Bluebonnet Notes: Contributing Factors For A Bluebonnet Bloom (Part III): South Texas

January 16, 2020  |  South Texas

In Part I of this series we discussed the factors which contribute to a successful bluebonnet season. In Part II we looked at weather data from the Big Bend in West Texas. In Part III, we will take a look at weather data for the 2010, 2015, and 2019 bluebonnet seasons in South Texas.

South Texas, like the Big Bend, is a very large area and it would be difficult to make generalizations about the entire area. Instead, we will examine data from Atascosa County, located south of San Antonio. Atascosa County is known to have widespread sandyland bluebonnet (Lupinus subcarnosus) blooms, provided conditions are favorable. It is imperative to note again that rainfall can vary significantly within fairly short distances, so the county-wide averages that are used herein may not be accurate for any particular place.

Atascosa County weather chart
Rainfall and temperature data from Atascosa County for the 2010, 2015, and 2019 bluebonnet seasons. Green indicates a wetter than average month, red indicates a drier than average month. Blue indicates a cooler than average month, orange indicates a warmer than average month.

The 2010 bluebonnet season in South Texas is regarded as an outstanding bloom season, and for many, the best in memory. Rainfall was so abundant during the fall of 2009 and the winter of 2010 that it is difficult to pinpoint specifically in which months it is most important. Totals were above average every month except for March, a month in South Texas which I would equate with February in the Big Bend. Sandyland bluebonnets begin blooming about mid-March and it is probably the least important month in terms of rainfall needs.

The story is the same for temperature, which was below average every single month. The only conclusions we can draw from the 2009/2010 data is that above average rainfall and below average temperatures are both favorable conditions for a sandyland bluebonnet bloom.

The 2019 wildflower season was a very good season in Atascosa County, particularly in the area surrounding the town of Poteet. As it turns out, the rainfall data from this season is much more informative than the data from the 2010 season. We can see that rainfall was way above average during the seed germination months of September and October and only slightly below average in November. During the rosette development months, rain in December was above average but then in the following three months rainfall was significantly below average. This seems to indicate that above average rainfall during the seed germination period is significant enough to overcome poor rainfall during the rosette development period, meaning that September and October, and perhaps to a lesser extent November, are the most important months for rainfall.

A look at the temperatures during the fall of 2018 and the winter of 2019 show average to below average temperatures during the seed germination period and above average temperatures during the rosette development period. Even though we know that ideal conditions for rosette development include cold weather, this data seems to indicate that significant rain during the seed germination period can overcome not only below average rainfall during the rosette development period but above average temperatures as well.

2019032601, Texas paintbrushes and sandyland bluebonnets at dusk

A line of Texas paintbrush (Castilleja indivisa) in a field of sandyland bluebonnets (Lupinus subcarnosus) at dusk near Poteet in Atascosa County in South Texas during the 2019 bluebonnet season. Prints and licensing available.

The 2015 season was also a good season in Atascosa County, and the data from the fall of 2014 and the winter of 2015 presents an interesting variation of conditions which can produce a successful bloom. Rainfall was slightly above average in September, well below average in October, then significantly above average in November, which was sufficient for germination of a large quantity of bluebonnet seed. Though rainfall during rosette development was below average overall (January rainfall was about 40 percent above average) it was sufficient for the development of a large number of bluebonnet plants. No clear pattern of rainfall is readily identifiable in this data, so an analysis of it doesn’t particularly advance the objective of this article. It does go to show, however, that there is no single prescription for a good bluebonnet bloom.

In contrast to rainfall, temperatures were more constant, mostly above average during seed germination and mostly below average during rosette development, which does support what we already believe to be the true: warm temperatures promote germination and cool temperatures promote rosette development.

In the final part of this series, we will look at the 2010, 2012, and 2019 blooms in Llano County. Stay tuned…


Climate at a Glance. (n.d.). Retrieved from

Merlo, J. (2020, January 11). Contributing factors for a bluebonnet bloom (Part I). Retrieved from