When does poison hemlock bloom in California?

Most often in June. Across 1,536 dated, research-grade observations of Conium maculatum in California, the flowering season runs roughly May to August.

Peak June In flower 1,536 Examined 2,475 State California

Flowering 1,536 in flower of 2,475 examined

20%

39%

45%

65%

83%

89%

80%

76%

53%

31%

24%

22%

Proportion of examined Conium maculatum in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	41	208	20%	15% to 26%
Feb	42	109	39%	30% to 48%
Mar	83	184	45%	38% to 52%
Apr	269	415	65%	60% to 69%
May	530	635	83%	80% to 86%
Jun	274	307	89%	85% to 92%
Jul	83	104	80%	71% to 86%
Aug	53	70	76%	65% to 84%
Sep	100	188	53%	46% to 60%
Oct	14	45	31%	20% to 46%
Nov	15	63	24%	15% to 36%
Dec	32	147	22%	16% to 29%

Peak flowering in Jun. Each bar is the share of Conium maculatum 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. 1,536 of 2,475 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 Conium maculatum 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

Conium maculatum

poison hemlock

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.