When does castor bean bloom in California?

Most often in May. Across 194 dated, research-grade observations of Ricinus communis in California, the flowering season runs roughly April to July.

Peak May In flower 194 Examined 630 State California

Flowering 194 in flower of 630 examined

17%

29%

28%

39%

42%

36%

37%

31%

24%

25%

32%

Proportion of examined Ricinus communis in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	9	53	17%	9% to 29%
Feb	12	41	29%	18% to 44%
Mar	19	68	28%	19% to 40%
Apr	31	80	39%	29% to 50%
May	18	43	42%	28% to 57%
Jun	17	47	36%	24% to 50%
Jul	16	43	37%	24% to 52%
Aug	14	45	31%	20% to 46%
Sep	14	59	24%	15% to 36%
Oct	14	57	25%	15% to 37%
Nov	15	47	32%	20% to 46%
Dec	15	47	32%	20% to 46%

Peak flowering in May. Each bar is the share of Ricinus communis 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. 194 of 630 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 Ricinus communis 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

Ricinus communis

castor bean

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.