  The vegetation's ``red edge'', an intensity bump in the Earth's spectrum near
700 $nm$ when sunlight is reflected from greenery, is often suggested as a tool
in the search for life in terrestrial-like extrasolar planets. Here, through
ground-based observations of the Earth's spectrum, satellite observations of
clouds, and an advanced atmospheric radiative transfer code, we determine the
temporal evolution of the vegetation signature of Earth. The excellent
agreement between models and observations motivated us to probe more deeply
into the red edge detectability using real cloud observations at longer time
scales. Overall, we find the evolution of the red edge signal in the
globally-averaged spectra to be weak, and only attributable to vegetation
changes when the real land and cloud distributions for the day are known.
However, it becomes prominent under certain Sun-Earth-Moon orbital geometries,
which are applicable to the search for life in extrasolar planets. Our results
indicate that vegetation detection in Earth-like planets will require a
considerable level of instrumental precision and will be a difficult task, but
not as difficult as the normally weak earthshine signal might seem to suggest.
