When does great mullein bloom in Colorado?

Most often in July. Across 179 dated, research-grade observations of Verbascum thapsus in Colorado, the flowering season runs roughly July to August.

Peak July In flower 179 Examined 512 State Colorado

Flowering 179 in flower of 512 examined

36%

73%

63%

52%

43%

Proportion of examined Verbascum thapsus in Colorado in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	1	26	4%	1% to 19%
Feb	0	5	0%	0% to 43%
Mar	0	17	0%	0% to 18%
Apr	0	37	0%	0% to 9%
May	1	23	4%	1% to 21%
Jun	15	42	36%	23% to 51%
Jul	43	59	73%	60% to 83%
Aug	29	46	63%	49% to 75%
Sep	35	67	52%	40% to 64%
Oct	51	120	43%	34% to 51%
Nov	4	47	9%	3% to 20%
Dec	0	23	0%	0% to 14%

Peak flowering in Jul. Each bar is the share of Verbascum thapsus in Colorado 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. 179 of 512 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 Colorado found Verbascum thapsus in flower, not a forecast. It is computed only from observations made in Colorado, 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

Verbascum thapsus

great mullein

Sourcesevery claim on this page

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