The forests, mostly represented by secondary spruce monocultures, cover a territory of almost 53% of the total area of the Slavkovský Les PLA. Natural beech forests, fir and beech forests, or mountain spruce forests have been preserved only in fragments and in extreme conditions.
An impressive component are relict pine forests, which grow on extensive serpentinite sub-grade rock. It is one of the major serpetinite complexes in central Europe. Quite unique species may be found on serpentine, e.g. ladder spleenwort (Asplenium adulterinum), serpentine spleenwort (Asplenium cuneifolium), mouse-ear chickweed (Cerastium alsinifolium), or serpentinite rocks, e.g. the winter heath (Erica carnea) or the box-leaved milkwort (Polygala chamaebuxus).
In the south-western part of the Slavkov Forest, there are numerous raised bogs with stands of Swiss mountain pine (Pinus rotundata) in both scrub and tree form. Specific peat vegetation is typical for peat bogs and its surrounding wetland meadows are home to the Siberian iris (Iris sibirica).
From the non-original species, the most common is the obtrusive and difficult-to-liquidate giant hogweed (Heracleum mantegazzianum), which invaded the area during World War II, probably from spa parks. Specific bryophyte and algae species may be found around sulphur and carbon springs.