When does Japanese barberry bloom in Massachusetts?

Most often in May. Across 64 dated, research-grade observations of Berberis thunbergii in Massachusetts, the flowering season runs roughly May.

Peak May In flower 64 Examined 618 State Massachusetts

Flowering 64 in flower of 618 examined

10%

51%

Proportion of examined Berberis thunbergii in Massachusetts in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	18	0%	0% to 18%
Feb	0	12	0%	0% to 24%
Mar	0	33	0%	0% to 10%
Apr	11	109	10%	6% to 17%
May	53	104	51%	41% to 60%
Jun	0	47	0%	0% to 8%
Jul	0	20	0%	0% to 16%
Aug	0	16	0%	0% to 19%
Sep	0	44	0%	0% to 8%
Oct	0	120	0%	0% to 3%
Nov	0	52	0%	0% to 7%
Dec	0	43	0%	0% to 8%

Peak flowering in May. Each bar is the share of Berberis thunbergii in Massachusetts 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. 64 of 618 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 Massachusetts found Berberis thunbergii in flower, not a forecast. It is computed only from observations made in Massachusetts, 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

Berberis thunbergii

Japanese barberry

Sourcesevery claim on this page

GBIF (iNaturalist Research-grade Observations). Dated flowering annotations in Massachusetts. 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.