The exotic species Bidens frondosa is rapidly spreading in northern Italian arable fields, while Bidens
tripartita, a native congeneric, shows less invasive capacity. To characterise and model the phenology of
the two species, a two-year study was conducted involving sowings at 15-day intervals between March
and August. The length of the sowing-emergence phase was well described by a simple thermal model,
with a base temperature of 4.5 ◦C, for both species. A multiplicative photothermal model was found to be
the best for the emergence-flowering phase. B. frondosa resulted as being less macrothermal and more
sensitive to photoperiod. The flowering-maturity phase lasted for 30–32 days, on average, and was well
described by a model based on soil water availability: this phase shortens when there is a lack of water,
particularly in B. tripartita. The more relevant differences between the two species seem to be related to
the vegetative phase length, which varied from 150 to 46 days and from 123 to 42 days in B. frondosa and
B. tripartita, respectively. Thanks to the high elasticity of its vegetative phase, B. frondosa can grow taller
than B. tripartita and acquire higher competitivity and seed production potential, both relevant factors
in determining the success of a species.
