Growing all Winter and Adored by Wild Monarchs: Florida's Native, EVERGREEN Milkweeds

Florida has native, evergreen milkweeds growing in winter, and monarchs are using these plants year-round.

There is a common misconception that all native milkweeds in the southeast are dormant during the winter months. 

The point of this article is to inform the public that current discussions and debates regarding “artificial winter populations” of monarch butterflies in Florida and the southeastern United States are not appealing to an important fact:  

Large, wild, evergreen populations of native milkweed are available to monarch butterflies throughout the deep south. The native species of milkweed at the center of this exciting information is aquatic milkweed - Asclepias perennis. 

A large, roadside population of aquatic milkweed in Taylor County, Florida

On on December 28th, 2014, Scott Davis, a park ranger at St. Marks National Wildlife Refuge, was leaving for the day when they noticed a plant flowering along the roadside. Upon further investigation, it was determined to be a flowering aquatic milkweed (Asclepias perennis).

As bewildering as it was to observe a native milkweed flowering in North Florida in December, it was more intriguing to the ranger to observe monarch butterfly caterpillars were actively feeding on the milkweed!

December 28th, 2014 - Monarchs and Native Milkweeds are Observed as Winter Active

 The ranger reported the winter milkweed observation to entomologists studying the annual monarch migration and was surprised to learn that none of them were aware that a native milkweed, in its native habitat, was actively growing (and providing larval food) in the winter dormant season. 

The park ranger’s observations were considered by various researchers and incorporated into publications.

Ø Aquatic Milkweed Surveys lead to BIG Discoveries

 Prompted by the discovery of aquatic milkweed’s evergreen nature, the Milkweed Foundation organized largescale surveys for aquatic milkweed. Populations of aquatic milkweed were documented and investigated across Florida, ranging from temperature zone 8a in the Florida Panhandle to temperature zone 10b in southwest Florida, at the southern boundary of aquatic milkweed’s distribution.

Milkweed Foundation - Aquatic Milkweed Survey Points

Key takeaways from our aquatic milkweed survey results are as follows:

o    Aquatic milkweed is a year-round, evergreen milkweed across its entire range in the State of Florida and       surrounding coastal plain states.

o    Monarch butterflies were documented utilizing aquatic milkweed as a larval host species 12 months of the year in Florida, statewide.

o    Aquatic milkweed is currently providing large biomasses of material for monarch butterflies due to the large populations present within its range in Florida and surrounding states.

o     Winter observations of monarch butterflies utilizing aquatic milkweed demonstrate that monarch populations are not inherently artificial or associated with plantings or colonies of non-native milkweeds. 

Further Support for the Presence of Winter Milkweeds and Indigenous Winter Monarchs 

The Milkweed Foundation has spent the last eight years surveying milkweeds across the deep south, notably in Florida. A full survey of native milkweed phenologies - i.e., their growing and flowering periods - has been recorded. 

Milkweed Foundation - Florida Native Milkweed Survey Points

Key takeaways from our multi-species milkweed survey results are as follows:

South of  approximately Orlando, Florida, the following milkweed species have consistently demonstrated dormant season growing phenologies: 

o   - Asclepias tuberosa var. rolfsii

o   - Asclepias lanceolata

o   - Asclepias longifolia

o   - Asclepias perennis

o   - Asclepias tomentosa

o   - Asclepias verticillata, specifically the wet prairie ecotype only known from Peninsular Florida

o   - Asclepias viridis

North of Orlando, Florida, the only native milkweed that is consistently phenologically-active all winter is aquatic milkweed. Traveling north from central Florida, populations of aquatic milkweed become larger at the landscape scale, and their distributions more prevalent.

Florida's Winter Milkweeds

Reasons for Aquatic Milkweed's Impacts on Monarch Butterfly Populations of the Southeast:

As has been consistently observed since the beginnings of our survey endeavors, Aquatic milkweed does not senesce (defoliate) for the winter in Temperature Zones 8-10, and minimally (in cold winters) for Temperature Zone 7B+. We have not attempted winter observational data for this species north of 7B. Other native milkweed species in the southeastern U.S., with the exception of milkweed ecotypes in Central and Southern Florida, senesce in the winter months.

