10 -Year Florida Milkweed Study Suggests Immediate Intervention Required to Save Remaining Native Milkweed Populations

A Roadside "Bottleneck" of Butterfly Milkweed

Takeaway: Native milkweeds and their habitats have undergone great declines due to habitat loss from development and fire-suppression, and populations under powerlines or roadsides comprise a bulk of remaining milkweed resources in many places. Roadside mismanagement with overmowing and herbicide utilization and roadside construction are drastically-impacting these remaining populations. Aggressive and immediate actions are necessary to prevent a majority of Florida’s milkweeds from vanishing.

Between 2013 and 2023, The Milkweed Foundation conducted extensive surveys across Florida and surrounding states. 23 milkweed and 6 milkweed vine species were documented and mapped. Milkweed populations were assessed (among other considerations) for the following:

-          Phenologies – e.g., when milkweed species flower, fruit, etc.

-          Status – e.g., are populations secure, increasing, declining, etc.

-          Distribution patterns

-          Recruitment (seedling germination)

-          Current threats

-          Soil characteristics

-          Associated and/or sympatric species

-          Monarch butterfly utilization rates

One of the more concerning facts arising from our study relates to the current distributions of milkweeds across Florida and surrounding coastal plain states.

The vast majority of milkweed population strongholds are confined to roads or powerline easements, with a minority of strongholds being found in cattle pastures, pine plantations, and cemeteries. At the landscape scale, roadside and powerline populations are the only remaining native milkweed population in many districts; this fact holds true on both public and private lands. A great example to demonstrate our findings can be found by reviewing survey data for three of Florida’s most important native milkweed species:

Population Geography Statistics for Three Important Milkweeds in Florida

Of the 616 populations in the table above, only 72 (11.69%) of the populations were detected within intact natural communities. In many places, roadside populations are linear, and surrounded by roads or development activities.

To review baseline data used to build this table, go here.

Asclepias humistrata and Asclepias tuberosa are upland species dependent upon fire disturbance and a healthy native plant community. In much of the their ranges in Florida, populations end at the “mow line” along the backslope of road right-of-ways (ROWs). Even where native plant communities persist adjacent to the ROWs, this populations only persist in the ROWs due to a lack of consistent fire management in the adjacent native plant communities.

Asclepias perennis typically occurs in forested wetlands disturbed by water flow, and has incurred less anthropogenic impacts than the aforementioned upland species. Despite this, many ROW populations of Asclepias perennis are also roadside-only because of modifications to the hydrology of surrounding wetlands.

The resulting phenomenon are "linear distributions" of milkweeds along the linear facilities of virtually every region in the state. Here are what these roadside bottlenecks look like:

Roadside-Only Population of Asclepias lanceolata

Roadside-Only Population of Asclepias perennis

Roadside Bottleneck of Asclepias viridis

Roadside-Only Population of Asclepias humistrata

Typical Roadside Milkweed Bottleneck along "Mow Line" Backslope of ROW

In the time since our survey efforts began in 2013, more than half of the populations that The Milkweed Foundation has mapped and monitored have been impacted or eliminated by road-widening projects, excessive mowing, or herbicide applications!

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

Generalization: Native milkweeds and their habitats have undergone great declines due to habitat loss from development and fire-suppression, and populations under powerlines or roadsides comprise a bulk of remaining milkweed resources in many places. Roadside mismanagement and herbicide utilization are drastically-impacting these remaining populations time. Aggressive and immediate actions are necessary to prevent a majority of Florida’s milkweeds from vanishing.