The biology and control of Azolla filiculoides Lam. and Lemna minuta Kunth.
The aim of this study was to improve our understanding of the biology of two
alien, invasive, free-floating species, the Water Fern Azollafiliculoides and the Least
Duckweed Lemna minUla, which cause severe weed problems in Britain. Hence,
control practices, which had been based upon anecdotal information, could be given a
An appraisal of the known distributions of the two species in Britain was
undertaken. From an analysis of spread to date, it can be predicted that L. minUla will
be present in 150 and A.filiculoides in 500, 10 x 10 km grid squares by the year 2000.
Neither A. filiculoides nor L. minuta produce specialised overwintering
structures and both rely heavily on ordinary, vegetative plants to overwinter. It was
found that vegetative plants of both species can survive encasement in ice and
laboratory studies show that they can withstand short exposure to sub-zero
temperatures above approximately -4 °C. However, field evidence suggests that both
species can survive much more severe temperature conditions, so both are considerably
less cold-sensitive than previously suggested.
L. minuta is not known to reproduce sexually in Britain. However, A.
filiculoides sporulates regularly over a wide geographical range to produce numerous,
viable sporocarps. These sporocarps form a 'seed' bank in the sediment and may
ensure population survival because they are more freeze- and desiccation- tolerant than
adult plants. Sporocarps require temperatures of not less than 10 °C and a short burst
of light to germinate.
L. min uta plants vary very little seasonally (although larger summer fronds
can be easily confused with the common, native duckweed Lemna minor). In contrast,
three forms of A. jilicuJoides can be identified; survival, mat and colonising.
Competition experiments suggested the following tentative order of decreasing
competitive ability; A. filiculoides > L. minuta > L. minor. This was probably a result
of the more erect A filiculoides plants growing over the Lemna fronds. Not all field
evidence supports this finding.
Floating mats of A.filiculoides and L. minuta cause similar ecological problems
because they reduce light, pH and oxygen and increase detrit~:s and probably alter
nutrient loading. Laboratory studies showed that morphologically and physiologically
plastic species of submerged plants, ego Elodea nuttall;;, could withstand these
conditions better than less adaptable species, ego Potamogeton crispus.
Four chemical control methods were tested over a range of dosages in the
laboratory; diquat (as both spray and sub-surface injection), glyphosate (as spray) and
terbutryn (as sub-surface application). These treatments were unsuccessful in
controlling L. minuta, in contrast to anecdotal field evidence where terbutryn is
considered effective. Glyphosate and diquat sprayed at 1.8 kg ai ha-1 and 1 kg ai ha-1
controlled A. jiliculoides. A. jiliculoides sporocarps were resistant to all chemical