Plants can cause mechanical weathering by sprouting and growing in soil that has collected in the cracks of a rock. As the plant grows, the roots extend and spread the crack until the rock eventually breaks.
Any plant seed that begins to grow inside a crack or fissure in a rock has the potential to cause mechanical weathering. Even plants with small root structures can eventually crumble a rock. Trees have much larger root systems and can break apart even large rocks and rock structures over time. These seeds may arrive in a variety of ways that range from birds carrying and dropping them to being blown by the wind or traveling via water.
Mechanical weathering is also referred to as physical weathering and occurs anytime there is a physical breakdown caused by animals, plants, water, salt or other factors. Mechanical weathering is what causes rocks to crumble or break. This type of weathering is constantly ongoing in nature, but is it generally a slow process that occurs over time. Nature is in a constant state of flux.
Other examples of mechanical weathering function in similar ways. Fluctuating temperatures can cause stress in pre-existing cracks that eventually cause the rock to break. Water collected in the cracks can freeze and thaw repeatedly, enlarging the cracks over time.