1. The phylogenetic distance of an introduced plant species to a resident native community may play a role in determining its establishment success. While Darwin’s naturalization hypothesis predicts a positive relationship, the preadaptation hypothesis predicts a negative relationship. Rigorous tests of this now so-called Darwin’s naturalization conundrum require not only information on establishment successes but also of failures, which is frequently not available. Such essential information, however, can be provided by experimental introductions. 2. Here, we analysed three datasets from two field experiments in Germany and Switzerland. In the Swiss experiment, alien and native grassland species were introduced as seeds only with and without disturbance (tilling). In the German experiment, alien and native grassland species were introduced both as seeds and as seedlings with and without disturbance (tilling), and with and without fungicide application. For the seedling introduction experiment, there was an additional herbivore-exclusion treatment. 3. Phylogenetic distance affected establishment in the three datasets differently, with success peaking at intermediate distances for the seed datasets, but decreasing with increasing distances in the seedling dataset. Disturbance favored seedling survival, most likely by weakening the resident community. 4. Synthesis: By analyzing experimental introductions, we show that the relationship between phylogenetic distance and establishment, at least for seedling emergence, may actually be non-linear with an optimum at intermediate distances. Therefore, Darwin´s naturalization hypothesis and the preadaptation hypothesis need not be in conflict. Rather, the mechanisms underlying them can operate simultaneously or alternately depending on the life stage and on the environmental conditions of the resident community.