Vegetable production in open greenhouses is often associated with high inputs of synthetic pesticides. Introducing flowering plants into such greenhouses may promote the top-down pest suppression by natural enemies and reduce the reliance on pesticide use. However, it is not known how effective this practice is in organically and conventionally managed greenhouse crops. We assessed the influence of introducing flowering plants into open greenhouses with organically and conventionally managed tomato crops on the abundance of pests, natural enemies (NE), pollinators, and crop yield. We monitored tomato crops during two years in two greenhouses at four organic farms and four conventional farms that used integrated pest management (IPM). On each farm one greenhouse contained flower islands of basil (Ocimum bacilicum), marigold (Tagetes patula) and alyssum (Lobularia maritima), and the other greenhouse served as a control. Organic farms had yields comparable to conventional farms, a lower abundance of pests, less pest injury, and a higher abundance of NE. The cumulative pest:NE ratio was 9 for organic and 38 for conventional management. The effect of introducing flowering plants on arthropods depended significantly on the type of farm management. Conventionally managed tomato crops in greenhouses with seven flower islands per 100 m2 had 18% lower pest abundance compared to the control greenhouses without flowers, while flower islands did not significantly influence arthropod abundances in organically managed tomato crops. Tomato plants had a higher abundance of pests than the three introduced plant species in conventionally managed greenhouses, while marigold had a higher abundance of pests than tomato in organically managed greenhouses. Alyssum supported a relatively low pest abundance and high abundance of NE and pollinators. Our findings indicate that NE in IPM-conventionally managed greenhouses can benefit from resources provided by flowering plants, resulting in reduced pest abundance, while in organically managed greenhouses the conditions are already favourable for the suppression of pests and addition of floral resources does not further improve the effectiveness of NE. This finding highlights the potential of agroecological and organic management to reduce the reliance on synthetic pesticides without yield reduction.