Bluebonnet Notes: Contributing Factors For A Bluebonnet Bloom (Part II): The Big Bend Of Texas

January 12, 2020  |  Big Bend

In Part I of this series we discussed the factors which contribute to a successful bluebonnet season. In Part II, we will take a look at the weather data from the Big Bend in West Texas for the 2016 and 2019 bluebonnet seasons. Those years were chosen because they are the only two years that I have been to Big Bend National Park during bluebonnet season.

Big Bend comprises a very large portion of West Texas, and therefore it is imperative to look at the rainfall in specific locations rather than to look at data for the entire area, since rainfall can vary drastically within fairly short distances. With that in mind, the rainfall data we will examine was collected at Castolon, which, more than any of the other data locations available, was right in the midst of where the bluebonnets were blooming in abundance in 2019. It is unclear from the source where exactly the temperature was measured, though I feel it is fairly safe to assume that if the temperature was above or below average in one area of the Big Bend, it probably was so in most of or the rest of the entire region.

Big Bend weather chart
Big Bend weather data for the 2016 and 2019 bluebonnet seasons. Rainfall data collected at Castolon. Temperature recorded in unknown location in Big Bend National Park. 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. "i" after a number indicates that the data is incomplete.

The bluebonnet bloom of 2019 in the Big Bend is widely regarded as the best in memory. Even though Big Bend bluebonnets (Lupinus havardii), being desert plants, do not grow in densely clustered groups like sandyland bluebonnets (Lupinus subcarnosus) or Texas bluebonnets (Lupinus texensis), one was able to see hillside after hillside covered with blue, something that no one can remember ever seeing. What led to that?

September 2018 saw more than double the average amount of rain and October 2018 saw almost double. But note that in November 2018 there was no rainfall whatsoever, which does not appear to have affected either germination or the early stages of rosette development. But from this data alone it isn’t possible to determine if either September or October is the most important month for rainfall in terms of seed germination. The available rainfall data was incomplete for both December and January, but even so, we know that rainfall was over four times the average in December and at least average in January. Big Bend bluebonnets typically begin to bloom in mid-February, and the complete lack of rain in February 2019 indicates that rosette development has completed by that time and that it, along with November, are probably the least important months in terms of rainfall needs.

If we look at the temperature data, we can see that below average temperatures in September, October, and November were sufficient for germination of a very large number of bluebonnets. During the rosette development period temperatures were mostly below average except for December, which saw average temperatures. February’s above average temperature had no adverse effect on the bloom, again indicating that rosette development has completed by the end of January and that temperature relative to the average probably only affects bloom timing (warmer temperatures would provoke an earlier bloom).

2019022402, Big Bend bluebonnets and the Sierra Ponce at sunrise

Big Bend bluebonnets (Lupinus havardii) above Castolon during the 2019 superbloom in Big Bend National Park in West Texas. Prints and licensing available.

In contrast to 2019, 2016 was a very poor year for bluebonnets in the Big Bend. Aside from roadside bluebonnets, which tend to grow and bloom differently than other bluebonnets due to their position next to asphalt, I was only able to locate two small stands of bluebonnets in the entire national park despite extensive searching, and those that I did find were past peak and/or forming seed pods in very early March 2016.

If we look at the rainfall data for 2015 and 2016, we note that September 2015 data, though incomplete, shows a total that is well below average. October 2015, also incomplete, shows slightly above average rainfall. Rainfall in November 2015 through February 2016 was well below average every month (no greater than 50% of average). Unfortunately, the data for this bloom season is not particularly helpful since the almost complete lack of bluebonnets that year does not allow a meaningful analysis of the importance of one month’s rainfall over another. The lack of rainfall also makes an analysis of the September 2015 to February 2016 temperatures moot.

Based on the 2019 data we can identify September, October, December, and January as more important months than November and February for rain with regard to Big Bend bluebonnets. We can also deduce that cooler than average temperatures throughout periods of germination and rosette development are at least sufficient, if not optimal, for Big Bend bluebonnets. In addition, February’s above average temperature means that we can most likely entirely discount the weather in the month of February from having any effect on the bluebonnet bloom in the Big Bend.

In Part III of this series, we will look at the 2010, 2015, and 2019 blooms in Atascosa County in South Texas. Stay tuned…


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

NOAA. (n.d.). Castolon Cooperative Precipitation Data. Retrieved from

US Climate Data. (n.d.). Retrieved from