All good vineyard soils are well drained, whether naturally or by artificial drainage. Permanent waterlogging or prolonged waterlogging after the start of spring growth is lethal to vine roots. Even marginal waterlogging can be harmful, by causing restriction of root and soil microbial activity, and consequent starvation of the vine for nutrients and root-produced growth substances (see cytokinin). Soils that are cold and wet at the time of flowering are a major factor in poor berry setting, or coulure. However, if waterlogging is temporary and the soil dries out at or just after flowering, with mild water stress before veraison, it is still possible to produce fine wines, especially on alluvial soils when sand or gravel is layered over finer sediment. Such waterlogging may reduce root depth but will not necessarily affect eventual wine quality.