This study dealt with the effect of two different processes on the mechanical performances of lightweight insulating plant-based concretes using hemp hurd or rice husk and a lime-based binder. The first process was a lime-water treatment of plant aggregates prior to incorporating them in the mix. The second one consisted in exposing concrete materials to moist curing and elevated temperature. Chemical and textural modifications of plant aggregates after treatment were investigated by infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Compression tests were conducted on concretes using raw or treated aggregates after 28 days of curing under standard conditions (20 °C–50%RH). Effect of high relative humidity (95%RH) and elevated curing temperature (50 °C) was first studied on lime-based mortars. Binder hardening was investigated by means of thermogravimetric analysis (TGA) and back-scattered electron imaging (BSE) on polished cross sections. Then, the same curing process was performed on plant-based concretes and the transition zone between plant aggregates and binder was analyzed by SEM. Compression tests were conducted after 28 days for both materials. Results show that lime-water treatment improved compressive strength of rice husk-based concrete but halved that of hemp concrete. As regards the impact of curing conditions, it was shown that compressive strength of hydraulic lime mortars after 28 days can be strongly increased with moist curing and elevated temperature. However, plant-based concretes suffer from a high leaching of polysaccharides in the binder and from a transition zone between aggregates and binder of very poor quality when they are subjected to moist curing. This led to unchanged or even lower mechanical performances of plant-based concretes in the case of high RH and elevated temperature compared to 20 °C–50%RH.