As in the case of Montecristo, the absence of permanent settlements on the island has allowed to preserve the original vegetation of Giannutri more than in the other islands of the Tuscan Archipelago, where the anthropization has had a disruptive impact on the natural balance. Until the fifties of the twentieth century in the central and northern part of Giannutri there remained traces of small cultivated areas, a remain of the agricultural enterprise of the Garibaldian Gualtiero Adami. With the subsequent transformation in the tourist-bathing sense of Giannutri, all traces of the arable land disappeared while the ancient olive trees planted by Adami in the Ischiaiola – Cala Maestra locality were preserved (they can be observed walking through the paths that run along the villas in this part of the island); the pines present in the gardens of the houses around Cala Spalmatoio were instead introduced by the owners in more recent times. Following the disappearance of the cultivated fields and the cessation of any tree-felling activity to derive timber (formally impeded following the establishment of the Tuscan Archipelago National Park), a natural reforestation process spread throughout the island with the exception of of its southernmost part, more exposed to the winds coming from the open sea and therefore less suitable for the development of a dense thicket. In general, the photographs of the early twentieth century show a far more barren island than we can observe today.
A true forest of holm oaks (Quercus ilex) is found only on the northern slope of Monte Mario, presumably the last remnant of more extensive wooded areas that characterized Giannutri before the abatements, operated by man since ancient times, reduced its extension to about 1% of the total area of the island; in the rest of the north-central part of Giannutri, the bush is dominant, with the ruling presence of the turbinated juniper (Juniperus turbinata), an arboreal or shrub species particularly suitable to the rocky soils and to the Mediterranean climate of Giannutri and characterized by a pleasant resinous smell; next to the junipers it is often possible to find plants of lentisk (Pistacia lentiscus), a shrub with a pleasant aroma that produces vivid red berries, or arbutus (Arbutus unedo), with red berries larger than lentisk and similar delicate odor. The shrubs are more common in the central-southern part of Giannutri where they produce wonderful effects of color combination: the candor enriched by bright yellow flowers of the silver ragwort (Senecio cineraria) are flanked by the beautiful violet flowers of the heather (Erica multiflora) and the yellow ones of the helichrysum of the coasts (Helichrysum litoreum or “perpetuini” in Italian language) or of the tree spurge (Euphorbia dendroides), which loses its leaves in summer and takes on a fascinating skeletal appearance; widespread are also rosemary (Rosmarinus officinalis), myrtle (Myrtus communis) and the tree germander (Teucrium fruticans) with pink flowers. Among the herbaceous plants it should be reported the Friar’s Cowl (Arisarum vulgare), with large leaves and with a large flower in the form of a tube folded to a hood.
Along the coasts of the island the vegetation becomes variegated, with the presence of sporadic species landed on the island thanks to the seeds carried by the winds or by birds; there are interesting botanical rarities endemic to the southern islands of the Tuscan Archipelago: among the rocks overlooking the sea it is easy to find the Sommier’s limonium (Limonium sommierianum), a plant present only in Giannutri, Giglio and Montecristo which creates green cushions of glossy leaves and slightly folded from which emerge long green branches (it is more frequent to find them already dry) with at the ends small flowers of light colors tending to purple.
A recent problem that has affected the flora of Giannutri is the introduction by the holidaymakers of exotic and weed species, first of all the Hottentot fig (Carpobrotus acinaciformis and edulis, also known as sour fig), favorite for the magnificent flower with bright colors (yellow or pink the most common varieties). These plants have altered the botanical balance of Giannutri beginning to spread rapidly along the rocky perimeter of the island to the detriment of native vegetation: to obviate the infestation, the Tuscan Archipelago National Park Authority has launched a campaign of eradication of the Hottentot fig and other alien species from the island that seems to have obtained the expected results thanks to the use of dark drapes, spread in infested places, which do not allow the passage of solar rays necessary for the survival of the plants.