Milkweed germination

Because monarch butterfly (Danaus plexippus) larvae depend exclusively on milkweed (plants belonging to the genus Asclepias) for survival, one of the leading hypotheses linked to monarch butterfly decline is the loss of milkweed habitat. But protecting this species is not an easy task, as milkweed and monarchs do not exist in isolation.  There are 73 species of milkweed native to North America (29 native to Arizona), with only about half used by monarchs. Many species are already threatened or endangered, along with the ecosystem they depend upon. In the United States alone, it is estimated that we have lost nearly 99% of our native prairies, leaving only fragmented pieces of the 1% intact. 

The decline of monarch butterflies has captured international attention, inspiring innovative approaches such as the use of citizen science to track and monitor its yearly migration. Milkweed is in increasingly high demand, and the interest in reestablishing our fragmented grasslands and prairies to support pollinator habitat is growing. From the grassroots to the national level, people with access to green spaces are planting milkweed. Some have a few potted milkweeds on an apartment patio, others are able to convert their entire yards into monarch way stations. Schools and communities are developing pollinator gardens to help educate the public and encourage participation in conservation, while native plants are becoming more popular in city landscapes.  Collaborative restoration projects are reestablishing milkweed throughout its native range- seeding it back on acres of degraded land. Monarch butterflies have become a unifying cause for the restoration community- encouraging involvement across age groups, backgrounds, and even nations. One of the most well known organizations in the fight for the monarch is Xerces Society- which provides major funding and support for milkweed cultivation, as well as many other resources for restoring and conserving pollinator habitats.   

Local knowledge is incredibly important in horticulture and I have had the privilege of working with and meeting commercial, non-profit, and hobby milkweed growers in both Texas and Arizona who were enthusiastic and eager to share their stories of propagating milkweed. The most common theme I encountered is that germination is not always consistent. In fact, milkweed is consistently inconsistent. While one stratification process works for one grower, it might not work for another. Results can vary from year to year, and also vary according to genetic origin, age, and what environmental conditions they have been exposed to. In my previous work as a conservation horticulturist, I remember trying a number of stratification procedures on seeds, only to find that the highest germination occurred with the seeds that fell into the gravel on our greenhouse floor- or ones that I mindlessly scattered in my backyard (and completely ignored). I have never been able to understand why. From the available scientific literature on growing milkweed, germination requirements do not appear to be same across species-  or even among populations of the same species (can I scream yet?). While there are some commonly used practices - such as cold-wet stratification, or prolonged soaking- many questions are still left unanswered for the majority of milkweed species. With water as one of the biggest determining factors in breaking seed dormancy, what I aim to find out is exactly how much moisture is required for germination. 

As arid ecosystems experience drastically fluctuating precipitation from year to year and the increasing intensity of drought, there is growing concern as to how climate change  might affect the establishment and future survival of milkweed. I am excited to be working with the Gornish lab and Fehmi lab at the University of Arizona to investigate the specific moisture requirements (and minimum thresholds for germination) of milkweed. By using a germination chamber, I am able to develop a controlled environment with specific moisture levels, humidity, light, and temperature, to help isolate the necessary conditions to break seed dormancy. This study will then be used to inform an experimental design in the field in which I will investigate seeding strategies that enhance their  germination and establishment in real world conditions.  With this research, I look forward to collaborating with local growers and organization such as Borderlands Restoration, Rainforest Rising, and Desert Seed Resource Center, to help enhance and inform future restoration efforts in planting milkweed, as well as provide a milkweed germination protocol for anyone interested in participating in bringing this important species back.

Philippa Johnstone is a masters student at the University of Arizona studying Ecology, Management, and Restoration of Rangeland Ecosystems. She is originally from Houston, Texas where she worked as a teacher, native landscaper, and conservation horticulturist. She moved to Tucson to build her knowledge and gain a better understanding of large scale ecological restoration practices. In particular, she is interested in seeding strategies that enhance the success of native vegetation on degraded land. She is also passionate about cultivating people-plant relationships through a practice known as eco-therapy. She hopes to bring these two fields together to help people reconnect with nature through ecological restoration work.