Porous or dry soils are normally light and loose, there is no stagnant water but relatively quickly flows in deeper layers; such ground are more airy and warmer, yet drier and usually contain less humus and for such undergrowth it is often to for drought to appear (e. g. rockgardens, walls, by paths and roads, on gravel, also on gravel surface in towns and close to buildings ...), plants of such undergrowth need well-drained soil, they tolerate drought but cannot tolerate constant moisture or even flooding.