Seed Production and Viability of Eight Porterweed Selections Grown In Northern and Southern Florida

Blue porterweed (Stachytarpheta urticifolia) has been classified as a Florida Exotic Pest Plant Council (FLEPPC) Category II invasive.  The native porterweed (Stachytarpheta jamaicensis) and its closely related species may offer possible landscape alternatives to blue porterweed.  Eight selections of porterweed were planted in north and south Fla., and evaluated monthly for visual quality and flowering.  Mature inflorescences were counted and removed from each plant each month for seed viability and germination tests; and at 0 and 28 weeks, the perpendicular plant widths and heights were measured to generate growth indices.  Pre-germination viability from south Fla. seed ranged from 0% (S. mutabilis ‘Violacea’) to 93% (S. jamaicensis).   After 28 days at 30/20 °C, germination was 5% or less for S. mutabilis (1%), S. mutabilis ‘Violacea’ (0%), S. ‘Naples Lilac’ (1%), S. speciosa ‘J.P.’s Pink’ (5%) and S. speciosa ‘Red Compact’ (5%).  Germination was greatest for S. jamaicensis (85%) and S. urticifolia (69%).  Stachytarpheta urticifolia also had high germination under a wider range of temperatures (20/10, 25/15, 30/20, and 35/25 °C) with or without provision of light.  Visual quality and flowering varied among cultivars and sites.  Regardless of site, S. ‘Naples Lilac’ and S. speciosa ‘Red Compact’ had exceptional flowering and form, respectively, throughout most of the study.   After 28 weeks, north Fla. plants were 1.2 to 1.8 times larger and produced 1.2 to 4.7 times more flower spikes than south Fla. plants.  Ploidy analysis using flow cytometry indicates that all of the selections evaluated in this study are polyploids, with the exception of S. speciosa ‘J.P.’s Pink’ and S. speciosa ‘Red Compact’, that are diploids.  Efforts to determine sexual compatibility between S. jamaicensis and S. urticifolia through artificial hybridizations are currently underway.