While species may coexist at population level, their individuals may still segregate, at least temporarily, and hence interact little. Aggregation among heterospecific individuals may require particular during-life traits, for example, traits limiting competition for space or resources. But is aggregation also facilitated by after-life traits accelerating decomposition of plant litters, notably through synergy among co-decomposing litters? We investigated the role of leaf traits, litter traits and litter-mixture decomposition in the spatial aggregation of individuals of pairs of grassland species in a long-term mesocosm experiment. We related aggregation between pairs of species to mean and dissimilarity in (a) during-life traits related to resource-acquisition and clonal dispersal, (b) after-life traits related to decomposition and (c) rate and synergy of decomposition. In most years, aggregation of heterospecific individuals strongly increased with means of (a) during-life clonal traits reducing competition for space, and (b) after-life traits increasing litter decomposition. Trait dissimilarities had comparatively less influence. Interestingly, in the last year, synergistic decomposition among litters improved species aggregation. Overall, after-life interactions due to fast and synergistic decomposition appear to increase during-life aggregation between individuals of different plant species. We introduce the concept of 'associational decomposition', equivalent to associational resistance, but mediated via decomposers rather than enemies. A free Plain Language Summary can be found within the Supporting Information of this article.