Abstract The use of the genotype mixtures, including species mixtures of trees, may be indicated on forest planting sites of variable or uncertain quality and/or when the reactions of components of the mixture to these sites are imperfectly known; that is, in cases where predictability is low. Four examples are given of planting practice in the Netherlands with species mixtures on such sites. The development of stands was monitored. This led to increased knowledge of the sites and of the reaction of the mixture components to them. On certain sites the planting of some species had been superfluous and a waste of money, as they rapidly disappeared from the mixture. Increased knowledge reduced the need for mixtures. Where the site or its vital weather characteristics remained unpredictable, the planting of a mixture retained its value. It proved difficult to maintain a particular mixture over an appreciable part of the rotation, in spite of careful thinnings. Some individual mixtures developed into pure stands, others into a mosaic of more or less pure stands of different species. In Dutch poplar culture, enough is known about sites and clones and their interaction to make the planting of single clones in pure stands attractive. If several clones are suited to the site, a mosaic of pure stands is preferred over an individual mixture.