Natural fiber reinforced plastics (NFRP) becoming increasingly popular as an alternative for fully synthetic construction materials. They offer specific advantages like an excellent lightweight potential and a smaller ecological footprint which makes them part of bioeconomic strategies to achieve climate targets and to reduce dependencies from non-renewable resources. The recent developments related to bio-based polymer systems led to synergetic effects since their usage for NFRPs further increases the content of renewable resources while providing good mechanical performance. In this context, liquid composite molding (LCM) processes like resin transfer molding (RTM) enable the production of high quality, durable composites based on thermoset resin systems. The present work addresses the characterization of the impregnation properties of common flax fiber reinforcements with different textile architectures. Besides the state-of-the-art permeability determination, the analysis of the capillary driven saturation effects is focused. The reinforcing fabrics are prone to flow-induced void formation during impregnation due to their complex pore morphology. These voids can detrimentally affect the mechanical properties of the resulting composite. Therefore, the main objective in this work is the development of a methodical approach to derive optimal impregnation velocities for natural fiber reinforcements in LCM processes. Experimental series were conducted based on a specially developed test stand for capillary rise experiments. The textile-specific dynamic flow was modeled based on an adjusted version of the Ludwig-Washburn equation, which offers high accuracies for the description of the capillary driven saturation processes. Based on the experimental results, an approach to determine optimal impregnation velocities was applied and verified via an RTM test series. The comparison between the optimization approach and the empirical test results indicates the velocity range for optimal impregnation conditions for the considered flax fiber reinforcements. Therefore, the present investigations on the methodical optimization of the textile impregnation make a considerable contribution to the manufacturing of NFRP products with high quality and reliability.