When does alder buckthorn bloom in Massachusetts?

Most often in June. Across 248 dated, research-grade observations of Frangula alnus in Massachusetts, the flowering season runs roughly May to June.

Peak June In flower 248 Examined 2,044 State Massachusetts

Flowering 248 in flower of 2,044 examined

36%

37%

Proportion of examined Frangula alnus in Massachusetts in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	6	0%	0% to 39%
Feb	0	9	0%	0% to 30%
Mar	0	9	0%	0% to 30%
Apr	0	35	0%	0% to 10%
May	64	178	36%	29% to 43%
Jun	144	385	37%	33% to 42%
Jul	25	557	4%	3% to 7%
Aug	8	316	3%	1% to 5%
Sep	3	239	1%	0% to 4%
Oct	4	217	2%	1% to 5%
Nov	0	82	0%	0% to 4%
Dec	0	11	0%	0% to 26%

Peak flowering in Jun. Each bar is the share of Frangula alnus 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. 248 of 2,044 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 Frangula alnus 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

Frangula alnus

alder buckthorn

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.