When does birdcage evening primrose bloom in California?

Most often in May. Across 2,005 dated, research-grade observations of Oenothera deltoides in California, the flowering season runs roughly January to August.

Peak May In flower 2,005 Examined 2,325 State California

Flowering 2,005 in flower of 2,325 examined

81%

85%

96%

89%

100%

86%

83%

14%

69%

76%

Proportion of examined Oenothera deltoides in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	197	244	81%	75% to 85%
Feb	447	523	85%	82% to 88%
Mar	845	879	96%	95% to 97%
Apr	132	148	89%	83% to 93%
May	34	34	100%	90% to 100%
Jun	17	17	100%	82% to 100%
Jul	12	14	86%	60% to 96%
Aug	5	6	83%	44% to 97%
Sep	1	7	14%	3% to 51%
Oct	1	17	6%	1% to 27%
Nov	159	231	69%	63% to 74%
Dec	155	205	76%	69% to 81%

Peak flowering in May. Each bar is the share of Oenothera deltoides 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. 2,005 of 2,325 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 Oenothera deltoides 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

Oenothera deltoides

birdcage evening primrose

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.