Abstract: A mycorrhiza have growth-retardant activity for Poa annua and may be used to control the growth of P. annua in high quality sport or amenity turf consisting mainly of Agrostis or Festuca species. Strains of VA mycorrhiza selected from genera Glomus, Acaulospora, Entrophosphora, Gigaspora, Scutellospora and Scierocytis may be used. The VA mycorrhiza is applied to the soil as a formulated product in which the VA mycorrhizae constitute the principal component having biological activity on the growth of turf grass. Dose rates of inoculum having an MPN of from about 200 To 1000 or more propagules per gram are effective and may be formulated with a clay or other solid carrier. The amount of formulated product applied to the turf may be from about 5 g/m2 to about 1 kg/m2. The infective prop are selected from fungal spores, mycelium, hyphae, and fragments of mycorrhiza infected roots. These may be applied to sport or amenity turf, or other grassy area in need of P.