The present work focuses on providing a better evaluation and understanding of the swelling and shrinkage behavior of plant aggregates in response to moisture variation. The morphology of the aggregates is analyzed using an optical microscope to measure the percentage of swelling and shrinkage response to the relative humidity variation. The aggregates are exposed to a relative humidity cycle with varying stages allowing swelling and shrinking of the aggregates. An investigation on the effect of salt treatments with NaHCO3 and NaCl, is also conducted to evaluate their impact on the swelling behavior of the plant aggregates studied (wheat straw, rapeseed straw, sunflower bark and sunflower pith). The results show a high percentage of swelling of the sunflower pith, followed by wheat straw, rape straw, and sunflower bark. Treatment with NaHCO3 seems to be effective to reduce the swelling and shrinkage of aggregates with a decrease of 53% unlike NaCl, which has no effect on the change in chemical components. The effect of the lime binder is also studied in order to enhance the understanding of the behaviour and design of biocomposites.