The phenology and growth of water hyacinth (Eichhornia crassipes (Mart.) Solms) in a eutrophic north-central Florida lake

Abstract A population of water hyacinth in north-central Florida was monitored at weekly intervals to determine annual changes in maximum leaf length, lamina and petiole area, lamina area index, number of leaves per plant, plant density, leaf density, biomass distribution, root/shoot ratio and standing crop. Three distinct phases were apparent in the annual re-development of the population. The first was characterized by a reapportioning of the biomass distribution following winter freeze damage to the emergent shoot. The second phase began in early spring and was characterized by increased branching and ramet production, high leaf densities, and high foliar height diversity. The third phase began in late spring and was characterized by increases in leaf size, a stable number of leaves per rosette, a loss of the smaller plants resulting in a lower absolute density, and maximal standing crop values. The lamina area index began to increase during the first phase and continued through the third phase. All three phases are interpreted as mechanisms enabling an increase in the leaf area index (increases in leaves per plant, leaf density and leaf size, respectively) which closely paralleled solar radiation intensities. The maximum rate of biomass accumulation was estimated to be ca. 20 g m −2 day −1 , and the maximum relative growth rate was ca. 1.50% day −1 . Peak efficiency of tissue storage relative to solar input was estimated to be ca. 1.4%. The period of maximum growth occurred in April before the attainment of the peak standing crop (ca. 2.3–2.5 kg m −2 ). Growth appeared to be a function of an interaction between the lamina area ratio (lamina area index: standing crop) and solar radiation. Plant density was inversely related to plant size and appeared to be a function of the degree of intraspecific competition for light and space. Peak densities as high as 180 plants m −2 occurred in April but this was reduced to stable values of 70–80 during the summer. Root, stem and leaf ratios were very constant during the frost-free period of the year. Following the attainment of the ceiling yield in June a gradual decline began during which standing crop values appeared to track climatic conditions in a ‘steady-state’ situation. As the standing crop declined, so did the average height of the canopy and the population responded through an increase in plant density. We suggest that the response of water hyacinth to various environmental conditions through changes in plant proportions, leaf size and form, sexual or vegetative reproduction, etc. is the result of its adaptation to tropical riverine systems in which the plant is likely to encounter frequent expansions or contractions of the available habitat with alternating wet and dry seasons. These adaptations permit the plant to persist upstream following flooding, rapidly recolonize areas after being washed out, and to establish new populations if swept downstream. This ability to rapidly re-establish populations following extreme perturbations makes permanent control of this weed difficult in its adventive range.