Helianthus annuus is the most common species of sunflower in North America and is commonly found growing and flowering year-round in South Texas. In previous studies, we found no differences in growth and photosynthetic characteristics between progeny from summer-active and winter-active plants from South Texas during each season. We also found no differences in growth and photosynthetic characteristics during a winter experiment in which we grew progeny from both winter-active South Texas plants and progeny from summer-active plants from Central Texas. In addition, we found that plants from seed produced by Central Texas plants can produce viable seed when grown during the winter in South Texas. Currently, H. annuus does not grow during the winter in Central Texas. Our results suggest that in a future warmer climate when winters in Central Texas are more similar to current winters in South Texas, plants already in the Central Texas area would be able to grow and reproduce during that season. In this study, we further test this idea by determining if seeds produced by Central Texas in our previous experiment can produce another generation of plants during the winter in South Texas.
For the period of 23 October 2016 to 21 February 2017, germination percentage of seeds produced by South Texas plants was 11%, 60%, 67% and 60% in four replicate 128 cell trays. Germination percentages for seeds produced by Central Texas plants during the previous winter in South Texas were 4%, 7%, 6% and 20%. Germination percentages for seeds produced by Kansas plants during the previous summer were 2%, 17%, 2%, and 0% for the remaining three replicate 128 cell trays. In general, South Texas plants produced seeds that had a higher germination percentage than did Central Texas plants. However, at least some Central Texas seeds produced during the previous winter can germinate under what may be similar to future winter conditions in Central Texas. Some seeds from Kansas, where it will not be warm enough during the winter to allow for growth in the near future, did germinate, indicating that the ability to develop an additional growing season may be common in this species. These results, along with our previous results, indicate that H. annuus could extend its growing season in regions farther north of South Texas if winters become warmer in these areas.