Sphagnum cultivation is a type of paludiculture and a way to use formerly drained peatlands productively but under wet and therefore climate-friendly conditions. Where Sphagnum mosses are cultivated other plant species will also establish and possibly compete with the Sphagnum. The aim of this study was to determine which factors influence vascular plant cover as well as plant species numbers at Sphagnum cultivation sites and to derive recommendations for their management. Two cultivation sites were studied in northwest Germany. One of these was established directly after peat extraction while the other was rewetted seven years prior to establishment. Irrigation ditches for water management were installed at both sites. The cover of vascular plants and the number of plant species present were determined in systematically positioned plots. Six variables were tested for their influence on the assessed data by applying boosted regression tree models. The main factors influencing vascular plant cover at the two Sphagnum cultivation sites were the distance to an irrigation ditch (m), the site (location) and Sphagnum cover (%). The number of species per plot was influenced mainly by Sphagnum cover (%), the distance to an irrigation ditch (m) and the donor species used for initiating the cultivation sites. A sufficient supply of nutrient-poor water and optimal Sphagnum growth can reduce vascular plant cover and the number of plant species potentially present at a site. Insufficient water distribution and uneven Sphagnum establishment lead to inhomogeneous site conditions and thus to a higher number of plant species. The number and cover of plant species at a cultivation site are influenced by the vegetation of the sites’ surroundings and the selection of the donor site.