There are two reasons for the biodiversity of the La Palma flora: the geographic isolation of the island, on the one hand, and the abundance of microclimates, on the other. When the island emerged from the ocean, around two million years ago, many of the species from the Mediterranean, North Africa and the older Canary Islands occupied this new territory. They continued to evolve here, until they became different species from the original ones.

The rugged topography of the island creates different areas with different conditions of humidity, temperature and exposure to the sun. Hence, the influence of the trade winds divides the island into two clearly differentiated parts: the rainier east, and the drier and hotter west
The variation of climate with altitude divides island's flora into different layers, determined by the topography of the area and the predominance of some species over others.
Since most of the island's coastline consists of cliffs, with few sandy areas, there are only a few halophytic (salt loving) species, like the dwarf sea lavender (Limonium pectinatum) and the Canary Island samphire (Astydamia latifolia). These are some of the few species capable of living under extreme conditions, like saline moisture, poor soil and marine abrasion.

Between 100 and 400 metres above sea level, the vegetation is mainly dominated by two spurges, Canary Island Spurge or Hercules Club (Euphorbia canariensis) and Euphorbia balsamifera, known locally as Tabaiba. Other species are: the candle plant (Kleinia neriifolia), the sorrel (Rumex lunaria), and the wax plant (Ceropegia dichotoma).Further up, between 300 and 700 metres, the first trees appear, such as the dragon tree, the juniper, the Canary palm tree, the mastic tree, the mocan tree, the Canary bindweed and the Canary St.John’s wort. The best examples in the Canary Islands of the famous dragon trees, indigenous to Macaronesia, can be seen in La Palma, especially in the Breñas, Garafía, and Puntagorda, the most spectacular ones being those located in La Tosca (Barlovento) and Buracas (Las Tricias). 

The damp mountain layer lies above 700 m, and is much wider in those parts of the island affected by the trade winds (the north and northeast).
Its most characteristic ecosystems are the so-called Monteverde (evergreen species) with species such as the wax myrtle and the tree heather, and the laurel forest, which is dominated by members of the laurel family, including the Azores laurel (Laurus azorica), the Greenheart (Ocotea foetens), the Canary Island avocado (Persea indica), the Canary Island laurel (Apollonias barbujana) and the southern olive (Picconia excelsa). One of the best places to find this type of vegetation is the Reserva Natural Especial de Guelguén (Guelguen Natural Special Reserve)in Garafía and Barlovento whose ravines, running out to the sea, are home to plenty of laurel species.
The pine forests grow higher up, where the climate is dryer, consisting mostly of Canary pine trees (Pinus canariensis), which can thrive in poor soils or on the steepest slopes and can resist forest fires. In the south of the island, the pine forest extends almost down to sea level, while in the more humid north they can be found above the laurel forests. Very few shrubs can survive under pine trees. However, we find: Canary rock rose (Cistus sympithifolius) and trefoil (Lotus hillebrandii) as well as tree heathers in more humid areas and brooms in highest peaks.
Above 2000m, conditions are extremely difficult for plants. They are exposed to high doses of ultraviolet radiation and, since they are almost always above the clouds, very little moisture.  The Canary Island cedar (Juniperus cedrus) is the only tree growing at these heights, together with bushes such as sticky brooms (Adenocarpus viscosus), brooms (Genista benehoavensis), Teide white broom (Spartocytisus supranubius), savoir (Micromeri herpyllomorpha), tansy mustard (Descurainia gilva) and wall flowers (Erysimum scoparium). But the crowning jewel is the La Palma violet (Viola palmensis) which is less delicate than it looks.

Tajinaste (Viper’s bugloss) is the name the Benahoaritas (aboriginals of the island) gave to the members of a whole genus: Echium webbii, Echium gentianoides, Echium wildpretii... which have adapted to growing in different altitudes: some can grow at sea level, while others in high altitudes or among pine trees.
Among those flora species growing on rocks and slopes, the best known are the stonecrops or pinwheels (Aeonium also known as Greenovia), which have adapted to growing on slopes all over the island. Thus we can see them on coastal cliffs or on the steepest walls of the Caldera de Taburiente National Park.  Sometimes they even grow on the roofs of traditional houses. Of the dozen varieties found on the island, the most spectacular is the bejeque rojo (Aeonium nobile) or noble stonecrop, which takes years to bloom, but is spectacular when it does. The Lotus pyranthus, or kestrel beak, is also a beautiful species, but it is threatened. 

Further information on the most representative flora species on La Palma