Spanish Broom bloom in California?

Most often in May. Across 394 dated, research-grade observations of Spartium junceum in California, the flowering season runs roughly March to June.

Peak May In flower 394 Examined 466 State California

Flowering 394 in flower of 466 examined

45%

55%

87%

96%

98%

95%

78%

67%

14%

33%

15%

Proportion of examined Spartium junceum in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	5	11	45%	21% to 72%
Feb	6	11	55%	28% to 79%
Mar	20	23	87%	68% to 95%
Apr	81	84	96%	90% to 99%
May	151	154	98%	94% to 99%
Jun	80	84	95%	88% to 98%
Jul	29	37	78%	63% to 89%
Aug	14	21	67%	45% to 83%
Sep	2	14	14%	4% to 40%
Oct	4	12	33%	14% to 61%
Nov	0	2	too few examined
Dec	2	13	15%	4% to 42%

Peak flowering in May. Each bar is the share of Spartium junceum 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. 394 of 466 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. One month has fewer than 5 examined observations, so no proportion is drawn for it. 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 Spartium junceum 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

Spartium junceum

Spanish Broom

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.