Aquatic milkweed is tough! Populations are subject to freezing conditions nearly-annually throughout much of their range in the deep south. Winter frost and frozen-over surface waters do not defoliate these plants. When the ice thaws or melts, the evergreen nature of aquatic milkweed is unscathed by the cold conditions. Hydric soils figure into this phenomenon, as plants cultivated in upland garden conditions will experience leaf dehydrating (leaf shriveling) similar to what occurs with other familiar winter evergreen species, such as Catawba rhododendron.

Photo-documented occurrences of monarch larvae (at multiple instars) and of females ovipositing eggs have been documented on aquatic milkweed every month of the year, statewide across Florida. Many of these observations have occurred in very remote locations, far away from human settlements; and thus, no urban heat zone or vector for artificial populations is anywhere nearby.

February 10th, 2023 - Surveys Detect Monarchs and Aquatic Milkweeds Statewide

Aside from being evergreen, aquatic milkweed is a preferred milkweed by monarchs:

Butterfly milkweed (Asclepias tuberosa) and sandhill milkweed (Asclepias humistrata) are important milkweeds to comparatively assess in the southeast when attempting to determine the importance of aquatic milkweed to monarch butterflies. Butterfly milkweed is considered one of the most widespread and secure milkweed species in North America, and sandhill milkweed is considered one of the most important larval host milkweeds for monarchs in the southeastern United States. 

A largescale species assessment for all three species was conducted over the last ten years, and the results are very interesting!

Monarch Utilization Statistics for Three Important Milkweeds in Florida

The results of our survey are generalized in the table above. To review the large dataset behind this table, go to our Data page, or click here.

In a nutshell, monarchs utilize aquatic milkweed all year, and they love it! Our survey data suggests that monarchs show preference for aquatic milkweed, with 76% (48 discrete populations) having monarchs being documented at least once. Out of 248 wild populations of butterfly milkweed, monarchs were only documented at twice!

Butterfly milkweed and sandhill milkweed (like most milkweed species in Florida) are sharply declining and mostly located along roads or powerline easements. Although also declining and frequently roadside-only, aquatic milkweed maintains larger population numbers and distributions due to its forested wetland habitat.

Recently, roadside aquatic milkweed populations have begun to vanish due to the increase in herbicide utilization by agencies and linear facilities companies. The issue must be addressed soon!

7-19-2023 - Levy County, Florida: Mass Herbicide Kill of Roadside Asclepias perennis

 

The Best Place in Florida to Study Aquatic Milkweed and Winter Utilization by Monarchs:

Although aquatic milkweed populations can be found easily across its range, several hot spots for observing winter behaviors of aquatic milkweeds and monarch butterflies have been identified. Florida’s Big Bend coastal district is one-such region. The Big Bend is a phytogeographical interface between the Florida Panhandle and Peninsular Florida, and holds very high densities of aquatic milkweed. We refer to this region at Florida’s “monarch belt” as it is a friction-interaction zone between non-migratory and migratory monarch butterflies; and, the region holds Florida’s most voluminous populations of native milkweeds.

Monarchs travel through the Big Bend during their spring and fall migrations. Beyond this point, it is unclear where (all) the monarchs go. In the autumn migration, we do not know how much of the monarch population disperses west along the gulf coast toward their hibernaculums in Mexico, or how much of the population disperses south into peninsular Florida. The Big Bend is one-such area where large populations of evergreen aquatic milkweed provide resources and incentives for monarchs to hang-out, and exit reproductive diapause.

On the north end of this "milkweed belt", the monarch populations are presumed to be migratory, but on the south end, (from Citrus County and south) the butterflies are presumed non-migratory.

Are monarchs following this belt of milkweed down into the peninsula, much as they form non-migratory populations in cities? Is the Big Bend a geographical interface at which the migratory and non-migratory populations are interacting?

An example of a publication that references the aforementioned information and the park ranger’s observations can be found here.

One thing is for sure - Florida is providing monarchs with year-round resource potential. If the climate were to transition into a long-term warming trend, it would follow that there would be increasing numbers of monarchs documented overwintering in the region. Please reference our other pages on Asclepias perennis for more detail on the facts we have documented regarding the species.